# CVE-2026-66034: libssh2 heap out-of-bounds read in publickey_list_fetch

Source: https://junglewise.ai/threats/cve-2026-66034-libssh2-heap-out-of-bounds-read-in-publickey-list-fetch  
Publisher: Junglewise Threat Intelligence, https://junglewise.ai/threats  
Published: 2026-07-24. Updated: 2026-07-24  
License: CC BY 4.0 with a link back

CVE-2026-66034 · Severity: high · CVSS 7.5 · Published 2026-07-24

libssh2 is a widely used library that allows applications to communicate securely using the SSH protocol. A vulnerability in how the library handles public key lists allows a malicious SSH server to crash an application or potentially steal sensitive information from its memory. This could lead to a loss of data confidentiality or a complete service outage for software relying on this library.

Technologies: [Libssh2](https://junglewise.ai/threats/technologies/libssh2). Vendors: [Libssh2](https://junglewise.ai/threats/vendors/libssh2).

## Quick answers

**Is CVE-2026-66034 exploited in the wild?** No. Junglewise Threat Intelligence has no report of CVE-2026-66034 being exploited in the wild.

**How severe is CVE-2026-66034?** High severity, with a CVSS score of 7.5, published 24 July 2026.

**Which products and versions are affected?** Libssh2 through 1.11.1.

**Is there a fix for CVE-2026-66034?** A patch was published on 2026-07-04: Fix submitted via GitHub pull request 2202 Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.

**How is CVE-2026-66034 attacked?** It is attacked over the network, in a software library.

**Am I affected by CVE-2026-66034?** Check whether you run Libssh2 at an affected version. Junglewise does that check for you around the clock: connect your code, cloud and domains, and it matches new vulnerabilities like this one against what you actually run and tells you when one reaches you.

## Technical details

A vulnerability exists in the libssh2_publickey_list_fetch() function within the publickey subsystem. The version 1 response parser fails to validate a server-controlled 'comment_len' value against the remaining buffer size before advancing the parse pointer. This leads to an arbitrary-length heap out-of-bounds read, which can be used to leak heap pointers and bypass ASLR. Additionally, the error cleanup path may attempt to free an uninitialized pointer from a non-zeroed realloc() region, resulting in heap corruption. Exploitation requires a user to connect to a malicious SSH server. The issue is fixed in commit a13bb6c.

## Affected products

- libssh2 libssh2 through 1.11.1

## Timeline

- 2026-07-04: patched: Fix submitted via GitHub pull request 2202
- 2026-07-24: disclosed: CVE-2026-66034 published

## References

- [https://github.com/libssh2/libssh2/commit/a13bb6c773f0d55ad1628cede57e99803cd898d9](https://github.com/libssh2/libssh2/commit/a13bb6c773f0d55ad1628cede57e99803cd898d9)
- [https://github.com/libssh2/libssh2/pull/2202](https://github.com/libssh2/libssh2/pull/2202)
- [https://www.vulncheck.com/advisories/libssh2-heap-out-of-bounds-read-via-publickey-subsystem](https://www.vulncheck.com/advisories/libssh2-heap-out-of-bounds-read-via-publickey-subsystem)

## Related threats

- [libssh2 heap buffer overflow in transport layer during EtM negotiation](https://junglewise.ai/threats/cve-2026-66035-libssh2-heap-buffer-overflow-in-transport-layer-during-etm)
- [libssh2 integer underflow in ssh2_cipher_crypt](https://junglewise.ai/threats/cve-2026-66033-libssh2-integer-underflow-in-ssh2-cipher-crypt)
- [libssh2 double free in sftp_open](https://junglewise.ai/threats/cve-2026-66032-libssh2-double-free-in-sftp-open)
- [libssh2 uninitialized pointer free in publickey list cleanup](https://junglewise.ai/threats/cve-2026-58051-libssh2-uninitialized-pointer-free-in-publickey-list-cleanup)
- [libssh2 heap buffer overflow in publickey-subsystem attribute allocation](https://junglewise.ai/threats/cve-2026-58050-libssh2-heap-buffer-overflow-in-publickey-subsystem-attribute)