Vulnerability 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.
CVSS Score
HIGH
Affected Products
| Vendor | Product | Versions |
|---|---|---|
| Libssh2 | Libssh2 | <= 1.11.1 |
Related Weaknesses (CWE)
References
- https://github.com/libssh2/libssh2/commit/42e33d81577ed4b95d4b4f6f845e5ee8efe5eePatch
- https://github.com/libssh2/libssh2/pull/2198Issue TrackingPatch
- https://www.vulncheck.com/advisories/libssh2-heap-buffer-overflow-via-etm-cipherPatchThird Party Advisory
FAQ
What is CVE-2026-66035?
CVE-2026-66035 is a vulnerability with a CVSS score of 7.5 (HIGH). 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 b...
How severe is CVE-2026-66035?
CVE-2026-66035 has been rated HIGH with a CVSS base score of 7.5/10. Review the CVSS metrics above for detailed severity breakdown.
Is there a patch for CVE-2026-66035?
Check the references section above for vendor advisories and patch information. Affected products include: Libssh2 Libssh2.