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 ma

highCVSS 7.5CVE-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.

Details

Source: NVD Recent CVEs (US · database · site)
Severity: high — CVSS 7.5
Published: 2026-07-24
Last updated: 2026-07-30
Exploitation: Not in CISA KEV at last sync

Original advisory: https://nvd.nist.gov/vuln/detail/CVE-2026-66033

Exploitation outlook

EPSS (FIRST.org) estimates each CVE’s probability of exploitation in the next 30 days — here is the CSIRTS.com read on those numbers.

Low exploitation riskCVE-2026-660330.37% 30-day exploitation probability — currently an unlikely target, but scores change as exploit code circulates. Riskier than 30% of all scored CVEs.

Referenced CVEs

CVE	CSIRTS overview	External
CVE-2026-66033	coverage & exploitation status	NVD · CVE.org

Same CVEs, other sources

How other CERTs, PSIRTs and databases cover the vulnerabilities in this advisory.

high[NEW] [high] libssh2: Multiple vulnerabilitiescert-bund
highCVE-2026-66033: libssh2 Integer Underflow DoS via AES-GCM Cipher Negotiationmsrc

Recent advisories for libssh2

A cluster of recent advisories against the same product widens the attack surface — attackers routinely chain freshly published CVEs on one product, so review these together.

high[NEW] [high] libssh2: Multiple vulnerabilitiescert-bund · 2026-07-28
medium[UPDATE] [medium] libssh2: Multiple vulnerabilitiescert-bund · 2026-07-28
highCVE-2026-66035: libssh2 through 1.11.1, fixed in commit 42e33d8, contains a pre-authentication heap buffer ove…nvd · 2026-07-24
highCVE-2026-66034: libssh2 through 1.11.1, fixed in commit a13bb6c, contains a missing bounds check vulnerability…nvd · 2026-07-24
highCVE-2026-66032: libssh2 through 1.11.1, fixed in commit 5e47761, contains a double-free vulnerability in the s…nvd · 2026-07-24
high[UPDATE] [high] libssh2: Multiple vulnerabilitiescert-bund · 2026-07-17