libssh2 up to 1.11.1 Encrypt-then-MAC Cipher Negotiation src/transport.c fullpacket heap-based overflow

Summaryinfo

A vulnerability has been found in libssh2 up to 1.11.1 and classified as critical. Affected by this vulnerability is the function fullpacket of the file src/transport.c of the component Encrypt-then-MAC Cipher Negotiation. This manipulation causes heap-based overflow. This vulnerability is tracked as CVE-2026-66035. The attack is possible to be carried out remotely. No exploit exists.

Detailsinfo

A vulnerability classified as critical was found in libssh2 up to 1.11.1. Affected by this vulnerability is the function fullpacket of the file src/transport.c of the component Encrypt-then-MAC Cipher Negotiation. The manipulation with an unknown input leads to a heap-based overflow vulnerability. The CWE definition for the vulnerability is CWE-122. A 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(). As an impact it is known to affect confidentiality, integrity, and availability. The summary by CVE is:

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.

It is possible to read the advisory at github.com. This vulnerability is known as CVE-2026-66035 since 07/23/2026. The exploitation appears to be easy. The attack can be launched remotely. The exploitation doesn't need any form of authentication. Technical details of the vulnerability are known, but there is no available exploit.

Applying the patch 42e33d81577ed4b95d4b4f6f845e5ee8efe5eeb4 is able to eliminate this problem.

Statistical analysis made it clear that VulDB provides the best quality for vulnerability data.

Productinfo

Name

• libssh2

Version

• 1.11.0
• 1.11.1

Website

• Product: https://github.com/libssh2/libssh2/

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