CVE-2026-66035: libssh2 Heap-based Buffer Overflow

Vulnerability summary

CVE-2026-66035 is a high (CVSS 7.7) Heap-based Buffer Overflow vulnerability in libssh2. EPSS estimates a 0.47% chance of exploitation in the next 30 days, higher than 39.4% of scored vulnerabilities.

Timeline

1. 24-07-2026
   PublishedVulnerability first disclosed.
2. 28-07-2026
   Last modifiedVulnerability information updated.

Description

libssh2 through 1.11.1, fixed in commit 42e33d8, contains a pre-authentication heap buffer overflow vulnerability that allows a malicious SSH server to corrupt heap metadata in any connecting client by sending a packet with a packet_length smaller than the cipher's block size during Encrypt-then-MAC cipher negotiation. In the fullpacket() function in src/transport.c, the ETM path allocates a buffer of packet_length bytes but copies blocksize minus one bytes via memcpy, causing an overflow that on 32-bit glibc writes attacker-controlled bytes into an adjacent chunk's SIZE field, enabling tcache bin confusion, overlapping live objects, and function pointer overwrite during the session handshake before authentication.

• CWE-122Heap-based Buffer OverflowA heap overflow condition is a buffer overflow, where the buffer that can be overwritten is allocated in the heap portion of memory, generally meaning that the buffer was allocated using a routine such as malloc().