Out-of-bounds Read in Libssh2
CVE-2026-66033
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.
Vulnerability class: Buffer Overflow
Published · last modified .
CVSS v3 metric
CVSS v3 base score 7.5 (High). Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H.
EPSS exploit prediction
EPSS: 0.006 (47.2th percentile), scored .
Very low probability of exploitation in the next 30 days; routine patching cadence is appropriate. 47th percentile — 47.2% of CVEs in the catalogue have a lower EPSS than this one. How to read EPSS.
EPSS over last 30 days · oldest: 0.007 · newest: 0.006 · change: -0.001
Affected products
- Libssh2 — versions 0, a2ed82d40964bbc0d64cd717aa0a5a892117d2e6
Weakness classification (CWE)
References
- github.com/libssh2/libssh2/pull/2401 (issue-tracking, Patch, patch, Issue Tracking)
- github.com/libssh2/libssh2/commit/a2ed82d40964bbc0d64cd717aa0a5a892117d2e6 (Patch, patch)
- www.vulncheck.com/advisories/libssh2-integer-underflow-dos-via-aes-gcm-cipher-n… (Patch, Third Party Advisory, third-party-advisory)
Frequently asked questions
- What is CVE-2026-66033?
- CVE-2026-66033 is a high-severity vulnerability in Libssh2, classified under Out-of-bounds Read. CVSS score: 7.5/10. Published 2026-07-24.
- How severe is CVE-2026-66033?
- High severity. CVSS v3 base score is 7.5 out of 10.