CVE-2026-66035

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 v3 7.5 HIGH

7.5^/10

CVSS v3 : HIGH

Vector :

Exploitability : 1.6 / Impact : 5.9

Attack Vector NETWORK

Attack Complexity HIGH

Privileges Required NONE

User Interaction REQUIRED

Confidentiality Impact HIGH

Integrity Impact HIGH

Availability Impact HIGH

Scope UNCHANGED

References

Link	Resource
https://github.com/libssh2/libssh2/commit/42e33d81577ed4b95d4b4f6f845e5ee8efe5eeb4	Patch
https://github.com/libssh2/libssh2/pull/2198	Issue Tracking Patch
https://www.vulncheck.com/advisories/libssh2-heap-buffer-overflow-via-etm-cipher-negotiation	Patch Third Party Advisory

Configurations

Configuration 1 (hide)

cpe:2.3:a:libssh2:libssh2:*:*:*:*:*:*:*:*

History

No history.

Information

Published : 2026-07-24 17:17

Updated : 2026-07-30 15:41

NVD link : CVE-2026-66035

Mitre link : CVE-2026-66035

CVE.ORG link : CVE-2026-66035

JSON object : View

Products Affected

libssh2

libssh2

CWE

CWE-122

Heap-based Buffer Overflow

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