libssh2 Integer Underflow DoS via AES-GCM Cipher Negotiation
Description
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Affected software
pkg:deb/ubuntu/libssh2?arch=source&distro=esm-infra-legacy/trustypkg:deb/ubuntu/libssh2?arch=source&distro=esm-apps/xenialpkg:deb/ubuntu/libssh2?arch=source&distro=bionicpkg:deb/ubuntu/libssh2?arch=source&distro=focalpkg:deb/ubuntu/libssh2?arch=source&distro=jammypkg:deb/ubuntu/libssh2?arch=source&distro=noblepkg:deb/ubuntu/libssh2?arch=source&distro=resoluteRun on your own infrastructure? Check whether these packages are installed with threat-finder — our free open-source scanner.
Weaknesses
AI-Powered Analysis
Machine-generated threat intelligence
Technical Analysis
The vulnerability in libssh2 up to version 1.11.1 is a pre-authentication integer underflow in the ssh2_cipher_crypt() function located in src/openssl.c. When a malicious SSH server negotiates AES-GCM ciphers during the handshake, it exploits an underflow in the calculation of blocksize minus aadlen minus authentication tag length. This leads to an out-of-bounds read and a memcpy operation with a near-SIZE_MAX length argument, resulting in an immediate crash of the client process before any authentication occurs. The issue was fixed in commit a2ed82d.
Potential Impact
Exploitation of this vulnerability allows a malicious SSH server to cause a denial of service by crashing any connecting libssh2 client before authentication. There is no impact on confidentiality or integrity, but availability is affected due to the forced process crash.
Mitigation Recommendations
A fix for this vulnerability is available in libssh2 as of commit a2ed82d. Users should upgrade to a version that includes this fix to prevent denial of service. Patch status is confirmed by the fix commit; no further mitigation is required.
Technical Details
- Gcve Source
- db.gcve.eu
- Osv Id
- UBUNTU-CVE-2026-66033
- Osv Schema Version
- 1.7.0
- Ecosystems
- ["Ubuntu:Pro:14.04:LTS","Ubuntu:Pro:16.04:LTS","Ubuntu:18.04:LTS","Ubuntu:20.04:LTS","Ubuntu:22.04:LTS","Ubuntu:24.04:LTS","Ubuntu:26.04:LTS"]
- Cvss Version
- 3.1
Threat ID: 6a984573acd9273b496ad57f
Added to database: 09/02/2026, 15:49:07 UTC
Last enriched: 09/07/2026, 22:09:04 UTC
Last updated: 10/15/2026, 18:48:22 UTC
Views: 44
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