MediumNET-APP-002Network ExposureNetwork

Application Server Ports Reachable Without a Reverse Proxy

Checks whether application servers and developer tools (Node.js, Tomcat, Django, Flask, Vite) serve HTTP on network-reachable addresses.

Why It Matters: Application servers are designed to sit behind a reverse proxy that adds TLS, security headers, logging and request limits. Reached directly they bypass all of that, and development servers on these ports often run with debug pages on. The port number alone does not prove a weakness, so this is rated Medium and should be reviewed.
How It Detects: Matches network-reachable listening TCP sockets from /proc/net against the default HTTP ports of common application servers and developer tools, recording how each is bound and whether a web server listens on port 80 or 443 on this machine (which suggests the application was meant to be proxied). No packets are sent.
How to Fix: Bind the application to 127.0.0.1 (Node.js: app.listen(3000, '127.0.0.1'); Gunicorn: --bind 127.0.0.1:8000; Tomcat: address="127.0.0.1" on the Connector) and publish it only through nginx or Apache.
How to Verify: ss -tlnp shows these ports on 127.0.0.1 or ::1 only.
CVSS v3.1 Vector: CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:L/I:L/A:L
Control Category: Network Exposure: CIS Controls v8 4, 12, 13; NIST CSF 2.0 PR.IR, PR.PS
Fix Effort: Low
CIS Controls v8 Safeguard: 4.8
NIST CSF 2.0 Subcategory: PR.PS-01, PR.IR-01
CWE: CWE-1188, CWE-1327
OWASP Top 10: A05:2021
NIST SP 800-53 Rev. 5: SC-7, CM-7
Benchmark Topic: Expose applications only through a hardened front end (topic only, no benchmark text)
References: OWASP Node.js Security Cheat Sheet
Gunicorn documentation, Deploying Gunicorn (behind a proxy)

Learn the theory behind this check: P13 Application Servers: Tomcat, Node.js and Python.

Source Code

The complete file that defines this check. Run it with openvapt scan --only NET-APP-002, or read it offline with openvapt explain NET-APP-002.

internal/checks/network/firewall_exposure.go · Lines 1-296View on GitHub at This Commit

1package network
2
3import (
"context"
"fmt"
"strings"
7
"github.com/openvapt/openvapt/internal/check"
"github.com/openvapt/openvapt/internal/checks/collect"
"github.com/openvapt/openvapt/internal/sys"
11)
12
13// Checks that compare what the host firewall allows with what is actually
14// listening. A firewall that is "on" can still let a database through, keep
15// holes open for services long gone, or be bypassed by Docker.
16
17// codePorts are debuggers, management protocols and shutdown ports of
18// application servers that give code execution or control to whoever
19// connects. Sources: Node.js, Apache Tomcat, PHP-FPM and Jupyter
20// documentation.
21var codePorts = map[int]string{
9229: "Node.js inspector (debugger)", 8009: "Tomcat AJP connector", 8005: "Tomcat shutdown port",
9000: "PHP-FPM FastCGI", 8888: "Jupyter Notebook",
24}
25
26// appPorts are the default HTTP ports of application servers and developer
27// tools that are meant to sit behind a reverse proxy, never face the
28// network. Reaching one directly bypasses TLS, headers and limits, but the
29// port number alone does not prove code execution, so the severity is lower
30// than codePorts. Sources: Node.js, Django, Flask, Vite and Tomcat docs.
31var appPorts = map[int]string{
3000: "Node.js / Express app server", 3001: "Node.js app server", 4000: "app server", 4200: "Angular dev server",
5000: "Flask / Python dev server", 5173: "Vite dev server", 8000: "Django / Gunicorn / Uvicorn", 8008: "app server",
8080: "Tomcat / Java HTTP connector", 8081: "app server", 8088: "app server",
35}
36
37// sensitivePort names a data, administrative or application server port.
38func sensitivePort(p int) (string, bool) {
for _, m := range []map[int]string{dataPorts, controlPorts, managementPorts, codePorts, appPorts, aiPorts} {
if n, ok := m[p]; ok {
	return n, true
}
}
return "", false
45}
46
47var refNIST41 = check.Ref{Title: "NIST SP 800-41 Rev. 1, Guidelines on Firewalls and Firewall Policy", URL: "https://csrc.nist.gov/pubs/sp/800/41/r1/final"}
48
49func publicListeners(s sys.System) ([]collect.Listener, []check.Result) {
ls, err := collect.Listeners(s)
if err != nil {
return nil, []check.Result{check.IncompleteR("/proc/net", "One or more socket tables were unreadable or partial")}
}
var pub []collect.Listener
for _, l := range ls {
if l.Public() {
	pub = append(pub, l)
}
}
return pub, nil
61}
62
63func init() {
check.Register(&check.Check{
ID: "NET-FW-003", Title: "Firewall Allows Sensitive Services From Anywhere", Domain: check.DomainNetwork,
Severity: check.High, Category: check.CatNetwork, Effort: check.EffortLow,
CVSS:      "CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:H",
Summary:   "Checks whether the host firewall explicitly opens a listening database, administrative or application server port to every address on the internet.",
Rationale: "A default-deny firewall only protects what it does not allow. A single \"allow 3306 from anywhere\" rule, often added while debugging, makes a database or admin panel as exposed as if there were no firewall at all.",
Detection: "Reads the incoming allow rules of ufw (ufw status verbose), nftables (nft list ruleset, input-hook chains) or iptables (iptables -S, INPUT and ufw chains), and matches rules with no source restriction against network-reachable listening sockets on well-known data, administrative and application server ports.",
Fix:       "Delete the rule or restrict it to the addresses that need it, for example: ufw delete allow 3306 && ufw allow from 203.0.113.10 to any port 3306 proto tcp . Better, bind the service to 127.0.0.1 and reach it through an SSH tunnel or VPN.",
Verify:    "ufw status numbered  or  nft list ruleset  shows no rule opening that port to Anywhere.",
Refs:      []check.Ref{refNIST41, {Title: "ufw(8) manual (Ubuntu)", URL: "https://manpages.ubuntu.com/manpages/noble/man8/ufw.8.html"}},
Standards: check.Standards{CWE: []string{"CWE-923", "CWE-1327"}, OWASP: []string{"A05:2021"}, NIST: []string{"SC-7", "AC-4", "CM-7"}, CISTopic: "Restrict firewall rules to required sources"},
Run: func(ctx context.Context, s sys.System) []check.Result {
	fw := collect.ReadFirewall(ctx, s)
	if len(fw.Rules) == 0 {
		return []check.Result{check.SkipR("firewall", "No firewall allow rules found (see NET-FW-001 for whether a firewall is active)")}
	}
	pub, inc := publicListeners(s)
	if inc != nil {
		return inc
	}
	var bad []string
	for _, l := range pub {
		name, ok := sensitivePort(l.Port)
		if !ok {
			continue
		}
		for _, r := range fw.Rules {
			if r.Any() && r.Covers(l.Proto, l.Port) {
				bad = append(bad, fmt.Sprintf("%s (%s) allowed from anywhere by %s rule: %s", l.String(), name, r.Engine, r.Raw))
				break
			}
		}
	}
	if len(bad) > 0 {
		return []check.Result{check.FailR("firewall", "Sensitive services are open to every address", bad...)}
	}
	return []check.Result{check.PassR("firewall", "No sensitive listening service is opened to every address")}
},
})
103
check.Register(&check.Check{
ID: "NET-FW-004", Title: "Firewall Rules Open Ports With No Service", Domain: check.DomainNetwork,
Severity: check.Low, Category: check.CatNetwork, Effort: check.EffortLow,
CVSS:      "CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:L/I:N/A:N",
Summary:   "Checks for firewall allow rules whose port has no listening service on this machine.",
Rationale: "Stale rules are holes waiting for a service. Anything later started on that port, by an administrator, a package or an attacker, becomes reachable without anyone deciding it should be. Rule sets also become impossible to review when they no longer match reality.",
Detection: "Compares each single-port incoming allow rule from ufw, nftables or iptables with the listening sockets in /proc/net (tcp, tcp6, udp, udp6). Port ranges are not judged, because they are often used for passive FTP or media and may be idle.",
Fix:       "Remove rules for services that no longer exist (ufw status numbered, then ufw delete <n>), or note why a rule must stay. Review the firewall whenever a service is retired.",
Verify:    "Re-run this check; every allowed port should have a listening service.",
Refs:      []check.Ref{refNIST41},
Standards: check.Standards{CWE: []string{"CWE-1188"}, NIST: []string{"SC-7", "CM-7", "CM-3"}, CISTopic: "Review firewall rules periodically"},
Run: func(ctx context.Context, s sys.System) []check.Result {
	fw := collect.ReadFirewall(ctx, s)
	if len(fw.Rules) == 0 {
		return []check.Result{check.SkipR("firewall", "No firewall allow rules found")}
	}
	ls, err := collect.Listeners(s)
	if err != nil {
		return []check.Result{check.IncompleteR("/proc/net", "One or more socket tables were unreadable or partial")}
	}
	seen := map[string]bool{}
	var bad []string
	for _, r := range fw.Rules {
		if r.From != r.To {
			continue
		}
		used := false
		for _, l := range ls {
			if r.Covers(l.Proto, l.Port) {
				used = true
				break
			}
		}
		key := fmt.Sprintf("%s/%d", r.Proto, r.From)
		if !used && !seen[key] {
			seen[key] = true
			bad = append(bad, fmt.Sprintf("port %d%s allowed by %s but nothing listens on it: %s", r.From, protoSuffix(r.Proto), r.Engine, r.Raw))
		}
	}
	if len(bad) > 0 {
		return []check.Result{check.FailR("firewall", fmt.Sprintf("%d allow rule(s) open ports with no service", len(bad)), bad...)}
	}
	return []check.Result{check.PassR("firewall", "Every single-port allow rule matches a listening service")}
},
})
149
check.Register(&check.Check{
ID: "NET-FW-005", Title: "Container Ports Bypass the Host Firewall", Domain: check.DomainNetwork,
Severity: check.Medium, Category: check.CatNetwork, Effort: check.EffortMedium,
CVSS:      "CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:L/I:L/A:L",
Summary:   "Checks whether Docker publishes container ports through its own iptables chains with no DOCKER-USER filtering.",
Rationale: "Docker inserts its NAT and FORWARD rules ahead of ufw and of the INPUT chain, so a port published with -p 8080:80 is reachable from the internet even when ufw says the port is denied. Administrators routinely believe the firewall protects these containers when it does not.",
Detection: "Reads iptables -S and looks for ACCEPT rules with a destination port in the DOCKER chain (published container ports). Fails when there are such rules and the DOCKER-USER chain contains no DROP or REJECT rule.",
Fix:       "Publish container ports on loopback only (-p 127.0.0.1:8080:80) and put a reverse proxy in front, or add filtering to DOCKER-USER, for example: iptables -I DOCKER-USER -i eth0 ! -s 203.0.113.0/24 -j DROP (persist it with your firewall tooling). Caution: test from an allowed address first; a wrong interface name or source range cuts access to every published container.",
Verify:    "docker ps --format '{{.Ports}}'  shows 127.0.0.1 bindings, or iptables -S DOCKER-USER shows DROP rules.",
Refs:      []check.Ref{{Title: "Docker documentation, Packet filtering and firewalls", URL: "https://docs.docker.com/engine/network/packet-filtering-firewalls/"}, refNIST41},
Standards: check.Standards{CWE: []string{"CWE-923", "CWE-1327"}, OWASP: []string{"A05:2021"}, NIST: []string{"SC-7", "CM-7"}, CISTopic: "Filter container traffic"},
Run: func(ctx context.Context, s sys.System) []check.Result {
	fw := collect.ReadFirewall(ctx, s)
	if len(fw.Docker) == 0 {
		return []check.Result{check.PassR("iptables", "No published container ports found in the Docker chain")}
	}
	if fw.DockerUser {
		return []check.Result{check.PassR("iptables", "Published container ports are filtered in DOCKER-USER")}
	}
	return []check.Result{check.FailR("iptables", fmt.Sprintf("%d published container port rule(s) are not filtered by the host firewall", len(fw.Docker)), fw.Docker...)}
},
})
172
check.Register(&check.Check{
ID: "NET-FW-006", Title: "Firewall Exposure Map of Listening Services", Domain: check.DomainNetwork,
Severity: check.Info, Category: check.CatNetwork, Effort: check.EffortLow,
CVSS:      "CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:N",
Summary:   "Shows, for every network-reachable listening socket, what the host firewall does with traffic to it.",
Rationale: "Exposure is the combination of what listens and what the firewall lets through. Seeing both side by side is how assessors find services that are open by accident and confirm that the rest are blocked on purpose.",
Detection: "Combines the listening sockets in /proc/net with the firewall's default policy and allow rules (ufw, nftables or iptables). Each socket is labelled allowed from anywhere, allowed from restricted sources, blocked by default deny, or reachable because no filtering firewall is active.",
Fix:       "For each socket labelled allowed from anywhere or reachable, decide whether it must be public. If not, bind it to 127.0.0.1 or restrict the firewall rule.",
Verify:    "Re-run this check after changes and compare the map.",
Refs:      []check.Ref{refNIST41, {Title: "NIST SP 800-115, Technical Guide to Information Security Testing and Assessment", URL: "https://csrc.nist.gov/pubs/sp/800/115/final"}},
Standards: check.Standards{CWE: []string{"CWE-923"}, NIST: []string{"CM-7", "CM-8", "SC-7"}, CISTopic: "Network services inventory"},
Run: func(ctx context.Context, s sys.System) []check.Result {
	pub, inc := publicListeners(s)
	if inc != nil {
		return inc
	}
	fw := collect.ReadFirewall(ctx, s)
	engine := fw.Engine
	if engine == "" {
		engine = "none detected"
	}
	ev := []string{fmt.Sprintf("firewall: %s, default deny: %v", engine, fw.DefaultDeny)}
	open := 0
	for _, l := range pub {
		state := "reachable (no default-deny firewall)"
		if fw.DefaultDeny {
			state = "blocked by default deny"
		}
		for _, r := range fw.Rules {
			if r.Covers(l.Proto, l.Port) {
				if r.Any() {
					state = "allowed from anywhere"
				} else {
					state = "allowed from " + r.Source
				}
				break
			}
		}
		if strings.HasPrefix(state, "allowed from anywhere") || strings.HasPrefix(state, "reachable") {
			open++
		}
		ev = append(ev, l.String()+portName(l.Port)+": "+state)
	}
	return []check.Result{check.PassR("firewall", fmt.Sprintf("%d network-reachable sockets, %d open to every address", len(pub), open), ev...)}
},
})
219
check.Register(&check.Check{
ID: "NET-APP-001", Title: "Debugger or Application Control Ports Exposed", Domain: check.DomainNetwork,
Severity: check.High, Category: check.CatNetwork, Effort: check.EffortLow,
CVSS:      "CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:L/A:L",
Summary:   "Checks whether the Node.js inspector, Tomcat AJP or shutdown port, PHP-FPM FastCGI or Jupyter listen on network-reachable addresses.",
Rationale: "These ports are not web pages: the Node.js inspector and Jupyter run code for whoever connects, Tomcat AJP has been abused for file read and code execution (Ghostcat), the shutdown port stops the server, and PHP-FPM executes scripts handed to it. None of them is designed to face a network.",
Detection: "Matches network-reachable listening TCP sockets from /proc/net against the default ports of these debuggers and control protocols, recording how each is bound. Plain application HTTP ports are rated separately by NET-APP-002. No packets are sent.",
Fix:       "Bind each to 127.0.0.1 or a Unix socket (node --inspect=127.0.0.1:9229; Tomcat Connector address=\"127.0.0.1\" or remove the AJP connector; PHP-FPM listen = /run/php/php-fpm.sock; jupyter --ip=127.0.0.1) and never expose a debugger.",
Verify:    "ss -tlnp  shows these ports on 127.0.0.1 or ::1 only.",
Refs: []check.Ref{{Title: "Node.js, Inspector security implications", URL: "https://nodejs.org/en/learn/getting-started/debugging#security-implications"},
	{Title: "Apache Tomcat 9, AJP Connector", URL: "https://tomcat.apache.org/tomcat-9.0-doc/config/ajp.html"}},
Standards: check.Standards{CWE: []string{"CWE-489", "CWE-1327"}, OWASP: []string{"A05:2021"}, NIST: []string{"SC-7", "CM-7"}, CISTopic: "Expose applications only through a hardened front end"},
Run:       appPortRun(codePorts, "Debugger or application control ports are network-reachable", "No debugger or application control port is network-reachable"),
})
234
check.Register(&check.Check{
ID: "NET-APP-002", Title: "Application Server Ports Reachable Without a Reverse Proxy", Domain: check.DomainNetwork,
Severity: check.Medium, Category: check.CatNetwork, Effort: check.EffortLow,
CVSS:      "CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:L/I:L/A:L",
Summary:   "Checks whether application servers and developer tools (Node.js, Tomcat, Django, Flask, Vite) serve HTTP on network-reachable addresses.",
Rationale: "Application servers are designed to sit behind a reverse proxy that adds TLS, security headers, logging and request limits. Reached directly they bypass all of that, and development servers on these ports often run with debug pages on. The port number alone does not prove a weakness, so this is rated Medium and should be reviewed.",
Detection: "Matches network-reachable listening TCP sockets from /proc/net against the default HTTP ports of common application servers and developer tools, recording how each is bound and whether a web server listens on port 80 or 443 on this machine (which suggests the application was meant to be proxied). No packets are sent.",
Fix:       "Bind the application to 127.0.0.1 (Node.js: app.listen(3000, '127.0.0.1'); Gunicorn: --bind 127.0.0.1:8000; Tomcat: address=\"127.0.0.1\" on the Connector) and publish it only through nginx or Apache.",
Verify:    "ss -tlnp  shows these ports on 127.0.0.1 or ::1 only.",
Refs: []check.Ref{{Title: "OWASP Node.js Security Cheat Sheet", URL: "https://cheatsheetseries.owasp.org/cheatsheets/Nodejs_Security_Cheat_Sheet.html"},
	{Title: "Gunicorn documentation, Deploying Gunicorn (behind a proxy)", URL: "https://gunicorn.org/deploy/"}},
Standards: check.Standards{CWE: []string{"CWE-1188", "CWE-1327"}, OWASP: []string{"A05:2021"}, NIST: []string{"SC-7", "CM-7"}, CISTopic: "Expose applications only through a hardened front end"},
Run:       appPortRun(appPorts, "Application server ports are network-reachable", "No application server port is network-reachable"),
})
249}
250
251// appPortRun reports network-reachable TCP listeners on the given ports,
252// noting whether a web server (80 or 443) is present to front them.
253func appPortRun(ports map[int]string, failMsg, passMsg string) func(context.Context, sys.System) []check.Result {
return func(_ context.Context, s sys.System) []check.Result {
pub, inc := publicListeners(s)
if inc != nil {
	return inc
}
var bad []string
web := false
for _, l := range pub {
	if l.Port == 80 || l.Port == 443 {
		web = true
	}
}
for _, l := range pub {
	if n, ok := ports[l.Port]; ok && strings.HasPrefix(l.Proto, "tcp") {
		bad = append(bad, l.String()+" "+n+", bound to "+exposure(l))
	}
}
if len(bad) > 0 {
	if web {
		bad = append(bad, "a web server listens on port 80 or 443 on this machine: bind the application to 127.0.0.1 and publish it through that proxy")
	} else {
		bad = append(bad, "no web server listens on port 80 or 443 on this machine")
	}
	return []check.Result{check.FailR("sockets", failMsg, bad...)}
}
return []check.Result{check.PassR("sockets", passMsg)}
}
281}
282
283// portName labels a port with its well-known service, if any.
284func portName(p int) string {
if n, ok := sensitivePort(p); ok {
return " (" + n + ")"
}
return label(p)
289}
290
291func protoSuffix(p string) string {
if p == "" {
return ""
}
return "/" + p
296}

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Cite This Page

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APA 7

OpenVAPT. (2026). NET-APP-002 Application Server Ports Reachable Without a Reverse Proxy (Version 0.1.0-alpha.6). OpenVAPT Check Catalogue. Retrieved October 16, 2026, from https://openvapt.com/checks/NET-APP-002.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_net_app_002,
  author       = {{OpenVAPT}},
  title        = {{NET-APP-002 Application Server Ports Reachable Without a Reverse Proxy}},
  year         = {2026},
  howpublished = {\url{https://openvapt.com/checks/NET-APP-002.html}},
  note         = {OpenVAPT Check Catalogue, version 0.1.0-alpha.6. Accessed 16 October 2026}
}

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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 network
2
3	import (
4	"context"
5	"fmt"
6	"strings"
7
8	"github.com/openvapt/openvapt/internal/check"
9	"github.com/openvapt/openvapt/internal/checks/collect"
10	"github.com/openvapt/openvapt/internal/sys"
11	)
12
13	// Checks that compare what the host firewall allows with what is actually
14	// listening. A firewall that is "on" can still let a database through, keep
15	// holes open for services long gone, or be bypassed by Docker.
16
17	// codePorts are debuggers, management protocols and shutdown ports of
18	// application servers that give code execution or control to whoever
19	// connects. Sources: Node.js, Apache Tomcat, PHP-FPM and Jupyter
20	// documentation.
21	var codePorts = map[int]string{
22	9229: "Node.js inspector (debugger)", 8009: "Tomcat AJP connector", 8005: "Tomcat shutdown port",
23	9000: "PHP-FPM FastCGI", 8888: "Jupyter Notebook",
24	}
25
26	// appPorts are the default HTTP ports of application servers and developer
27	// tools that are meant to sit behind a reverse proxy, never face the
28	// network. Reaching one directly bypasses TLS, headers and limits, but the
29	// port number alone does not prove code execution, so the severity is lower
30	// than codePorts. Sources: Node.js, Django, Flask, Vite and Tomcat docs.
31	var appPorts = map[int]string{
32	3000: "Node.js / Express app server", 3001: "Node.js app server", 4000: "app server", 4200: "Angular dev server",
33	5000: "Flask / Python dev server", 5173: "Vite dev server", 8000: "Django / Gunicorn / Uvicorn", 8008: "app server",
34	8080: "Tomcat / Java HTTP connector", 8081: "app server", 8088: "app server",
35	}
36
37	// sensitivePort names a data, administrative or application server port.
38	func sensitivePort(p int) (string, bool) {
39	for _, m := range []map[int]string{dataPorts, controlPorts, managementPorts, codePorts, appPorts, aiPorts} {
40	if n, ok := m[p]; ok {
41	return n, true
42	}
43	}
44	return "", false
45	}
46
47	var refNIST41 = check.Ref{Title: "NIST SP 800-41 Rev. 1, Guidelines on Firewalls and Firewall Policy", URL: "https://csrc.nist.gov/pubs/sp/800/41/r1/final"}
48
49	func publicListeners(s sys.System) ([]collect.Listener, []check.Result) {
50	ls, err := collect.Listeners(s)
51	if err != nil {
52	return nil, []check.Result{check.IncompleteR("/proc/net", "One or more socket tables were unreadable or partial")}
53	}
54	var pub []collect.Listener
55	for _, l := range ls {
56	if l.Public() {
57	pub = append(pub, l)
58	}
59	}
60	return pub, nil
61	}
62
63	func init() {
64	check.Register(&check.Check{
65	ID: "NET-FW-003", Title: "Firewall Allows Sensitive Services From Anywhere", Domain: check.DomainNetwork,
66	Severity: check.High, Category: check.CatNetwork, Effort: check.EffortLow,
67	CVSS: "CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:H",
68	Summary: "Checks whether the host firewall explicitly opens a listening database, administrative or application server port to every address on the internet.",
69	Rationale: "A default-deny firewall only protects what it does not allow. A single \"allow 3306 from anywhere\" rule, often added while debugging, makes a database or admin panel as exposed as if there were no firewall at all.",
70	Detection: "Reads the incoming allow rules of ufw (ufw status verbose), nftables (nft list ruleset, input-hook chains) or iptables (iptables -S, INPUT and ufw chains), and matches rules with no source restriction against network-reachable listening sockets on well-known data, administrative and application server ports.",
71	Fix: "Delete the rule or restrict it to the addresses that need it, for example: ufw delete allow 3306 && ufw allow from 203.0.113.10 to any port 3306 proto tcp . Better, bind the service to 127.0.0.1 and reach it through an SSH tunnel or VPN.",
72	Verify: "ufw status numbered or nft list ruleset shows no rule opening that port to Anywhere.",
73	Refs: []check.Ref{refNIST41, {Title: "ufw(8) manual (Ubuntu)", URL: "https://manpages.ubuntu.com/manpages/noble/man8/ufw.8.html"}},
74	Standards: check.Standards{CWE: []string{"CWE-923", "CWE-1327"}, OWASP: []string{"A05:2021"}, NIST: []string{"SC-7", "AC-4", "CM-7"}, CISTopic: "Restrict firewall rules to required sources"},
75	Run: func(ctx context.Context, s sys.System) []check.Result {
76	fw := collect.ReadFirewall(ctx, s)
77	if len(fw.Rules) == 0 {
78	return []check.Result{check.SkipR("firewall", "No firewall allow rules found (see NET-FW-001 for whether a firewall is active)")}
79	}
80	pub, inc := publicListeners(s)
81	if inc != nil {
82	return inc
83	}
84	var bad []string
85	for _, l := range pub {
86	name, ok := sensitivePort(l.Port)
87	if !ok {
88	continue
89	}
90	for _, r := range fw.Rules {
91	if r.Any() && r.Covers(l.Proto, l.Port) {
92	bad = append(bad, fmt.Sprintf("%s (%s) allowed from anywhere by %s rule: %s", l.String(), name, r.Engine, r.Raw))
93	break
94	}
95	}
96	}
97	if len(bad) > 0 {
98	return []check.Result{check.FailR("firewall", "Sensitive services are open to every address", bad...)}
99	}
100	return []check.Result{check.PassR("firewall", "No sensitive listening service is opened to every address")}
101	},
102	})
103
104	check.Register(&check.Check{
105	ID: "NET-FW-004", Title: "Firewall Rules Open Ports With No Service", Domain: check.DomainNetwork,
106	Severity: check.Low, Category: check.CatNetwork, Effort: check.EffortLow,
107	CVSS: "CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:L/I:N/A:N",
108	Summary: "Checks for firewall allow rules whose port has no listening service on this machine.",
109	Rationale: "Stale rules are holes waiting for a service. Anything later started on that port, by an administrator, a package or an attacker, becomes reachable without anyone deciding it should be. Rule sets also become impossible to review when they no longer match reality.",
110	Detection: "Compares each single-port incoming allow rule from ufw, nftables or iptables with the listening sockets in /proc/net (tcp, tcp6, udp, udp6). Port ranges are not judged, because they are often used for passive FTP or media and may be idle.",
111	Fix: "Remove rules for services that no longer exist (ufw status numbered, then ufw delete <n>), or note why a rule must stay. Review the firewall whenever a service is retired.",
112	Verify: "Re-run this check; every allowed port should have a listening service.",
113	Refs: []check.Ref{refNIST41},
114	Standards: check.Standards{CWE: []string{"CWE-1188"}, NIST: []string{"SC-7", "CM-7", "CM-3"}, CISTopic: "Review firewall rules periodically"},
115	Run: func(ctx context.Context, s sys.System) []check.Result {
116	fw := collect.ReadFirewall(ctx, s)
117	if len(fw.Rules) == 0 {
118	return []check.Result{check.SkipR("firewall", "No firewall allow rules found")}
119	}
120	ls, err := collect.Listeners(s)
121	if err != nil {
122	return []check.Result{check.IncompleteR("/proc/net", "One or more socket tables were unreadable or partial")}
123	}
124	seen := map[string]bool{}
125	var bad []string
126	for _, r := range fw.Rules {
127	if r.From != r.To {
128	continue
129	}
130	used := false
131	for _, l := range ls {
132	if r.Covers(l.Proto, l.Port) {
133	used = true
134	break
135	}
136	}
137	key := fmt.Sprintf("%s/%d", r.Proto, r.From)
138	if !used && !seen[key] {
139	seen[key] = true
140	bad = append(bad, fmt.Sprintf("port %d%s allowed by %s but nothing listens on it: %s", r.From, protoSuffix(r.Proto), r.Engine, r.Raw))
141	}
142	}
143	if len(bad) > 0 {
144	return []check.Result{check.FailR("firewall", fmt.Sprintf("%d allow rule(s) open ports with no service", len(bad)), bad...)}
145	}
146	return []check.Result{check.PassR("firewall", "Every single-port allow rule matches a listening service")}
147	},
148	})
149
150	check.Register(&check.Check{
151	ID: "NET-FW-005", Title: "Container Ports Bypass the Host Firewall", Domain: check.DomainNetwork,
152	Severity: check.Medium, Category: check.CatNetwork, Effort: check.EffortMedium,
153	CVSS: "CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:L/I:L/A:L",
154	Summary: "Checks whether Docker publishes container ports through its own iptables chains with no DOCKER-USER filtering.",
155	Rationale: "Docker inserts its NAT and FORWARD rules ahead of ufw and of the INPUT chain, so a port published with -p 8080:80 is reachable from the internet even when ufw says the port is denied. Administrators routinely believe the firewall protects these containers when it does not.",
156	Detection: "Reads iptables -S and looks for ACCEPT rules with a destination port in the DOCKER chain (published container ports). Fails when there are such rules and the DOCKER-USER chain contains no DROP or REJECT rule.",
157	Fix: "Publish container ports on loopback only (-p 127.0.0.1:8080:80) and put a reverse proxy in front, or add filtering to DOCKER-USER, for example: iptables -I DOCKER-USER -i eth0 ! -s 203.0.113.0/24 -j DROP (persist it with your firewall tooling). Caution: test from an allowed address first; a wrong interface name or source range cuts access to every published container.",
158	Verify: "docker ps --format '{{.Ports}}' shows 127.0.0.1 bindings, or iptables -S DOCKER-USER shows DROP rules.",
159	Refs: []check.Ref{{Title: "Docker documentation, Packet filtering and firewalls", URL: "https://docs.docker.com/engine/network/packet-filtering-firewalls/"}, refNIST41},
160	Standards: check.Standards{CWE: []string{"CWE-923", "CWE-1327"}, OWASP: []string{"A05:2021"}, NIST: []string{"SC-7", "CM-7"}, CISTopic: "Filter container traffic"},
161	Run: func(ctx context.Context, s sys.System) []check.Result {
162	fw := collect.ReadFirewall(ctx, s)
163	if len(fw.Docker) == 0 {
164	return []check.Result{check.PassR("iptables", "No published container ports found in the Docker chain")}
165	}
166	if fw.DockerUser {
167	return []check.Result{check.PassR("iptables", "Published container ports are filtered in DOCKER-USER")}
168	}
169	return []check.Result{check.FailR("iptables", fmt.Sprintf("%d published container port rule(s) are not filtered by the host firewall", len(fw.Docker)), fw.Docker...)}
170	},
171	})
172
173	check.Register(&check.Check{
174	ID: "NET-FW-006", Title: "Firewall Exposure Map of Listening Services", Domain: check.DomainNetwork,
175	Severity: check.Info, Category: check.CatNetwork, Effort: check.EffortLow,
176	CVSS: "CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:N",
177	Summary: "Shows, for every network-reachable listening socket, what the host firewall does with traffic to it.",
178	Rationale: "Exposure is the combination of what listens and what the firewall lets through. Seeing both side by side is how assessors find services that are open by accident and confirm that the rest are blocked on purpose.",
179	Detection: "Combines the listening sockets in /proc/net with the firewall's default policy and allow rules (ufw, nftables or iptables). Each socket is labelled allowed from anywhere, allowed from restricted sources, blocked by default deny, or reachable because no filtering firewall is active.",
180	Fix: "For each socket labelled allowed from anywhere or reachable, decide whether it must be public. If not, bind it to 127.0.0.1 or restrict the firewall rule.",
181	Verify: "Re-run this check after changes and compare the map.",
182	Refs: []check.Ref{refNIST41, {Title: "NIST SP 800-115, Technical Guide to Information Security Testing and Assessment", URL: "https://csrc.nist.gov/pubs/sp/800/115/final"}},
183	Standards: check.Standards{CWE: []string{"CWE-923"}, NIST: []string{"CM-7", "CM-8", "SC-7"}, CISTopic: "Network services inventory"},
184	Run: func(ctx context.Context, s sys.System) []check.Result {
185	pub, inc := publicListeners(s)
186	if inc != nil {
187	return inc
188	}
189	fw := collect.ReadFirewall(ctx, s)
190	engine := fw.Engine
191	if engine == "" {
192	engine = "none detected"
193	}
194	ev := []string{fmt.Sprintf("firewall: %s, default deny: %v", engine, fw.DefaultDeny)}
195	open := 0
196	for _, l := range pub {
197	state := "reachable (no default-deny firewall)"
198	if fw.DefaultDeny {
199	state = "blocked by default deny"
200	}
201	for _, r := range fw.Rules {
202	if r.Covers(l.Proto, l.Port) {
203	if r.Any() {
204	state = "allowed from anywhere"
205	} else {
206	state = "allowed from " + r.Source
207	}
208	break
209	}
210	}
211	if strings.HasPrefix(state, "allowed from anywhere") \|\| strings.HasPrefix(state, "reachable") {
212	open++
213	}
214	ev = append(ev, l.String()+portName(l.Port)+": "+state)
215	}
216	return []check.Result{check.PassR("firewall", fmt.Sprintf("%d network-reachable sockets, %d open to every address", len(pub), open), ev...)}
217	},
218	})
219
220	check.Register(&check.Check{
221	ID: "NET-APP-001", Title: "Debugger or Application Control Ports Exposed", Domain: check.DomainNetwork,
222	Severity: check.High, Category: check.CatNetwork, Effort: check.EffortLow,
223	CVSS: "CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:L/A:L",
224	Summary: "Checks whether the Node.js inspector, Tomcat AJP or shutdown port, PHP-FPM FastCGI or Jupyter listen on network-reachable addresses.",
225	Rationale: "These ports are not web pages: the Node.js inspector and Jupyter run code for whoever connects, Tomcat AJP has been abused for file read and code execution (Ghostcat), the shutdown port stops the server, and PHP-FPM executes scripts handed to it. None of them is designed to face a network.",
226	Detection: "Matches network-reachable listening TCP sockets from /proc/net against the default ports of these debuggers and control protocols, recording how each is bound. Plain application HTTP ports are rated separately by NET-APP-002. No packets are sent.",
227	Fix: "Bind each to 127.0.0.1 or a Unix socket (node --inspect=127.0.0.1:9229; Tomcat Connector address=\"127.0.0.1\" or remove the AJP connector; PHP-FPM listen = /run/php/php-fpm.sock; jupyter --ip=127.0.0.1) and never expose a debugger.",
228	Verify: "ss -tlnp shows these ports on 127.0.0.1 or ::1 only.",
229	Refs: []check.Ref{{Title: "Node.js, Inspector security implications", URL: "https://nodejs.org/en/learn/getting-started/debugging#security-implications"},
230	{Title: "Apache Tomcat 9, AJP Connector", URL: "https://tomcat.apache.org/tomcat-9.0-doc/config/ajp.html"}},
231	Standards: check.Standards{CWE: []string{"CWE-489", "CWE-1327"}, OWASP: []string{"A05:2021"}, NIST: []string{"SC-7", "CM-7"}, CISTopic: "Expose applications only through a hardened front end"},
232	Run: appPortRun(codePorts, "Debugger or application control ports are network-reachable", "No debugger or application control port is network-reachable"),
233	})
234
235	check.Register(&check.Check{
236	ID: "NET-APP-002", Title: "Application Server Ports Reachable Without a Reverse Proxy", Domain: check.DomainNetwork,
237	Severity: check.Medium, Category: check.CatNetwork, Effort: check.EffortLow,
238	CVSS: "CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:L/I:L/A:L",
239	Summary: "Checks whether application servers and developer tools (Node.js, Tomcat, Django, Flask, Vite) serve HTTP on network-reachable addresses.",
240	Rationale: "Application servers are designed to sit behind a reverse proxy that adds TLS, security headers, logging and request limits. Reached directly they bypass all of that, and development servers on these ports often run with debug pages on. The port number alone does not prove a weakness, so this is rated Medium and should be reviewed.",
241	Detection: "Matches network-reachable listening TCP sockets from /proc/net against the default HTTP ports of common application servers and developer tools, recording how each is bound and whether a web server listens on port 80 or 443 on this machine (which suggests the application was meant to be proxied). No packets are sent.",
242	Fix: "Bind the application to 127.0.0.1 (Node.js: app.listen(3000, '127.0.0.1'); Gunicorn: --bind 127.0.0.1:8000; Tomcat: address=\"127.0.0.1\" on the Connector) and publish it only through nginx or Apache.",
243	Verify: "ss -tlnp shows these ports on 127.0.0.1 or ::1 only.",
244	Refs: []check.Ref{{Title: "OWASP Node.js Security Cheat Sheet", URL: "https://cheatsheetseries.owasp.org/cheatsheets/Nodejs_Security_Cheat_Sheet.html"},
245	{Title: "Gunicorn documentation, Deploying Gunicorn (behind a proxy)", URL: "https://gunicorn.org/deploy/"}},
246	Standards: check.Standards{CWE: []string{"CWE-1188", "CWE-1327"}, OWASP: []string{"A05:2021"}, NIST: []string{"SC-7", "CM-7"}, CISTopic: "Expose applications only through a hardened front end"},
247	Run: appPortRun(appPorts, "Application server ports are network-reachable", "No application server port is network-reachable"),
248	})
249	}
250
251	// appPortRun reports network-reachable TCP listeners on the given ports,
252	// noting whether a web server (80 or 443) is present to front them.
253	func appPortRun(ports map[int]string, failMsg, passMsg string) func(context.Context, sys.System) []check.Result {
254	return func(_ context.Context, s sys.System) []check.Result {
255	pub, inc := publicListeners(s)
256	if inc != nil {
257	return inc
258	}
259	var bad []string
260	web := false
261	for _, l := range pub {
262	if l.Port == 80 \|\| l.Port == 443 {
263	web = true
264	}
265	}
266	for _, l := range pub {
267	if n, ok := ports[l.Port]; ok && strings.HasPrefix(l.Proto, "tcp") {
268	bad = append(bad, l.String()+" "+n+", bound to "+exposure(l))
269	}
270	}
271	if len(bad) > 0 {
272	if web {
273	bad = append(bad, "a web server listens on port 80 or 443 on this machine: bind the application to 127.0.0.1 and publish it through that proxy")
274	} else {
275	bad = append(bad, "no web server listens on port 80 or 443 on this machine")
276	}
277	return []check.Result{check.FailR("sockets", failMsg, bad...)}
278	}
279	return []check.Result{check.PassR("sockets", passMsg)}
280	}
281	}
282
283	// portName labels a port with its well-known service, if any.
284	func portName(p int) string {
285	if n, ok := sensitivePort(p); ok {
286	return " (" + n + ")"
287	}
288	return label(p)
289	}
290
291	func protoSuffix(p string) string {
292	if p == "" {
293	return ""
294	}
295	return "/" + p
296	}