Pre-auth SSH DoS via unbounded zlib inflate

Weakness Type (CWE)

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

CAPEC

CVSS 4.0 Score

6.9

MEDIUM

CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:L/SC:N/SI:N/SA:N

Vulnerability 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 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

`pkg:otp/ssh`

Module	Source File	Routine
`ssh_transport`	`src/ssh_transport.erl`	`ssh_transport:decompress/2`
		`ssh_transport:handle_packet_part/4`

Status	Type	Version	Changes / Fixed in
affected	otp ^ⓘ	`3.0.1`	unaffected at `5.5.1` unaffected at `5.2.11.6` unaffected at `5.1.4.14`

`pkg:github/erlang/otp`

Module	Source File	Routine
`ssh_transport`	`lib/ssh/src/ssh_transport.erl`	`ssh_transport:decompress/2`
		`ssh_transport:handle_packet_part/4`

Status	Type	Version	Changes / Fixed in
affected	otp ^ⓘ	`17.0`	unaffected at `28.4.1` unaffected at `27.3.4.9` unaffected at `26.2.5.18`
affected	git ^ⓘ	`07b8f441ca71`	unaffected at `43a87b949bdf` unaffected at `93073c3bd338` unaffected at `0c1c04b191f6`

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)
```

References

Credits

Reporter: Igor Morgenstern / Aisle Research
Remediation developer: Michał Wąsowski
Remediation reviewer: Jakub Witczak

CVE record as JSON: GET /cves/CVE-2026-23943.json
OSV record as JSON: GET /osv/EEF-CVE-2026-23943.json