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.

CVSS v3 7.5 HIGH

7.5^/10

CVSS v3 : HIGH

Vector :

Exploitability : 3.9 / Impact : 3.6

Attack Vector NETWORK

Attack Complexity LOW

Privileges Required NONE

User Interaction NONE

Confidentiality Impact NONE

Integrity Impact NONE

Availability Impact HIGH

Scope UNCHANGED

References

Link	Resource
https://github.com/libssh2/libssh2/commit/a2ed82d40964bbc0d64cd717aa0a5a892117d2e6	Patch
https://github.com/libssh2/libssh2/pull/2401	Issue Tracking Patch
https://www.vulncheck.com/advisories/libssh2-integer-underflow-dos-via-aes-gcm-cipher-negotiation	Patch Third Party Advisory

Configurations

Configuration 1 (hide)

cpe:2.3:a:libssh2:libssh2:*:*:*:*:*:*:*:*

History

30 Jul 2026, 15:44

Type	Values Removed	Values Added
First Time		Libssh2 libssh2 Libssh2
CPE		cpe:2.3:a:libssh2:libssh2::::::::
References	~~() https://github.com/libssh2/libssh2/commit/a2ed82d40964bbc0d64cd717aa0a5a892117d2e6 -~~	() https://github.com/libssh2/libssh2/commit/a2ed82d40964bbc0d64cd717aa0a5a892117d2e6 - Patch
References	~~() https://github.com/libssh2/libssh2/pull/2401 -~~	() https://github.com/libssh2/libssh2/pull/2401 - Issue Tracking, Patch
References	~~() https://www.vulncheck.com/advisories/libssh2-integer-underflow-dos-via-aes-gcm-cipher-negotiation -~~	() https://www.vulncheck.com/advisories/libssh2-integer-underflow-dos-via-aes-gcm-cipher-negotiation - Patch, Third Party Advisory

24 Jul 2026, 17:17

Type	Values Removed	Values Added
New CVE

Information

Published : 2026-07-24 17:17

Updated : 2026-07-30 15:44

NVD link : CVE-2026-66033

Mitre link : CVE-2026-66033

CVE.ORG link : CVE-2026-66033

JSON object : View

Products Affected

libssh2

libssh2

CWE

CWE-125

Out-of-bounds Read

CWE-191

Integer Underflow (Wrap or Wraparound)

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