# CVE-2026-55199: libssh2 CPU exhaustion in SSH_MSG_EXT_INFO handler

Source: https://junglewise.ai/threats/cve-2026-55199-libssh2-cpu-exhaustion-in-ssh-msg-ext-info-handler  
Publisher: Junglewise Threat Intelligence, https://junglewise.ai/threats  
Published: 2026-06-17. Updated: 2026-06-20  
License: CC BY 4.0 with a link back

CVE-2026-55199 · Severity: medium · CVSS 5.9 · Published 2026-06-17

libssh2 is a widely used library that allows applications to communicate securely using the SSH protocol. A vulnerability in how the library handles initial connection messages allows a malicious server to force a client application into an unresponsive state. This results in a denial of service where the client consumes excessive processor power for over a minute, potentially crashing the application or disrupting automated tasks.

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

## Quick answers

**Is CVE-2026-55199 exploited in the wild?** No. Junglewise Threat Intelligence has no report of CVE-2026-55199 being exploited in the wild.

**How severe is CVE-2026-55199?** Medium severity, with a CVSS score of 5.9, published 17 June 2026.

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

**Is there a fix for CVE-2026-55199?** A patch was published on 2026-04-21: Fix merged into master branch via commit 1762685 Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.

**How is CVE-2026-55199 attacked?** It is attacked over the network, in a software library.

**Am I affected by CVE-2026-55199?** 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 denial of service vulnerability exists in the SSH_MSG_EXT_INFO handler within src/packet.c of libssh2. The root cause is a failure to check the return values of _libssh2_get_string() when parsing extension name/value pairs. An attacker-controlled SSH server can send a crafted extension count (nr_extensions) of 0xFFFFFFFF; if the buffer is exhausted, the client enters a tight CPU loop performing no-op iterations. Because session timeouts do not apply to these CPU-bound loops, the client remains unresponsive for a significant duration. The issue is fixed in commit 1762685 by ensuring the loop breaks upon parsing failures.

## Affected products

- libssh2 libssh2 through 1.11.1

## Timeline

- 2026-04-15: disclosed: Initial pull request submitted by researcher
- 2026-04-21: patched: Fix merged into master branch via commit 1762685
- 2026-06-17: advisory: CVE-2026-55199 published

## References

- [https://github.com/libssh2/libssh2/commit/17626857d20b3c9a1addfa45979dadcee1cd84a4](https://github.com/libssh2/libssh2/commit/17626857d20b3c9a1addfa45979dadcee1cd84a4)
- [https://github.com/libssh2/libssh2/pull/1864](https://github.com/libssh2/libssh2/pull/1864)
- [https://www.vulncheck.com/advisories/libssh2-pre-authentication-dos-via-ssh-msg-ext-info-handler](https://www.vulncheck.com/advisories/libssh2-pre-authentication-dos-via-ssh-msg-ext-info-handler)

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