lowaccess-controlverified ✓rollback-safe
Disable SSH compression
ssh-compression · RHEL ≥ 8 · 2 impls
Description
The SSH daemon must not use compression for data transmitted over the encrypted channel. SSH compression can be exploited by oracle attacks similar to CRIME and BREACH that infer plaintext content from compressed ciphertext size variations.
Rationale
When compression is enabled before encryption, an attacker who can influence part of the plaintext (such as through a web request reflected in a terminal) can observe the compressed ciphertext size to extract secrets byte by byte. Disabling compression eliminates this side-channel attack vector entirely.
Check → Remediate
Checksshd_effective_config
- key:
- compression
- expected:
- no
Remediateconfig_set
- path:
- /etc/ssh/sshd_config
- key:
- Compression
- value:
- no
- separator:
- reload:
- sshd
Framework references
STIG
V-258002 / RHEL-09-255130
NIST 800-53
CM-6
Live verification
rhel9:check
#ssh#compression#performance