CVE-2026-66033: libssh2 Integer Underflow DoS via AES-GCM Cipher Negotiation
Last updated 4 September 2026
Other sources
libssh2 Integer Underflow DoS via AES-GCM Cipher Negotiation
— Microsoft
libssh2 through 1.11.1, fixed in commit a2ed82d, contains a pre-authentication integer underflow vulnerability in the ssh2ciphercrypt() 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-SIZEMAX length argument, causing immediate process crash before any authentication occurs.
— MITRE
Affected Software
Remediation
Recommended actions to resolve this vulnerability, in priority order.
- Upgrade
Upgrade to a fixed release to a version that resolves this vulnerability.
Fixed in 1.11.1-5 - Upgrade
Upgrade
debian/libssh2to a version that resolves this vulnerability.Fixed in 1.10.0-3Fixed in 1.11.1-6 - Upgrade
Upgrade
libssh2to a version that resolves this vulnerability.Fixed in 1.11.1Patch a2ed82d - Compensating control
Mitigate pre-authentication crash risk by limiting or filtering inbound SSH connections (e.g., via firewall/ACL) to reduce exposure to malicious servers performing AES-GCM cipher negotiation.
Event History
Frequently Asked Questions
What is the severity of CVE-2026-66033?
CVE-2026-66033 has a severity rating of high with a score of 7.5.
What type of vulnerability is CVE-2026-66033?
CVE-2026-66033 is classified as an integer underflow vulnerability.
How can CVE-2026-66033 be exploited?
CVE-2026-66033 can be exploited by a malicious SSH server to crash a client by negotiating AES-GCM ciphers during the handshake.
How do I fix CVE-2026-66033?
To fix CVE-2026-66033, upgrade to libssh2 version 1.11.2 or later, where the vulnerability has been addressed.
What software is affected by CVE-2026-66033?
CVE-2026-66033 affects libssh2 versions up to and including 1.11.1.