CVE-2026-66035

What it 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.

At a glance

Exploitability

Can be exploited over the network without logging in, but needs a user to act. High risk of data exposure, data tampering, service disruption.

Exploit odds

0.5% chance of exploitation in the next 30 days (EPSS).

Fix

A patch, advisory or fixed version is referenced. Upgrade or apply it.

Timeline

1. 2026-07-24 17:17 UTC PublishedDisclosed publicly on CVE.org and NVD
2. 2026-07-30 15:41 UTC Last updatedMost recent change to the record

Details

Published

2026-07-24 17:17 UTC

Last modified

2026-07-30 15:41 UTC

Exploit probability

0.5% · EPSS, next 30 days

Weakness

CWE-122

Record status

Analyzed

Affected

• libssh2 ≤ 1.11.1

Attack profile CVSS 4.0 · 7.7

Attack vector

Network

Complexity

High

Attack requirements

None

Privileges needed

None

User interaction

Passive

Confidentiality

High

Integrity

High

Availability

High