Pre-auth SSH DoS via unbounded zlib inflate

Open Source Vulnerabilities CVE-2026-23943

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 * [email protected]: 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.

Open Source Vulnerabilities EEF-CVE-2026-23943

## Summary 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 \* [email protected]: 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. ## Workaround 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 [email protected]): {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) ## Configuration The SSH server or client must advertise zlib or [email protected] compression. Both are enabled by default. With zlib, the attack is pre-authentication; with [email protected], authentication is required first.