Copyright © 2012 Red Hat, Inc. All rights reserved. Moderate 2026-07-24T16:35:28 libssh2: libssh2: Denial of Service via integer underflow in AES-GCM cipher negotiation 6.5 CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:N/I:N/A:H CWE-125

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.

A flaw was found in libssh2. A pre-authentication integer underflow vulnerability exists in the `ssh2_cipher_crypt()` function when negotiating AES-GCM ciphers during the handshake. A malicious SSH server can exploit this to trigger an out-of-bounds read and a memory copy operation with an extremely large length argument, leading to an immediate client process crash. This vulnerability results in a Denial of Service (DoS) for any connecting client.

This Moderate impact flaw in libssh2 allows a malicious SSH server to trigger a client-side denial of service. The vulnerability, an integer underflow during AES-GCM cipher negotiation, can cause a connecting client to crash before authentication. Exploitation requires the client to connect to a specially crafted, untrusted SSH server. To mitigate the vulnerability, developers or administrators must configure the specific applications utilizing libssh2 to explicitly exclude AES-GCM from their preferred cipher lists. Red Hat Hardened Images 2026-07-27T00:00:00Z RHSA-2026:46927 libssh2-main-1.11.1-10.2.hum1 Red Hat Enterprise Linux 6 Out of support scope libssh2 Red Hat Enterprise Linux 7 Affected libssh2 https://www.cve.org/CVERecord?id=CVE-2026-66033 https://nvd.nist.gov/vuln/detail/CVE-2026-66033 https://github.com/libssh2/libssh2/commit/a2ed82d40964bbc0d64cd717aa0a5a892117d2e6 https://github.com/libssh2/libssh2/pull/2401 https://www.vulncheck.com/advisories/libssh2-integer-underflow-dos-via-aes-gcm-cipher-negotiation