Pre-auth SSH DoS via unbounded zlib inflate

Improper Handling of Highly Compressed Data (Compression Bomb) vulnerability in Erlang OTP ssh (ssh_transport modules) allows Denial of Service via Resource Depletion.

The SSH transport layer advertises legacy zlib compression by default and inflates attacker-controlled payloads pre-authentication without any size limit, enabling reliable memory exhaustion DoS.

Two compression algorithms are affected:

• zlib: Activates immediately after key exchange, enabling unauthenticated attacks
• zlib@openssh.com: Activates post-authentication, enabling authenticated attacks

Each SSH packet can decompress ~255 MB from 256 KB of wire data (1029:1 amplification ratio). Multiple packets can rapidly exhaust available memory, causing OOM kills in memory-constrained environments.

This vulnerability is associated with program files lib/ssh/src/ssh_transport.erl and program routines ssh_transport:decompress/2, ssh_transport:handle_packet_part/4.

This issue affects OTP from OTP 17.0 before OTP 28.4.1, OTP 27.3.4.9 and OTP 26.2.5.18, corresponding to ssh from 3.0.1 before 5.5.1, 5.2.11.6 and 5.1.4.14.

Best workaround - Disable all compression:

{preferred_algorithms, [{compression, ['none']}]}

Alternative mitigations (less secure):

• Disable only pre-auth zlib compression (authenticated users can still exploit via zlib@openssh.com):

  {modify_algorithms, [{rm, [{compression, ['zlib']}]}]}

• Limit concurrent sessions (reduces attack surface but does not prevent exploitation):

  {max_sessions, N}  % Cap total concurrent sessions (default is infinity)

The SSH server or client must advertise zlib or zlib@openssh.com compression. Both are enabled by default. With zlib, the attack is pre-authentication; with zlib@openssh.com, authentication is required first.