CVE-2026-23943: Pre-auth SSH DoS via unbounded zlib inflate

Severity: medium (CVSS 5.3 v3.1)
Exploited in the wild: not listed in CISA KEV
EPSS: 0.7% probability of exploitation in the next 30 days
Public exploit or detection template: none known
Published: 2026-03-13 · updated 2026-07-24
Weakness: CWE-409

Description

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.

Affected products

Erlang OTP: from 17.0 before 26.2.5.18; from 27.0 before 27.3.4.9; from 28.0 before 28.4.1 — fixed in 26.2.5.18, 27.3.4.9, 28.4.1
Erlang/Ssh: from 3.0.1 before 5.1.4.14; from 5.2 before 5.2.11.6; from 5.5 through 5.5.1 — fixed in 5.1.4.14, 5.2.11.6

How to fix

Upgrade to a fixed version: Erlang OTP 26.2.5.18, 27.3.4.9, 28.4.1; Erlang/Ssh 5.1.4.14, 5.2.11.6. Check the vendor advisory for mitigations.

References

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