Hardening RHEL (and RHEL Based OSs)
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For obvious reasons RHEL is annoying, like needing to sign into their paywall to find this information. If you're ever needing to harden a RHEL based OS, specifically to disable SHA1 and CBC you can use the below and reboot the server.
These vulnerabilities are outlined below and the remedy is listed at the bottom. Mind any typo's I've copied the description out of a PDF and there may be some copy/paste artifacts or typos.
Medium (CVSS: 5.3) NVT: Weak Key Exchange (KEX) Algorithm(s) Supported (SSH) Product detection result cpe:/a:ietf:secure_shell_protocol Detected by SSH Protocol Algorithms Supported (OID: 1.3.6.1.4.1.25623.1.0.105565 →) Summary The remote SSH server is con gured to allow / support weak key exchange (KEX) algorithm(s). Quality of Detection (QoD): 80% Vulnerability Detection Result The remote SSH server supports the following weak KEX algorithm(s): KEX algorithm | Reason----------------------------------------------- diffie-hellman-group-exchange-sha1 | Using SHA-1 Impact An attacker can quickly break individual connections. Solution: Solution type: Mitigation Disable the reported weak KEX algorithm(s)- 1024-bit MODP group / prime KEX algorithms: Alternatively use elliptic-curve Di e-Hellmann in general, e.g. Curve 25519. Vulnerability Insight- 1024-bit MODP group / prime KEX algorithms: Millions of HTTPS, SSH, and VPN servers all use the same prime numbers for Diffie-Hellman key exchange. Practitioners believed this was safe as long as new key exchange messages were generated for every connection. However, the first step in the number field sieve-the most efficient algorithm for breaking a Diffie-Hellman connection-is dependent only on this prime. A nation-state can break a 1024-bit prime. Vulnerability Detection Method Checks the supported KEX algorithms of the remote SSH server. Currently weak KEX algorithms are defined as the following:- non-elliptic-curve Di e-Hellmann (DH) KEX algorithms with 1024-bit MODP group / prime- ephemerally generated key exchange groups uses SHA-1- using RSA 1024-bit modulus key Details: Weak Key Exchange (KEX) Algorithm(s) Supported (SSH) OID:1.3.6.1.4.1.25623.1.0.150713 Version used: 2024-06-14T05:05:48Z Product Detection Result Product: cpe:/a:ietf:secure_shell_protocol Method: SSH Protocol Algorithms Supported OID: 1.3.6.1.4.1.25623.1.0.105565) References url: https://weakdh.org/sysadmin.html url: https://www.rfc-editor.org/rfc/rfc9142 url: https://www.rfc-editor.org/rfc/rfc9142#name-summary-guidance-for-implem url: https://www.rfc-editor.org/rfc/rfc6194 url: https://www.rfc-editor.org/rfc/rfc4253#section-6.5And CBC
Medium (CVSS: 4.3) NVT: Weak Encryption Algorithm(s) Supported (SSH) Product detection result cpe:/a:ietf:secure_shell_protocol Detected by SSH Protocol Algorithms Supported (OID: 1.3.6.1.4.1.25623.1.0.105565 →) Summary The remote SSH server is con gured to allow / support weak encryption algorithm(s). Quality of Detection (QoD): 80% Vulnerability Detection Result The remote SSH server supports the following weak client-to-server encryption al →gorithm(s): aes128-cbc aes256-cbc The remote SSH server supports the following weak server-to-client encryption al →gorithm(s): aes128-cbc aes256-cbc Solution: Solution type: Mitigation Disable the reported weak encryption algorithm(s). . . . continues on next page ... 2 RESULTS PER HOST 6 . . . continued from previous page ... Vulnerability Insight- The 'arcfour' cipher is the Arcfour stream cipher with 128-bit keys. The Arcfour cipher is believed to be compatible with the RC4 cipher [SCHNEIER]. Arcfour (and RC4) has problems with weak keys, and should not be used anymore.- The 'none' algorithm specifies that no encryption is to be done. Note that this method provides no confidentiality protection, and it is NOT RECOMMENDED to use it.- A vulnerability exists in SSH messages that employ CBC mode that may allow an attacker to recover plaintext from a block of ciphertext. Vulnerability Detection Method Checks the supported encryption algorithms (client-to-server and server-to-client) of the remote SSH server. Currently weak encryption algorithms are de ned as the following:- Arcfour (RC4) cipher based algorithms- 'none' algorithm- CBC mode cipher based algorithms Details: Weak Encryption Algorithm(s) Supported (SSH) OID:1.3.6.1.4.1.25623.1.0.105611 Version used: 2024-06-14T05:05:48Z Product Detection Result Product: cpe:/a:ietf:secure_shell_protocol Method: SSH Protocol Algorithms Supported OID: 1.3.6.1.4.1.25623.1.0.105565) References url: https://www.rfc-editor.org/rfc/rfc8758 url: https://www.kb.cert.org/vuls/id/958563 url: https://www.rfc-editor.org/rfc/rfc4253#section-6.3Simply running
sudo update-crypto-policies --set DEFAULT:NO-SHA1:NO-CBCand rebooting the system in question removes these vulnerabilities.
