CVE-2026-66033

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.

The CVSS score(s) listed for this vulnerability do not reflect the associated product's status, and are included for informational purposes to better understand the severity of this vulnerability.

To mitigate the vulnerability, developers or administrators must configure the specific applications utilizing libssh2 to explicitly exclude AES-GCM from their preferred cipher lists.