CVE-2026-66033libssh2 Integer Underflow DoS via AES-GCM Cipher Negotiation
Updated under an hour ago ()
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No exploitation reported and no EPSS estimate yet.
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libssh2 through 1.11.1, fixed in commit a2ed82d, contains a pre-authentication integer underflow vulnerability in the ssh2_cipher_crypt() function in src/openssl.c that allows a malicious SSH server to crash any connecting client by negotiating AES-GCM ciphers during handshake. Attackers can exploit the underflow in the expression computing blocksize minus aadlen minus authentication tag length to trigger an out-of-bounds read and a memcpy call with a near-SIZE_MAX length argument, causing immediate process crash before any authentication occurs.
- CVSS
- 7.5 HIGH (v3.1, CNA)CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
- Weakness (CWE)
- CWE-191, CWE-125
- Known exploited
- Not in CISA's KEV catalog
- EPSS (exploitation in the next 30 days)
- Not loaded for this CVE
- Assigned by
- VulnCheck
- NVD analysis
- Analyzed
- Record last updated
- Jul 27, 2026
- First seen by InfoSec Signals
- Oct 9, 2026
Affected products
libssh2 libssh2 libssh2:libssh2 (from the name) | through 1.11.1; a2ed82d40964bbc0d64cd717aa0a5a892117d2e6 not affected | CNA record |
libssh2 libssh2 libssh2:libssh2 | through 1.11.1 | NVD |
References
- https://github.com/libssh2/libssh2/pull/2401issue-tracking, patch
- https://github.com/libssh2/libssh2/commit/a2ed82d40964bbc0d64cd717aa0a5a892117d2e6patch
- https://www.vulncheck.com/advisories/libssh2-integer-underflow-dos-via-aes-gcm-cipher-negotiationthird-party-advisory, patch
The CVE record on cve.org NVD's entry
The CVE record is authoritative. Products marked “from the name” were identified from the CNA's free-text names, not a CPE, and are matched less certainly. This product uses the NVD API but is not endorsed or certified by the NVD.