The vulnerability is a missing bounds check in the publickey subsystem of libssh2. A malicious SSH server can cause the client to read an arbitrary number of bytes beyond the end of a buffer, leaking heap pointers from nearby allocations. This out-of-bounds read defeats ASLR protection and can lead to heap allocator corruption when the error cleanup path frees an uninitialized pointer from a non-zeroed realloc region. The weaknesses correspond to CWE-125 and CWE-908.

The flaw exists in libssh2 up to version 1.11.1. All builds of libssh2 before the commit a13bb6c773f0d55ad1628cede57e99803cd898d9 are affected.

The CVSS score of 7.7 indicates a high impact. An EPSS score of less than 1% shows that exploitation chances are currently low, and the vulnerability is not listed in CISA's KEV catalog. The attack requires a remote attacker to act as an SSH server, so the threat surface is limited to environments that allow an attacker to impersonate a server or compromise an authenticated SSH session. Because the bug is triggered by specially crafted server messages, automated exploitation is possible, but a successful exploit would need network reach to the vulnerable client.