What’s Vulnerable › Vulnerabilities

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

• Severity: high (CVSS 7.5 v3.1)
• Exploited in the wild: not listed in CISA KEV
• EPSS: 0.5% probability of exploitation in the next 30 days
• Public exploit or detection template: none known
• Published: 2026-07-24 · updated 2026-07-30
• Weakness: CWE-122

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.

Affected products

• libssh2: through 1.11.1

References

• https://github.com/libssh2/libssh2/commit/42e33d81577ed4b95d4b4f6f845e5ee8efe5eeb4
• https://github.com/libssh2/libssh2/pull/2198
• https://www.vulncheck.com/advisories/libssh2-heap-buffer-overflow-via-etm-cipher-negotiation

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