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CVE-2026-66035 — libssh2 Heap Buffer Overflow via ETM Cipher Negotiation

2026-10-07T04:18:00.090505+00:00

Affected: libssh2

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.

USN-8722-1 — libssh2 vulnerabilities

2026-10-07T04:18:00.090563+00:00

Affected: Ubuntu:24.04:LTS: libssh2, Ubuntu:26.04:LTS: libssh2

It was discovered that libssh2 incorrectly handled certain SFTP server responses. A remote attacker controlling an SSH server could use this issue to cause libssh2 to crash or possibly execute arbitrary code. (CVE-2026-66032)

It was discovered that libssh2 incorrectly handled AES-GCM cipher negotiation. A remote attacker controlling an SSH server could possibly use this issue to cause libssh2 to crash, resulting in a denial of service. (CVE-2026-66033)

It was discovered that libssh2 incorrectly handled Encrypt-then-MAC cipher negotiation. A remote attacker controlling an SSH server could use this issue to cause libssh2 to crash or possibly execute arbitrary code. (CVE-2026-66035)