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Vulnerability · Published Jul 24, 2026 · Assigned by VulnCheck

CVE-2026-66033libssh2 Integer Underflow DoS via AES-GCM Cipher Negotiation

High 7.5

In plain English

A high-severity out-of-bounds read in libssh2 from libssh2. There are no confirmed reports of exploitation so far. The chance it is exploited in the next 30 days is estimated at 0.6% (higher than 48% of all known vulnerabilities). Fixed versions are available - upgrade the affected package.

Severity (CVSS 3.1)7.5High
Exploit odds (30 days)0.6%top 52.0% of all CVEs
Exploited in the wildNo reportsCISA SSVC: none
Attack toolingNoneno public tooling known

Official description

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.

Affected packages & fixed versions

EcosystemPackageFixed inAdvisory
Alpine:v3.21libssh21.11.1-r3ALPINE-CVE-2026-66033
Alpine:v3.22libssh21.11.1-r3ALPINE-CVE-2026-66033
Alpine:v3.23libssh21.11.1-r4ALPINE-CVE-2026-66033
Alpine:v3.24libssh21.11.1-r4ALPINE-CVE-2026-66033
Debian:13libssh21.11.1-1+deb13u2DEBIAN-CVE-2026-66033
Debian:14libssh21.11.1-5DEBIAN-CVE-2026-66033
Ubuntu:24.04:LTSlibssh21.11.0-4.1ubuntu0.24.04.4USN-8722-1
Ubuntu:24.04:LTSlibssh21.11.0-4.1ubuntu0.24.04.4UBUNTU-CVE-2026-66033
Ubuntu:26.04:LTSlibssh21.11.1-1ubuntu0.26.04.4USN-8722-1
Ubuntu:26.04:LTSlibssh21.11.1-1ubuntu0.26.04.4UBUNTU-CVE-2026-66033

Affected products (as reported)

VendorProductVersions
libssh2 libssh2 0 – through 1.11.1

Weakness: CWE-191, CWE-125 · Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H

References