LowHOST-KRN-008Secure ConfigurationServer

TCP SYN Cookies Disabled

Checks that SYN cookies protect the server from SYN flood attacks.

Why It Matters: Without SYN cookies a simple flood of half-open connections can make every service unreachable.
How It Detects: Reads /proc/sys/net/ipv4/tcp_syncookies directly (no command is run) and compares it with the hardened value 1.
How to Fix: Create /etc/sysctl.d/60-openvapt-hardening.conf containing: net.ipv4.tcp_syncookies = 1 then apply with: sysctl --system
How to Verify: sysctl net.ipv4.tcp_syncookies should print 1.
CVSS v3.1 Vector: CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:L
Control Category: Secure Configuration: CIS Controls v8 4; NIST CSF 2.0 PR.PS
Fix Effort: Low
CIS Controls v8 Safeguard: 4.1
NIST CSF 2.0 Subcategory: PR.PS-01
CWE: CWE-770
NIST SP 800-53 Rev. 5: SC-5, CM-6
Benchmark Topic: Kernel parameters: net.ipv4.tcp_syncookies (topic only, no benchmark text)
References: Linux kernel documentation, IP sysctl

Learn the theory behind this check: P1 Linux Server Hardening.

Source Code

The complete file that defines this check. Run it with openvapt scan --only HOST-KRN-008, or read it offline with openvapt explain HOST-KRN-008.

internal/checks/host/kernel.go · Lines 1-362View on GitHub at This Commit

1package host
2
3import (
"context"
"errors"
"fmt"
"io/fs"
"sort"
"strings"
10
"github.com/openvapt/openvapt/internal/check"
"github.com/openvapt/openvapt/internal/checks/collect"
"github.com/openvapt/openvapt/internal/sys"
14)
15
16// sysctlRule describes one kernel parameter and its hardened value.
17type sysctlRule struct {
id, title, key, want, summary, rationale string
sev                                      check.Severity
cvss                                     string
ref                                      check.Ref
cwe                                      string
allowed                                  []string // values that also pass
24}
25
26// One table, one loop: each row becomes its own registered check, so every
27// kernel setting has its own catalogue page, id and result.
28var sysctlRules = []sysctlRule{
{"HOST-KRN-001", "Address Space Layout Randomisation Not Fully Enabled", "kernel.randomize_va_space", "2",
"Checks that the kernel randomises memory layout for stack, libraries, heap and VDSO.",
"ASLR makes memory-corruption exploits much harder because attackers cannot predict where code lives.",
check.Medium, "CVSS:3.1/AV:L/AC:H/PR:L/UI:N/S:U/C:H/I:N/A:N", refKernelDocs, "CWE-344", nil},
{"HOST-KRN-002", "Kernel Pointers Exposed to Unprivileged Users", "kernel.kptr_restrict", "1",
"Checks that kernel memory addresses are hidden from normal users.",
"Leaked kernel addresses defeat kernel ASLR and are the first step of most local kernel exploits.",
check.Low, "CVSS:3.1/AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:N/A:N", refKernelDocs, "CWE-497", []string{"2"}},
{"HOST-KRN-003", "Kernel Log Readable by Unprivileged Users", "kernel.dmesg_restrict", "1",
"Checks that only privileged users can read the kernel log (dmesg).",
"The kernel log can reveal addresses, hardware and driver details useful for local exploitation.",
check.Low, "CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:L/I:N/A:N", refKernelDocs, "CWE-497", nil},
{"HOST-KRN-004", "Setuid Programs May Dump Core", "fs.suid_dumpable", "0",
"Checks that privileged programs do not write core dumps.",
"A core dump of a setuid program can contain password hashes or keys and may be readable by the user who crashed it.",
check.Low, "CVSS:3.1/AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:N/A:N", check.Ref{Title: "Linux kernel documentation, /proc/sys/fs", URL: "https://docs.kernel.org/admin-guide/sysctl/fs.html"}, "CWE-528", nil},
{"HOST-KRN-005", "IP Forwarding Enabled", "net.ipv4.ip_forward", "0",
"Checks that the server does not route packets between networks.",
"A server that forwards packets can be used to pivot between networks. Only routers, VPN gateways and container hosts need this.",
check.Low, "CVSS:3.1/AV:A/AC:H/PR:N/UI:N/S:U/C:N/I:L/A:N", refKernelNet, "CWE-1188", nil},
{"HOST-KRN-006", "ICMP Redirects Accepted", "net.ipv4.conf.all.accept_redirects", "0",
"Checks that the kernel ignores ICMP redirect messages.",
"Forged ICMP redirects let an attacker on the local network reroute the server's traffic through themselves.",
check.Low, "CVSS:3.1/AV:A/AC:H/PR:N/UI:N/S:U/C:N/I:L/A:N", refKernelNet, "CWE-940", nil},
{"HOST-KRN-007", "Source-Routed Packets Accepted", "net.ipv4.conf.all.accept_source_route", "0",
"Checks that the kernel drops packets that dictate their own route.",
"Source routing lets an attacker bypass network filtering and spoof trusted addresses.",
check.Low, "CVSS:3.1/AV:A/AC:H/PR:N/UI:N/S:U/C:N/I:L/A:N", refKernelNet, "CWE-940", nil},
{"HOST-KRN-008", "TCP SYN Cookies Disabled", "net.ipv4.tcp_syncookies", "1",
"Checks that SYN cookies protect the server from SYN flood attacks.",
"Without SYN cookies a simple flood of half-open connections can make every service unreachable.",
check.Low, "CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:L", refKernelNet, "CWE-770", nil},
{"HOST-KRN-009", "Unprivileged eBPF Allowed", "kernel.unprivileged_bpf_disabled", "1",
"Checks that normal users cannot load eBPF programs into the kernel.",
"Unprivileged eBPF has been the entry point for many local privilege-escalation bugs.",
check.Medium, "CVSS:3.1/AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:L/A:L", refKernelDocs, "CWE-749", []string{"2"}},
{"HOST-KRN-010", "Process Tracing Not Restricted", "kernel.yama.ptrace_scope", "1",
"Checks that processes cannot attach a debugger to other processes of the same user.",
"Unrestricted ptrace lets any compromised process read the memory of others, for example to steal SSH agent keys or browser sessions.",
check.Low, "CVSS:3.1/AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:N/A:N", check.Ref{Title: "Linux kernel documentation, Yama LSM", URL: "https://docs.kernel.org/admin-guide/LSM/Yama.html"}, "CWE-749", []string{"2", "3"}},
{"HOST-KRN-011", "Symlink Protection Disabled", "fs.protected_symlinks", "1",
"Checks that the kernel blocks following symlinks owned by other users in world-writable sticky directories.",
"Without it, classic /tmp symlink races let local users trick root-run programs into overwriting files.",
check.Low, "CVSS:3.1/AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:N/A:N", check.Ref{Title: "Linux kernel documentation, /proc/sys/fs", URL: "https://docs.kernel.org/admin-guide/sysctl/fs.html"}, "CWE-59", nil},
{"HOST-KRN-012", "Hardlink Protection Disabled", "fs.protected_hardlinks", "1",
"Checks that users cannot create hard links to files they do not own.",
"Hard links to privileged files enable time-of-check races and keep vulnerable setuid binaries alive after updates.",
check.Low, "CVSS:3.1/AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:N/A:N", check.Ref{Title: "Linux kernel documentation, /proc/sys/fs", URL: "https://docs.kernel.org/admin-guide/sysctl/fs.html"}, "CWE-59", nil},
{"HOST-KRN-013", "Reverse Path Filtering Disabled", "net.ipv4.conf.all.rp_filter", "1",
"Checks that the kernel drops packets whose source address could not be routed back through the receiving interface.",
"Reverse path filtering blocks many spoofed-source packets used in attacks and amplification.",
check.Low, "CVSS:3.1/AV:A/AC:H/PR:N/UI:N/S:U/C:N/I:L/A:N", refKernelNet, "CWE-940", []string{"2"}},
{"HOST-KRN-014", "Martian Packets Not Logged", "net.ipv4.conf.all.log_martians", "1",
"Checks that packets with impossible source addresses are logged.",
"Logging martian packets helps detect spoofing and misrouting attempts.",
check.Low, "CVSS:3.1/AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:L/A:N", refKernelNet, "CWE-778", nil},
{"HOST-KRN-015", "Server Sends ICMP Redirects", "net.ipv4.conf.all.send_redirects", "0",
"Checks that the server does not send ICMP redirects, which only routers should do.",
"A non-router sending redirects can be abused to influence other hosts' routing on the local network.",
check.Low, "CVSS:3.1/AV:A/AC:H/PR:N/UI:N/S:U/C:N/I:L/A:N", refKernelNet, "CWE-1188", nil},
{"HOST-KRN-016", "Broadcast ICMP Echo Answered", "net.ipv4.icmp_echo_ignore_broadcasts", "1",
"Checks that the server ignores ping requests sent to broadcast addresses.",
"Answering broadcast pings makes the server a participant in Smurf-style amplification attacks.",
check.Low, "CVSS:3.1/AV:A/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:L", refKernelNet, "CWE-406", nil},
{"HOST-KRN-017", "IPv6 ICMP Redirects Accepted", "net.ipv6.conf.all.accept_redirects", "0",
"Checks that the kernel ignores IPv6 ICMP redirect messages.",
"Forged IPv6 redirects let an attacker on the local network reroute traffic through themselves.",
check.Low, "CVSS:3.1/AV:A/AC:H/PR:N/UI:N/S:U/C:N/I:L/A:N", refKernelNet, "CWE-940", nil},
{"HOST-KRN-018", "Performance Events Open to Unprivileged Users", "kernel.perf_event_paranoid", "2",
"Checks that unprivileged users cannot use kernel performance events to observe the system.",
"Unrestricted perf events expose kernel and other processes' activity and have been used in local exploits.",
check.Low, "CVSS:3.1/AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:N/A:N", refKernelDocs, "CWE-749", []string{"3", "4"}},
101}
102
103func init() {
for _, r := range sysctlRules {
registerSysctl(r)
}
107}
108
109// registerSysctl turns one kernel parameter rule into a registered check.
110// kernel_depth.go registers its own table through the same function.
111func registerSysctl(r sysctlRule) {
{
ok := append([]string{r.want}, r.allowed...)
check.Register(&check.Check{
	ID: r.id, Title: r.title, Domain: check.DomainHost, Severity: r.sev, CVSS: r.cvss,
	Summary:   r.summary,
	Rationale: r.rationale,
	Detection: sysctlDetection(r),
	Fix:       "Create /etc/sysctl.d/60-openvapt-hardening.conf containing:  " + r.key + " = " + r.want + "  then apply with: sysctl --system" + sysctlFixCaution[r.id],
	Verify:    "sysctl " + r.key + "  should print " + r.want + ".",
	Refs:      []check.Ref{r.ref},
	Standards: check.Standards{CWE: []string{r.cwe}, NIST: sysctlNIST(r.id, r.key), CISTopic: "Kernel parameters: " + r.key},
	Run: func(ctx context.Context, s sys.System) []check.Result {
		res := runSysctlRule(ctx, s, r, ok)
		if note := sysctlPassNote[r.id]; note != nil && hasFail(res) {
			if msg, ev, pass := note(ctx, s); pass {
				return []check.Result{check.PassR(r.key, msg, append(failEvidence(res), ev...)...)}
			}
		}
		return res
	},
})
}
134}
135
136// runSysctlRule evaluates one kernel parameter rule.
137func runSysctlRule(ctx context.Context, s sys.System, r sysctlRule, ok []string) []check.Result {
if combinedSysctl[r.id] != nil {
return runCombinedSysctl(s, r.id, r.key, r.want)
}
if perInterfaceSysctl[r.id] {
return runPerInterfaceSysctl(s, r.key, r.want, ok)
}
v, err := collect.Sysctl(s, r.key)
if err != nil {
return []check.Result{check.SkipR(r.key, "Parameter not available on this kernel")}
}
for _, good := range ok {
if v == good {
	return []check.Result{check.PassR(r.key, r.key+" = "+v)}
}
}
if r.id == "HOST-KRN-005" {
return []check.Result{runIPForward(ctx, s, r.key, v)}
}
return []check.Result{check.FailR(r.key, r.key+" = "+v+" (hardened value: "+r.want+")")}
157}
158
159func slash(k string) string {
b := []byte(k)
for i := range b {
if b[i] == '.' {
	b[i] = '/'
}
}
return string(b)
167}
168
169// perInterfaceSysctl lists the rules whose kernel parameter also exists per
170// network interface. The kernel combines "all" with each interface value, so
171// every one of all, default and each interface must hold the hardened value.
172var perInterfaceSysctl = map[string]bool{"HOST-KRN-006": true, "HOST-KRN-015": true, "HOST-KRN-017": true, "HOST-KRN-023": true, "HOST-KRN-024": true, "HOST-KRN-025": true}
173
174// combinedSysctl lists IPv4 rules where the kernel combines the "all" value
175// with each interface value in a rule-specific way (include/linux/inetdevice.h):
176// rp_filter uses the larger of the two, log_martians is on when either is on,
177// and accept_redirects needs both on when the interface forwards packets but
178// either one otherwise. good reports whether the combined result is hardened.
179var combinedSysctl = map[string]func(all, iface int, forwarding bool) bool{
"HOST-KRN-006": func(all, iface int, fwd bool) bool {
if fwd {
	return all == 0 || iface == 0
}
return all == 0 && iface == 0
},
"HOST-KRN-013": func(all, iface int, _ bool) bool { m := max(all, iface); return m == 1 || m == 2 },
"HOST-KRN-014": func(all, iface int, _ bool) bool { return all == 1 || iface == 1 },
188}
189
190var combinedDetection = map[string]string{
"HOST-KRN-006": "The kernel ignores redirects on an interface when both values are 0 or, on an interface that forwards packets (net.ipv4.ip_forward or the interface's forwarding setting), when either one is 0; any interface where redirects remain accepted fails.",
"HOST-KRN-013": "The kernel uses the larger of the two values, so an interface passes when that larger value is 1 (strict) or 2 (loose).",
"HOST-KRN-014": "The kernel logs martians on an interface when either value is 1, so an interface passes when all or the interface value is 1.",
194}
195
196func sysctlDetection(r sysctlRule) string {
return sysctlDetectionBase(r) + sysctlDetectionNote[r.id]
198}
199
200func sysctlDetectionBase(r sysctlRule) string {
if r.id == "HOST-KRN-005" {
return "Reads /proc/sys/net/ipv4/ip_forward directly and compares it with the hardened value 0. When a container runtime or Kubernetes node is detected read-only (runtime sockets such as /var/run/docker.sock, /run/containerd/containerd.sock or /run/crio/crio.sock, /etc/kubernetes, or bridge interfaces such as docker0, cni0, cbr0 or flannel.1 in /sys/class/net), forwarding is required: the check then passes with context when the FORWARD path is filtered (iptables or nftables forward policy drop, or DROP/REJECT rules in DOCKER-USER), fails when forwarded traffic is accepted by default, and is incomplete when the policy cannot be read."
}
if d, ok := combinedDetection[r.id]; ok {
g := strings.Replace(slash(r.key), "/all/", "/*/", 1)
return "Reads /proc/sys/" + g + " directly (all, default for future interfaces, and every interface except loopback; no command is run) and combines the all value with each interface value as the kernel does. " + d
}
if perInterfaceSysctl[r.id] {
g := strings.Replace(slash(r.key), "/all/", "/*/", 1)
return "Reads /proc/sys/" + g + " directly (all, default and every interface; no command is run) and fails if any of them differs from the hardened value " + r.want + "."
}
return "Reads /proc/sys/" + slash(r.key) + " directly (no command is run) and compares it with the hardened value " + r.want + "."
213}
214
215// sysctlNIST maps a kernel rule to NIST SP 800-53 Rev. 5 controls: SI-16 for
216// memory protection, SC-7 for network boundary settings, SC-5 for denial of
217// service protection and AC-6 for least privilege on local information.
218func sysctlNIST(id, key string) []string {
switch id {
case "HOST-KRN-001", "HOST-KRN-002", "HOST-KRN-009", "HOST-KRN-018":
return []string{"CM-6", "SI-16"}
case "HOST-KRN-008", "HOST-KRN-016":
return []string{"SC-5", "CM-6"}
}
if strings.HasPrefix(key, "net.") {
return []string{"SC-7", "CM-6"}
}
return []string{"AC-6", "CM-6"}
229}
230
231// runPerInterfaceSysctl evaluates a ".conf.all." parameter together with its
232// "default" and per-interface siblings.
233func runPerInterfaceSysctl(s sys.System, key, want string, ok []string) []check.Result {
base := "/proc/sys/" + slash(key)
pat := strings.Replace(base, "/all/", "/*/", 1)
paths, _ := s.Glob(pat)
seen := map[string]bool{}
for _, p := range paths {
seen[p] = true
}
for _, p := range []string{base, strings.Replace(base, "/all/", "/default/", 1)} {
if !seen[p] {
	paths = append(paths, p)
	seen[p] = true
}
}
sort.Strings(paths)
var bad []string
read, unreadable := 0, 0
for _, p := range paths {
// Read the path directly: interface names may contain dots
// (VLANs such as eth0.100), which a dotted sysctl key cannot express.
k := strings.TrimPrefix(p, "/proc/sys/")
rr, err := s.ReadFile(context.Background(), p)
if err != nil || !rr.Complete {
	if !errors.Is(err, fs.ErrNotExist) {
		unreadable++
	}
	continue
}
v := strings.TrimSpace(string(rr.Data))
read++
good := false
for _, g := range ok {
	if v == g {
		good = true
		break
	}
}
if !good {
	bad = append(bad, k+" = "+v)
}
}
if read == 0 {
return []check.Result{check.SkipR(key, "Parameter not available on this kernel")}
}
if len(bad) > 0 {
return []check.Result{check.FailR(key, fmt.Sprintf("%d of %d setting(s) differ from the hardened value %s", len(bad), read, want), bad...)}
}
if unreadable > 0 {
return []check.Result{check.IncompleteR(key, fmt.Sprintf("%d setting(s) could not be read; the readable ones hold the hardened value", unreadable))}
}
return []check.Result{check.PassR(key, fmt.Sprintf("%s = %s for all, default and every interface (%d settings)", key, want, read))}
284}
285
286// runCombinedSysctl evaluates an IPv4 ".conf.all." parameter the way the
287// kernel applies it: the all value combined with each interface value (and
288// with default, which new interfaces inherit). Loopback is skipped: these
289// settings concern packets arriving from a network.
290func runCombinedSysctl(s sys.System, id, key, want string) []check.Result {
good := combinedSysctl[id]
base := "/proc/sys/" + slash(key)
name := base[strings.LastIndex(base, "/")+1:]
read := func(p string) (int, bool, bool) { // value, ok, unreadable
rr, err := s.ReadFile(context.Background(), p)
if err != nil || !rr.Complete {
	return 0, false, !errors.Is(err, fs.ErrNotExist)
}
var n int
if _, err := fmt.Sscan(strings.TrimSpace(string(rr.Data)), &n); err != nil {
	return 0, false, true
}
return n, true, false
}
all, ok, _ := read(base)
if !ok {
return []check.Result{check.SkipR(key, "Parameter not available on this kernel")}
}
ipFwd, _, _ := read("/proc/sys/net/ipv4/ip_forward")
paths, _ := s.Glob(strings.Replace(base, "/all/", "/*/", 1))
def := strings.Replace(base, "/all/", "/default/", 1)
seen := map[string]bool{base: true}
ifaces := []string{}
for _, p := range append(paths, def) {
dir := strings.TrimSuffix(p, "/"+name)
ifn := dir[strings.LastIndex(dir, "/")+1:]
if seen[p] || ifn == "lo" {
	continue
}
seen[p] = true
ifaces = append(ifaces, p)
}
sort.Strings(ifaces)
var bad []string
checked, unreadable := 0, 0
for _, p := range ifaces {
v, ok, unr := read(p)
if !ok {
	if unr {
		unreadable++
	}
	continue
}
checked++
dir := strings.TrimSuffix(p, "/"+name)
fwd := ipFwd == 1
if f, ok, _ := read(dir + "/forwarding"); ok && f == 1 {
	fwd = true
}
if !good(all, v, fwd) {
	note := fmt.Sprintf(" (all = %d", all)
	if id == "HOST-KRN-006" && fwd {
		note += ", interface forwards packets"
	}
	bad = append(bad, strings.TrimPrefix(p, "/proc/sys/")+fmt.Sprintf(" = %d", v)+note+")")
}
}
if checked == 0 && unreadable == 0 {
// No interface or default value is visible: judge all by itself.
if !good(all, all, ipFwd == 1) {
	return []check.Result{check.FailR(key, fmt.Sprintf("%s = %d (hardened value: %s)", key, all, want))}
}
return []check.Result{check.PassR(key, fmt.Sprintf("%s = %d", key, all))}
}
if len(bad) > 0 {
return []check.Result{check.FailR(key, fmt.Sprintf("%d interface setting(s) are not hardened once combined with all (hardened value: %s)", len(bad), want), bad...)}
}
if unreadable > 0 {
return []check.Result{check.IncompleteR(key, fmt.Sprintf("%d setting(s) could not be read; the readable ones are hardened", unreadable))}
}
return []check.Result{check.PassR(key, fmt.Sprintf("%s = %d; hardened on every interface once combined (%d checked)", key, all, checked))}
362}

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APA 7

OpenVAPT. (2026). HOST-KRN-008 TCP SYN Cookies Disabled (Version 0.1.0-alpha.6). OpenVAPT Check Catalogue. Retrieved October 16, 2026, from https://openvapt.com/checks/HOST-KRN-008.html

In text: (OpenVAPT, 2026). Accessed: the date shown is today's date from your own browser (the site's build date without JavaScript); change it to the day you read the page.

BibTeX

@misc{openvapt_host_krn_008,
  author       = {{OpenVAPT}},
  title        = {{HOST-KRN-008 TCP SYN Cookies Disabled}},
  year         = {2026},
  howpublished = {\url{https://openvapt.com/checks/HOST-KRN-008.html}},
  note         = {OpenVAPT Check Catalogue, version 0.1.0-alpha.6. Accessed 16 October 2026}
}

Accessed: the date shown is today's date from your own browser (the site's build date without JavaScript); change it to the day you read the page.

Citing OpenVAPT does not mean OpenVAPT endorses, approves or has reviewed your work.

Cite This Work: OpenVAPT. (2026). OpenVAPT: An open academic vulnerability assessment tool (Version 0.1.0-alpha.6) [Computer software]. https://github.com/OpenVAPT/openvapt

1	package host
2
3	import (
4	"context"
5	"errors"
6	"fmt"
7	"io/fs"
8	"sort"
9	"strings"
10
11	"github.com/openvapt/openvapt/internal/check"
12	"github.com/openvapt/openvapt/internal/checks/collect"
13	"github.com/openvapt/openvapt/internal/sys"
14	)
15
16	// sysctlRule describes one kernel parameter and its hardened value.
17	type sysctlRule struct {
18	id, title, key, want, summary, rationale string
19	sev check.Severity
20	cvss string
21	ref check.Ref
22	cwe string
23	allowed []string // values that also pass
24	}
25
26	// One table, one loop: each row becomes its own registered check, so every
27	// kernel setting has its own catalogue page, id and result.
28	var sysctlRules = []sysctlRule{
29	{"HOST-KRN-001", "Address Space Layout Randomisation Not Fully Enabled", "kernel.randomize_va_space", "2",
30	"Checks that the kernel randomises memory layout for stack, libraries, heap and VDSO.",
31	"ASLR makes memory-corruption exploits much harder because attackers cannot predict where code lives.",
32	check.Medium, "CVSS:3.1/AV:L/AC:H/PR:L/UI:N/S:U/C:H/I:N/A:N", refKernelDocs, "CWE-344", nil},
33	{"HOST-KRN-002", "Kernel Pointers Exposed to Unprivileged Users", "kernel.kptr_restrict", "1",
34	"Checks that kernel memory addresses are hidden from normal users.",
35	"Leaked kernel addresses defeat kernel ASLR and are the first step of most local kernel exploits.",
36	check.Low, "CVSS:3.1/AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:N/A:N", refKernelDocs, "CWE-497", []string{"2"}},
37	{"HOST-KRN-003", "Kernel Log Readable by Unprivileged Users", "kernel.dmesg_restrict", "1",
38	"Checks that only privileged users can read the kernel log (dmesg).",
39	"The kernel log can reveal addresses, hardware and driver details useful for local exploitation.",
40	check.Low, "CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:L/I:N/A:N", refKernelDocs, "CWE-497", nil},
41	{"HOST-KRN-004", "Setuid Programs May Dump Core", "fs.suid_dumpable", "0",
42	"Checks that privileged programs do not write core dumps.",
43	"A core dump of a setuid program can contain password hashes or keys and may be readable by the user who crashed it.",
44	check.Low, "CVSS:3.1/AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:N/A:N", check.Ref{Title: "Linux kernel documentation, /proc/sys/fs", URL: "https://docs.kernel.org/admin-guide/sysctl/fs.html"}, "CWE-528", nil},
45	{"HOST-KRN-005", "IP Forwarding Enabled", "net.ipv4.ip_forward", "0",
46	"Checks that the server does not route packets between networks.",
47	"A server that forwards packets can be used to pivot between networks. Only routers, VPN gateways and container hosts need this.",
48	check.Low, "CVSS:3.1/AV:A/AC:H/PR:N/UI:N/S:U/C:N/I:L/A:N", refKernelNet, "CWE-1188", nil},
49	{"HOST-KRN-006", "ICMP Redirects Accepted", "net.ipv4.conf.all.accept_redirects", "0",
50	"Checks that the kernel ignores ICMP redirect messages.",
51	"Forged ICMP redirects let an attacker on the local network reroute the server's traffic through themselves.",
52	check.Low, "CVSS:3.1/AV:A/AC:H/PR:N/UI:N/S:U/C:N/I:L/A:N", refKernelNet, "CWE-940", nil},
53	{"HOST-KRN-007", "Source-Routed Packets Accepted", "net.ipv4.conf.all.accept_source_route", "0",
54	"Checks that the kernel drops packets that dictate their own route.",
55	"Source routing lets an attacker bypass network filtering and spoof trusted addresses.",
56	check.Low, "CVSS:3.1/AV:A/AC:H/PR:N/UI:N/S:U/C:N/I:L/A:N", refKernelNet, "CWE-940", nil},
57	{"HOST-KRN-008", "TCP SYN Cookies Disabled", "net.ipv4.tcp_syncookies", "1",
58	"Checks that SYN cookies protect the server from SYN flood attacks.",
59	"Without SYN cookies a simple flood of half-open connections can make every service unreachable.",
60	check.Low, "CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:L", refKernelNet, "CWE-770", nil},
61	{"HOST-KRN-009", "Unprivileged eBPF Allowed", "kernel.unprivileged_bpf_disabled", "1",
62	"Checks that normal users cannot load eBPF programs into the kernel.",
63	"Unprivileged eBPF has been the entry point for many local privilege-escalation bugs.",
64	check.Medium, "CVSS:3.1/AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:L/A:L", refKernelDocs, "CWE-749", []string{"2"}},
65	{"HOST-KRN-010", "Process Tracing Not Restricted", "kernel.yama.ptrace_scope", "1",
66	"Checks that processes cannot attach a debugger to other processes of the same user.",
67	"Unrestricted ptrace lets any compromised process read the memory of others, for example to steal SSH agent keys or browser sessions.",
68	check.Low, "CVSS:3.1/AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:N/A:N", check.Ref{Title: "Linux kernel documentation, Yama LSM", URL: "https://docs.kernel.org/admin-guide/LSM/Yama.html"}, "CWE-749", []string{"2", "3"}},
69	{"HOST-KRN-011", "Symlink Protection Disabled", "fs.protected_symlinks", "1",
70	"Checks that the kernel blocks following symlinks owned by other users in world-writable sticky directories.",
71	"Without it, classic /tmp symlink races let local users trick root-run programs into overwriting files.",
72	check.Low, "CVSS:3.1/AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:N/A:N", check.Ref{Title: "Linux kernel documentation, /proc/sys/fs", URL: "https://docs.kernel.org/admin-guide/sysctl/fs.html"}, "CWE-59", nil},
73	{"HOST-KRN-012", "Hardlink Protection Disabled", "fs.protected_hardlinks", "1",
74	"Checks that users cannot create hard links to files they do not own.",
75	"Hard links to privileged files enable time-of-check races and keep vulnerable setuid binaries alive after updates.",
76	check.Low, "CVSS:3.1/AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:N/A:N", check.Ref{Title: "Linux kernel documentation, /proc/sys/fs", URL: "https://docs.kernel.org/admin-guide/sysctl/fs.html"}, "CWE-59", nil},
77	{"HOST-KRN-013", "Reverse Path Filtering Disabled", "net.ipv4.conf.all.rp_filter", "1",
78	"Checks that the kernel drops packets whose source address could not be routed back through the receiving interface.",
79	"Reverse path filtering blocks many spoofed-source packets used in attacks and amplification.",
80	check.Low, "CVSS:3.1/AV:A/AC:H/PR:N/UI:N/S:U/C:N/I:L/A:N", refKernelNet, "CWE-940", []string{"2"}},
81	{"HOST-KRN-014", "Martian Packets Not Logged", "net.ipv4.conf.all.log_martians", "1",
82	"Checks that packets with impossible source addresses are logged.",
83	"Logging martian packets helps detect spoofing and misrouting attempts.",
84	check.Low, "CVSS:3.1/AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:L/A:N", refKernelNet, "CWE-778", nil},
85	{"HOST-KRN-015", "Server Sends ICMP Redirects", "net.ipv4.conf.all.send_redirects", "0",
86	"Checks that the server does not send ICMP redirects, which only routers should do.",
87	"A non-router sending redirects can be abused to influence other hosts' routing on the local network.",
88	check.Low, "CVSS:3.1/AV:A/AC:H/PR:N/UI:N/S:U/C:N/I:L/A:N", refKernelNet, "CWE-1188", nil},
89	{"HOST-KRN-016", "Broadcast ICMP Echo Answered", "net.ipv4.icmp_echo_ignore_broadcasts", "1",
90	"Checks that the server ignores ping requests sent to broadcast addresses.",
91	"Answering broadcast pings makes the server a participant in Smurf-style amplification attacks.",
92	check.Low, "CVSS:3.1/AV:A/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:L", refKernelNet, "CWE-406", nil},
93	{"HOST-KRN-017", "IPv6 ICMP Redirects Accepted", "net.ipv6.conf.all.accept_redirects", "0",
94	"Checks that the kernel ignores IPv6 ICMP redirect messages.",
95	"Forged IPv6 redirects let an attacker on the local network reroute traffic through themselves.",
96	check.Low, "CVSS:3.1/AV:A/AC:H/PR:N/UI:N/S:U/C:N/I:L/A:N", refKernelNet, "CWE-940", nil},
97	{"HOST-KRN-018", "Performance Events Open to Unprivileged Users", "kernel.perf_event_paranoid", "2",
98	"Checks that unprivileged users cannot use kernel performance events to observe the system.",
99	"Unrestricted perf events expose kernel and other processes' activity and have been used in local exploits.",
100	check.Low, "CVSS:3.1/AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:N/A:N", refKernelDocs, "CWE-749", []string{"3", "4"}},
101	}
102
103	func init() {
104	for _, r := range sysctlRules {
105	registerSysctl(r)
106	}
107	}
108
109	// registerSysctl turns one kernel parameter rule into a registered check.
110	// kernel_depth.go registers its own table through the same function.
111	func registerSysctl(r sysctlRule) {
112	{
113	ok := append([]string{r.want}, r.allowed...)
114	check.Register(&check.Check{
115	ID: r.id, Title: r.title, Domain: check.DomainHost, Severity: r.sev, CVSS: r.cvss,
116	Summary: r.summary,
117	Rationale: r.rationale,
118	Detection: sysctlDetection(r),
119	Fix: "Create /etc/sysctl.d/60-openvapt-hardening.conf containing: " + r.key + " = " + r.want + " then apply with: sysctl --system" + sysctlFixCaution[r.id],
120	Verify: "sysctl " + r.key + " should print " + r.want + ".",
121	Refs: []check.Ref{r.ref},
122	Standards: check.Standards{CWE: []string{r.cwe}, NIST: sysctlNIST(r.id, r.key), CISTopic: "Kernel parameters: " + r.key},
123	Run: func(ctx context.Context, s sys.System) []check.Result {
124	res := runSysctlRule(ctx, s, r, ok)
125	if note := sysctlPassNote[r.id]; note != nil && hasFail(res) {
126	if msg, ev, pass := note(ctx, s); pass {
127	return []check.Result{check.PassR(r.key, msg, append(failEvidence(res), ev...)...)}
128	}
129	}
130	return res
131	},
132	})
133	}
134	}
135
136	// runSysctlRule evaluates one kernel parameter rule.
137	func runSysctlRule(ctx context.Context, s sys.System, r sysctlRule, ok []string) []check.Result {
138	if combinedSysctl[r.id] != nil {
139	return runCombinedSysctl(s, r.id, r.key, r.want)
140	}
141	if perInterfaceSysctl[r.id] {
142	return runPerInterfaceSysctl(s, r.key, r.want, ok)
143	}
144	v, err := collect.Sysctl(s, r.key)
145	if err != nil {
146	return []check.Result{check.SkipR(r.key, "Parameter not available on this kernel")}
147	}
148	for _, good := range ok {
149	if v == good {
150	return []check.Result{check.PassR(r.key, r.key+" = "+v)}
151	}
152	}
153	if r.id == "HOST-KRN-005" {
154	return []check.Result{runIPForward(ctx, s, r.key, v)}
155	}
156	return []check.Result{check.FailR(r.key, r.key+" = "+v+" (hardened value: "+r.want+")")}
157	}
158
159	func slash(k string) string {
160	b := []byte(k)
161	for i := range b {
162	if b[i] == '.' {
163	b[i] = '/'
164	}
165	}
166	return string(b)
167	}
168
169	// perInterfaceSysctl lists the rules whose kernel parameter also exists per
170	// network interface. The kernel combines "all" with each interface value, so
171	// every one of all, default and each interface must hold the hardened value.
172	var perInterfaceSysctl = map[string]bool{"HOST-KRN-006": true, "HOST-KRN-015": true, "HOST-KRN-017": true, "HOST-KRN-023": true, "HOST-KRN-024": true, "HOST-KRN-025": true}
173
174	// combinedSysctl lists IPv4 rules where the kernel combines the "all" value
175	// with each interface value in a rule-specific way (include/linux/inetdevice.h):
176	// rp_filter uses the larger of the two, log_martians is on when either is on,
177	// and accept_redirects needs both on when the interface forwards packets but
178	// either one otherwise. good reports whether the combined result is hardened.
179	var combinedSysctl = map[string]func(all, iface int, forwarding bool) bool{
180	"HOST-KRN-006": func(all, iface int, fwd bool) bool {
181	if fwd {
182	return all == 0 \|\| iface == 0
183	}
184	return all == 0 && iface == 0
185	},
186	"HOST-KRN-013": func(all, iface int, _ bool) bool { m := max(all, iface); return m == 1 \|\| m == 2 },
187	"HOST-KRN-014": func(all, iface int, _ bool) bool { return all == 1 \|\| iface == 1 },
188	}
189
190	var combinedDetection = map[string]string{
191	"HOST-KRN-006": "The kernel ignores redirects on an interface when both values are 0 or, on an interface that forwards packets (net.ipv4.ip_forward or the interface's forwarding setting), when either one is 0; any interface where redirects remain accepted fails.",
192	"HOST-KRN-013": "The kernel uses the larger of the two values, so an interface passes when that larger value is 1 (strict) or 2 (loose).",
193	"HOST-KRN-014": "The kernel logs martians on an interface when either value is 1, so an interface passes when all or the interface value is 1.",
194	}
195
196	func sysctlDetection(r sysctlRule) string {
197	return sysctlDetectionBase(r) + sysctlDetectionNote[r.id]
198	}
199
200	func sysctlDetectionBase(r sysctlRule) string {
201	if r.id == "HOST-KRN-005" {
202	return "Reads /proc/sys/net/ipv4/ip_forward directly and compares it with the hardened value 0. When a container runtime or Kubernetes node is detected read-only (runtime sockets such as /var/run/docker.sock, /run/containerd/containerd.sock or /run/crio/crio.sock, /etc/kubernetes, or bridge interfaces such as docker0, cni0, cbr0 or flannel.1 in /sys/class/net), forwarding is required: the check then passes with context when the FORWARD path is filtered (iptables or nftables forward policy drop, or DROP/REJECT rules in DOCKER-USER), fails when forwarded traffic is accepted by default, and is incomplete when the policy cannot be read."
203	}
204	if d, ok := combinedDetection[r.id]; ok {
205	g := strings.Replace(slash(r.key), "/all/", "/*/", 1)
206	return "Reads /proc/sys/" + g + " directly (all, default for future interfaces, and every interface except loopback; no command is run) and combines the all value with each interface value as the kernel does. " + d
207	}
208	if perInterfaceSysctl[r.id] {
209	g := strings.Replace(slash(r.key), "/all/", "/*/", 1)
210	return "Reads /proc/sys/" + g + " directly (all, default and every interface; no command is run) and fails if any of them differs from the hardened value " + r.want + "."
211	}
212	return "Reads /proc/sys/" + slash(r.key) + " directly (no command is run) and compares it with the hardened value " + r.want + "."
213	}
214
215	// sysctlNIST maps a kernel rule to NIST SP 800-53 Rev. 5 controls: SI-16 for
216	// memory protection, SC-7 for network boundary settings, SC-5 for denial of
217	// service protection and AC-6 for least privilege on local information.
218	func sysctlNIST(id, key string) []string {
219	switch id {
220	case "HOST-KRN-001", "HOST-KRN-002", "HOST-KRN-009", "HOST-KRN-018":
221	return []string{"CM-6", "SI-16"}
222	case "HOST-KRN-008", "HOST-KRN-016":
223	return []string{"SC-5", "CM-6"}
224	}
225	if strings.HasPrefix(key, "net.") {
226	return []string{"SC-7", "CM-6"}
227	}
228	return []string{"AC-6", "CM-6"}
229	}
230
231	// runPerInterfaceSysctl evaluates a ".conf.all." parameter together with its
232	// "default" and per-interface siblings.
233	func runPerInterfaceSysctl(s sys.System, key, want string, ok []string) []check.Result {
234	base := "/proc/sys/" + slash(key)
235	pat := strings.Replace(base, "/all/", "/*/", 1)
236	paths, _ := s.Glob(pat)
237	seen := map[string]bool{}
238	for _, p := range paths {
239	seen[p] = true
240	}
241	for _, p := range []string{base, strings.Replace(base, "/all/", "/default/", 1)} {
242	if !seen[p] {
243	paths = append(paths, p)
244	seen[p] = true
245	}
246	}
247	sort.Strings(paths)
248	var bad []string
249	read, unreadable := 0, 0
250	for _, p := range paths {
251	// Read the path directly: interface names may contain dots
252	// (VLANs such as eth0.100), which a dotted sysctl key cannot express.
253	k := strings.TrimPrefix(p, "/proc/sys/")
254	rr, err := s.ReadFile(context.Background(), p)
255	if err != nil \|\| !rr.Complete {
256	if !errors.Is(err, fs.ErrNotExist) {
257	unreadable++
258	}
259	continue
260	}
261	v := strings.TrimSpace(string(rr.Data))
262	read++
263	good := false
264	for _, g := range ok {
265	if v == g {
266	good = true
267	break
268	}
269	}
270	if !good {
271	bad = append(bad, k+" = "+v)
272	}
273	}
274	if read == 0 {
275	return []check.Result{check.SkipR(key, "Parameter not available on this kernel")}
276	}
277	if len(bad) > 0 {
278	return []check.Result{check.FailR(key, fmt.Sprintf("%d of %d setting(s) differ from the hardened value %s", len(bad), read, want), bad...)}
279	}
280	if unreadable > 0 {
281	return []check.Result{check.IncompleteR(key, fmt.Sprintf("%d setting(s) could not be read; the readable ones hold the hardened value", unreadable))}
282	}
283	return []check.Result{check.PassR(key, fmt.Sprintf("%s = %s for all, default and every interface (%d settings)", key, want, read))}
284	}
285
286	// runCombinedSysctl evaluates an IPv4 ".conf.all." parameter the way the
287	// kernel applies it: the all value combined with each interface value (and
288	// with default, which new interfaces inherit). Loopback is skipped: these
289	// settings concern packets arriving from a network.
290	func runCombinedSysctl(s sys.System, id, key, want string) []check.Result {
291	good := combinedSysctl[id]
292	base := "/proc/sys/" + slash(key)
293	name := base[strings.LastIndex(base, "/")+1:]
294	read := func(p string) (int, bool, bool) { // value, ok, unreadable
295	rr, err := s.ReadFile(context.Background(), p)
296	if err != nil \|\| !rr.Complete {
297	return 0, false, !errors.Is(err, fs.ErrNotExist)
298	}
299	var n int
300	if _, err := fmt.Sscan(strings.TrimSpace(string(rr.Data)), &n); err != nil {
301	return 0, false, true
302	}
303	return n, true, false
304	}
305	all, ok, _ := read(base)
306	if !ok {
307	return []check.Result{check.SkipR(key, "Parameter not available on this kernel")}
308	}
309	ipFwd, _, _ := read("/proc/sys/net/ipv4/ip_forward")
310	paths, _ := s.Glob(strings.Replace(base, "/all/", "/*/", 1))
311	def := strings.Replace(base, "/all/", "/default/", 1)
312	seen := map[string]bool{base: true}
313	ifaces := []string{}
314	for _, p := range append(paths, def) {
315	dir := strings.TrimSuffix(p, "/"+name)
316	ifn := dir[strings.LastIndex(dir, "/")+1:]
317	if seen[p] \|\| ifn == "lo" {
318	continue
319	}
320	seen[p] = true
321	ifaces = append(ifaces, p)
322	}
323	sort.Strings(ifaces)
324	var bad []string
325	checked, unreadable := 0, 0
326	for _, p := range ifaces {
327	v, ok, unr := read(p)
328	if !ok {
329	if unr {
330	unreadable++
331	}
332	continue
333	}
334	checked++
335	dir := strings.TrimSuffix(p, "/"+name)
336	fwd := ipFwd == 1
337	if f, ok, _ := read(dir + "/forwarding"); ok && f == 1 {
338	fwd = true
339	}
340	if !good(all, v, fwd) {
341	note := fmt.Sprintf(" (all = %d", all)
342	if id == "HOST-KRN-006" && fwd {
343	note += ", interface forwards packets"
344	}
345	bad = append(bad, strings.TrimPrefix(p, "/proc/sys/")+fmt.Sprintf(" = %d", v)+note+")")
346	}
347	}
348	if checked == 0 && unreadable == 0 {
349	// No interface or default value is visible: judge all by itself.
350	if !good(all, all, ipFwd == 1) {
351	return []check.Result{check.FailR(key, fmt.Sprintf("%s = %d (hardened value: %s)", key, all, want))}
352	}
353	return []check.Result{check.PassR(key, fmt.Sprintf("%s = %d", key, all))}
354	}
355	if len(bad) > 0 {
356	return []check.Result{check.FailR(key, fmt.Sprintf("%d interface setting(s) are not hardened once combined with all (hardened value: %s)", len(bad), want), bad...)}
357	}
358	if unreadable > 0 {
359	return []check.Result{check.IncompleteR(key, fmt.Sprintf("%d setting(s) could not be read; the readable ones are hardened", unreadable))}
360	}
361	return []check.Result{check.PassR(key, fmt.Sprintf("%s = %d; hardened on every interface once combined (%d checked)", key, all, checked))}
362	}