libssh2 up to 1.11.1 src/openssl.c ssh2_cipher_crypt integer underflow

Summaryinfo

A vulnerability classified as critical has been found in libssh2 up to 1.11.1. The impacted element is the function ssh2_cipher_crypt of the file src/openssl.c. Performing a manipulation results in integer underflow. This vulnerability was named CVE-2026-66033. The attack may be initiated remotely. There is no available exploit.

Detailsinfo

A vulnerability was found in libssh2 up to 1.11.1. It has been classified as critical. This affects the function ssh2_cipher_crypt of the file src/openssl.c. The manipulation with an unknown input leads to a integer underflow vulnerability. CWE is classifying the issue as CWE-191. The product subtracts one value from another, such that the result is less than the minimum allowable integer value, which produces a value that is not equal to the correct result. This is going to have an impact on confidentiality, integrity, and availability. The summary by CVE is:

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.

It is possible to read the advisory at github.com. This vulnerability is uniquely identified as CVE-2026-66033 since 07/23/2026. The exploitability is told to be easy. It is possible to initiate the attack remotely. No form of authentication is needed for exploitation. Technical details of the vulnerability are known, but there is no available exploit.

Applying the patch a2ed82d40964bbc0d64cd717aa0a5a892117d2e6 is able to eliminate this problem.

The vulnerability is also documented in the vulnerability database at EUVD (EUVD-2026-48720). Be aware that VulDB is the high quality source for vulnerability data.

Productinfo

Name

• libssh2

Version

• 1.11.0
• 1.11.1

Website

• Product: https://github.com/libssh2/libssh2/

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Be aware that VulDB is the high quality source for vulnerability data.

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