Executive brief
libssh2 is a widely used library that allows applications to communicate securely using the SSH protocol. A vulnerability has been identified where a malicious SSH server can compromise a client application that connects to it. This could allow an attacker to take control of the connecting system or crash the application before the user even logs in.
Technical details
A heap-based buffer overflow exists in the fullpacket() function within src/transport.c of libssh2. During Encrypt-then-MAC (EtM) cipher negotiation, the code allocates a buffer based on a provided packet_length but proceeds to copy 'blocksize minus one' bytes. If the packet_length is smaller than the cipher's block size, a memcpy operation overflows the allocated buffer. On 32-bit systems using glibc, this can overwrite the SIZE field of adjacent heap chunks, leading to tcache bin confusion, overlapping objects, and potential arbitrary code execution via function pointer overwrites. The vulnerability is triggered pre-authentication when a client connects to a malicious server. A fix is available in commit 42e33d8.
Affected products
- libssh2 libssh2 through 1.11.1
Timeline
- 2026-07-03: patched: Fix merged in pull request 2198
- 2026-07-24: disclosed: CVE-2026-66035 published