TMA-COS-5.1.13
Ensure only strong Ciphers are used
Description
This variable limits the ciphers that SSH can use during communication. Weak ciphers that are used for authentication to the cryptographic module cannot be relied upon to provide confidentiality or integrity, and system data may be compromised The DES, Triple DES, and Blowfish ciphers, as used in SSH, have a birthday bound of approximately four billion blocks, which makes it easier for remote attackers to obtain cleartext data via a birthday attack against a long-duration encrypted session, aka a "Sweet32" attack The RC4 algorithm, as used in the TLS protocol and SSL protocol, does not properly combine state data with key data during the initialization phase, which makes it easier for remote attackers to conduct plaintext-recovery attacks against the initial bytes of a stream by sniffing network traffic that occasionally relies on keys affected by the Invariance Weakness, and then usi...
How it is checked
Audit method: Shell command
alt0() { command -v grep >/dev/null 2>&1 || { echo "step1: grep not installed"; return 1; }; command -v sshd >/dev/null 2>&1 || { echo "step1: sshd not installed"; return 1; }; o=$(bash -c 'sshd -T | grep ciphers' 2>&1); printf '%s\n' "$o" | tr -s ' \t' ' ' | sed 's/^ //;s/ $//' | grep -Fxq -- 'Weak Ciphers:' || { echo step1: missing 'Weak Ciphers:'; return 1; }; printf '%s\n' "$o" | tr -s ' \t' ' ' | sed 's/^ //;s/ $//' | grep -Fxq -- 3des-cbc || { echo step1: missing 3des-cbc; return 1; }; printf '%s\n' "$o" | tr -s ' \t' ' ' | sed 's/^ //;s/ $//' | grep -Fxq -- aes128-cbc || { echo step1: missing aes128-cbc; return 1; }; printf '%s\n' "$o" | tr -s ' \t' ' ' | sed 's/^ //;s/ $//' | grep -Fxq -- aes192-cbc || { echo step1: missing aes192-cbc; return 1; }; printf '%s\n' "$o" | tr -s ' \t' ' ' | sed 's/^ //;s/ $//' | grep -Fxq -- aes256-cbc || { echo step1: missing aes256-cbc; return 1; }; printf '%s\n' "$o" | tr -s ' \t' ' ' | sed 's/^ //;s/ $//' | grep -Fxq -- arcfour || { echo step1: missing arcfour; return 1; }; printf '%s\n' "$o" | tr -s ' \t' ' ' | sed 's/^ //;s/ $//' | grep -Fxq -- arcfour128 || { echo step1: missing arcfour128; return 1; }; printf '%s\n' "$o" | tr -s ' \t' ' ' | sed 's/^ //;s/ $//' | grep -Fxq -- arcfour256 || { echo step1: missing arcfour256; return 1; }; printf '%s\n' "$o" | tr -s ' \t' ' ' | sed 's/^ //;s/ $//' | grep -Fxq -- blowfish-cbc || { echo step1: missing blowfish-cbc; return 1; }; printf '%s\n' "$o" | tr -s ' \t' ' ' | sed 's/^ //;s/ $//' | grep -Fxq -- cast128-cbc || { echo step1: missing cast128-cbc; return 1; }; printf '%s\n' "$o" | tr -s ' \t' ' ' | sed 's/^ //;s/ $//' | grep -Fxq -- rijndael-cbc@lysator.liu.se || { echo step1: missing rijndael-cbc@lysator.liu.se; return 1; }; }; if alt0; then echo compliant; fi- Expected output
compliant
How it is fixed
Fix method: Guidance or infrastructure patch
Edit the /etc/ssh/sshd_config file add/modify the Ciphers line to contain a comma separated list of the site approved ciphers Example: Ciphers chacha20-poly1305@openssh.com,aes256-gcm@openssh.com,aes128- gcm@openssh.com,aes256-ctr,aes192-ctr,aes128-ctr
Properties
- Operating systems
- Any
- Tags
- container-optimized-oscos
- Reference
- CIS Google Container-Optimized OS Benchmark v1.2.0 - 5.1.13 Ensure only strong Ciphers are used
Check this host
Run this on a machine you are allowed to audit. The scan is read-only.
tmaegis scan --target cos