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CVE-2026-66034 — libssh2

Fix: libssh2/libssh2@a13bb6c

CVE-2026-66034 is a Out-of-bounds Read vulnerability. A fix is available — see the affected versions and patch details below.

libssh2 Heap Out-of-Bounds Read via publickey subsystem

Published
Updated
Affected
1 product
Patched
See advisory
Exploits
None indexed
Exploitation data as of Oct 22, 2026 · OSV.dev, NVD, FIRST.org (EPSS)

Exploitation Status

No confirmed exploitation observed yet

  • A successful exploit gives an attacker total control of the affected component, not partial access.
  • CISA’s own triage has not observed active exploitation or public proof-of-concept code for this CVE as of its last assessment.

Exploitation and automatability from CISA’s SSVC triage for CVE-2026-66034.

EPSS Exploitation Probability

via FIRST.org ↗
0.4%probability of exploitation in next 30 days
Lower Risk0.00%
Lower risk than most CVEs33th percentile — riskier than 33% of all scored CVEsHighest risk
0.00%0.31%0.61%0.92%0.3%0.4%0.4%0.4%Aug 26Oct 26Oct 26

Probability of exploitation in the next 30 days, from FIRST.org EPSS.

Description

libssh2 through 1.11.1, fixed in commit a13bb6c, contains a missing bounds check vulnerability that allows a malicious SSH server to trigger an arbitrary-length heap out-of-bounds read and a free of an uninitialized pointer via the publickey subsystem. In libssh2_publickey_list_fetch(), the version 1 response parser reads a server-controlled comment_len value and advances the parse pointer without verifying sufficient bytes remain in the buffer, causing the out-of-bounds read to leak heap pointers from adjacent allocations defeating ASLR, followed by heap allocator state corruption when the error cleanup path frees an uninitialized pointer from a non-zeroed realloc() region.

Affected Products

1 product · 1 configurations
Application
libssh2libssh2
≤ 1.11.1
range

Detection & mitigation playbook

Vulnerability

  1. Detect

    Identify every host running the affected component and compare the installed build against the fixed version below — for source-built or distro-packaged software the version string, not a lockfile, is the source of truth (`dpkg -l`, `rpm -q`, or the binary's own `--version`).

  2. Fix

    Upgrade the affected component to the fixed release for CVE-2026-66034, or apply your distribution's backported patch — distro builds are often patched at an older version number, so check your vendor's advisory rather than the upstream version alone.

  3. Workarounds

    Stop feeding it untrusted input: reject or quarantine files and payloads from unverified sources until you can upgrade, restrict accepted formats to the ones you actually need, and run the parsing or decoding step in a least-privileged sandbox or short-lived worker so a crash or corrupted read cannot reach the rest of the process.

Fixing This On Your OS

If you run this on a Linux distribution, patch through your package manager against the distro's own security advisory below — it tracks the exact backported fix for your release, which can ship on a different timeline (and sometimes a different severity) than the upstream project.

Red HatModerate

This Moderate impact flaw in libssh2 allows a malicious SSH server to trigger an out-of-bounds read on a connecting client. While requiring user interaction to connect to a compromised server and having high attack complexity, successful exploitation could leak heap pointers, potentially aiding in ASLR bypass and…

Workaround published by Red Hat
To mitigate this, strictly avoid connecting to untrusted SSH servers and enforce this policy using outbound network firewalls to block unknown IP addresses. For defense-in-depth, utilize OS-level memory protections and service sandboxing to contain any accidental exposure.
Source: Red Hat security advisory for CVE-2026-66034 (CC BY 4.0)
ProductFixed inAdvisory
Red Hat Enterprise Linux 7 Extended Lifecycle Supportlibssh2-0:1.8.0-4.el7_9.2RHSA-2026:61752
Red Hat Hardened Imageslibssh2-main-1.11.1-10.2.hum1RHSA-2026:46927
UbuntuHIGH
USN-8799-1Ubuntu

Frequently Asked Questions

libssh2 through 1.11.1, fixed in commit a13bb6c, contains a missing bounds check vulnerability that allows a malicious SSH server to trigger an arbitrary-length heap out-of-bounds read and a free of an uninitialized pointer via the publickey subsystem. In libssh2_publickey_list_fetch(), the version 1 response parser reads a server-controlled comment_len value and advances the parse pointer without verifying sufficient bytes remain in the buffer, causing the out-of-bounds read to leak heap pointers from adjacent allocations defeating ASLR, followed by heap allocator state corruption when the er
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CVE-2026-66034: libssh2 Memory Corruption