OpenVAPTMediumHOST-SSH-004Cryptography and TLSServer
SSH Offers Weak Cryptography
Checks the SSH ciphers, MACs, key exchange and host key algorithms for outdated choices.
- Why It Matters
- CBC-mode ciphers, RC4, MD5/SHA-1 MACs and 1024-bit Diffie-Hellman have known practical weaknesses. Offering them lets a network attacker downgrade or weaken the session.
- How It Detects
- Reads ciphers, macs, kexalgorithms and hostkeyalgorithms from sshd -T and flags any algorithm whose name contains a known-weak fragment, plus the SHA-1 based ssh-rsa host key signature algorithm.
- How to Fix
- Remove the listed algorithms. On OpenSSH 8.x and later the defaults are already strong: delete any custom Ciphers/MACs/KexAlgorithms lines, or set explicit modern lists such as Ciphers chacha20-poly1305@openssh.com,aes256-gcm@openssh.com,aes128-gcm@openssh.com then reload sshd.
- How to Verify
sshd -T | grep -Ei '^(ciphers|macs|kexalgorithms)' should list no cbc, arcfour, md5 or sha1 entries, and hostkeyalgorithms should not list ssh-rsa.- CVSS v3.1 Vector
CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:N/A:N- Control Category
- Cryptography and TLS: CIS Controls v8 3; NIST CSF 2.0 PR.DS
- Fix Effort
- Low
- CIS Controls v8 Safeguard
- 3.10
- NIST CSF 2.0 Subcategory
- PR.DS-02
- CWE
- CWE-327
- OWASP Top 10
- A02:2021
- NIST SP 800-53 Rev. 5
- SC-8, SC-13
- Benchmark Topic
- SSH server configuration: approved ciphers, MACs and KEX (topic only, no benchmark text)
- References
- OpenBSD, sshd_config(5) manual page
RFC 9142, Key Exchange Method Updates for SSH
NIST SP 800-131A Rev. 2, Transitioning Cryptographic Algorithms
Learn the theory behind this check: P2 SSH Security.
Source Code
The complete file that defines this check. Run it with openvapt scan --only HOST-SSH-004, or read it offline with openvapt explain HOST-SSH-004.
internal/checks/host/ssh_weak_crypto.go · Lines 1-65View on GitHub at This Commit
| 1 | package host |
| 2 | |
| 3 | import ( |
| 4 | "context" |
| 5 | "strings" |
| 6 | |
| 7 | "github.com/openvapt/openvapt/internal/check" |
| 8 | "github.com/openvapt/openvapt/internal/checks/collect" |
| 9 | "github.com/openvapt/openvapt/internal/sys" |
| 10 | ) |
| 11 | |
| 12 | // weakSSH lists algorithm name fragments considered weak. Sources: RFC 9142 |
| 13 | // (key exchange), RFC 8758 (RC4 deprecated), and the OpenSSH release notes |
| 14 | // that removed or disabled these algorithms by default. |
| 15 | var weakSSH = []string{"cbc", "arcfour", "3des", "blowfish", "hmac-md5", "hmac-sha1", "umac-64", |
| 16 | "diffie-hellman-group1-", "diffie-hellman-group14-sha1", "diffie-hellman-group-exchange-sha1", "ssh-dss"} |
| 17 | |
| 18 | // sshRSASHA1 reports the SHA-1 based RSA signature algorithm "ssh-rsa" |
| 19 | // (and its certificate form), disabled by default since OpenSSH 8.8. |
| 20 | func sshRSASHA1(alg string) bool { |
| 21 | return alg == "ssh-rsa" || alg == "ssh-rsa-cert-v01@openssh.com" |
| 22 | } |
| 23 | |
| 24 | func init() { |
| 25 | check.Register(&check.Check{ |
| 26 | ID: "HOST-SSH-004", |
| 27 | Title: "SSH Offers Weak Cryptography", |
| 28 | Domain: check.DomainHost, |
| 29 | Severity: check.Medium, |
| 30 | CVSS: "CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:N/A:N", |
| 31 | Summary: "Checks the SSH ciphers, MACs, key exchange and host key algorithms for outdated choices.", |
| 32 | Rationale: "CBC-mode ciphers, RC4, MD5/SHA-1 MACs and 1024-bit Diffie-Hellman have known practical weaknesses. Offering them lets a network attacker downgrade or weaken the session.", |
| 33 | Detection: "Reads ciphers, macs, kexalgorithms and hostkeyalgorithms from sshd -T and flags any algorithm whose name contains a known-weak fragment, plus the SHA-1 based ssh-rsa host key signature algorithm.", |
| 34 | Fix: "Remove the listed algorithms. On OpenSSH 8.x and later the defaults are already strong: delete any custom Ciphers/MACs/KexAlgorithms lines, or set explicit modern lists such as Ciphers chacha20-poly1305@openssh.com,aes256-gcm@openssh.com,aes128-gcm@openssh.com then reload sshd.", |
| 35 | Verify: "sshd -T | grep -Ei '^(ciphers|macs|kexalgorithms)' should list no cbc, arcfour, md5 or sha1 entries, and hostkeyalgorithms should not list ssh-rsa.", |
| 36 | Refs: []check.Ref{refSSHDManual, |
| 37 | {Title: "RFC 9142, Key Exchange Method Updates for SSH", URL: "https://www.rfc-editor.org/rfc/rfc9142.html"}, |
| 38 | {Title: "NIST SP 800-131A Rev. 2, Transitioning Cryptographic Algorithms", URL: "https://csrc.nist.gov/pubs/sp/800/131/a/r2/final"}}, |
| 39 | Standards: check.Standards{CWE: []string{"CWE-327"}, OWASP: []string{"A02:2021"}, NIST: []string{"SC-8", "SC-13"}, CISTopic: "SSH server configuration: approved ciphers, MACs and KEX"}, |
| 40 | Run: func(ctx context.Context, s sys.System) []check.Result { |
| 41 | cfg, ok := collect.SSHD(ctx, s) |
| 42 | if !ok { |
| 43 | return []check.Result{check.SkipR("sshd", "SSH daemon configuration not found")} |
| 44 | } |
| 45 | var bad []string |
| 46 | for _, key := range []string{"ciphers", "macs", "kexalgorithms", "hostkeyalgorithms"} { |
| 47 | for _, alg := range strings.Split(cfg[key], ",") { |
| 48 | for _, w := range weakSSH { |
| 49 | if alg != "" && (strings.Contains(alg, w) || key == "hostkeyalgorithms" && sshRSASHA1(alg)) { |
| 50 | bad = append(bad, key+": "+alg) |
| 51 | break |
| 52 | } |
| 53 | } |
| 54 | } |
| 55 | } |
| 56 | if len(bad) > 0 { |
| 57 | return []check.Result{check.FailR("sshd", "Weak SSH algorithms are offered", bad...)} |
| 58 | } |
| 59 | if cfg["ciphers"] == "" { |
| 60 | return []check.Result{check.SkipR("sshd", "Algorithm lists not available (sshd -T not readable); OpenSSH defaults assumed")} |
| 61 | } |
| 62 | return []check.Result{check.PassR("sshd", "No weak SSH algorithms offered")} |
| 63 | }, |
| 64 | }) |
| 65 | } |
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APA 7
OpenVAPT. (2026). HOST-SSH-004 SSH Offers Weak Cryptography (Version 0.1.0-alpha.6). OpenVAPT Check Catalogue. Retrieved October 16, 2026, from https://openvapt.com/checks/HOST-SSH-004.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_ssh_004,
author = {{OpenVAPT}},
title = {{HOST-SSH-004 SSH Offers Weak Cryptography}},
year = {2026},
howpublished = {\url{https://openvapt.com/checks/HOST-SSH-004.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.