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
  • 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 until OTP 28.4.1, 27.3.4.9 and 26.2.5.18 corresponding to ssh from 3.0.1 until 5.5.1, 5.2.11.6 and 5.1.4.14.

Affected Software

VendorProductVersion RangeStatus
ErlangOTP3.0.1 < *affected
ErlangOTP17.0 < *affected
ErlangOTP07b8f441ca711f9812fad9e9115bab3c3aa92f79 < *affected

Weaknesses

  • CWE-409: CWE-409 Improper Handling of Highly Compressed Data (Data Amplification)

Workarounds

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)

ADP Enrichment

CISA ADP Vulnrichment

  • SSVC:
  • Exploitation: none
    • Automatable: yes
    • Technical Impact: partial

References