# CVE-2026-55199: libssh2 - Pre-Authentication DoS via SSH_MSG_EXT_INFO Handler

CVE-2026-55199 is fixed in 7 products on 1 platform, first by libssh2-1t64 1.11.1-1ubuntu0.26.04.2, released June 18, 2026. It is not reported as exploited. Patch Radar priority: High.

- Web page: https://patchblox.com/security/cve/CVE-2026-55199
- Priority: High
- CVSS: 7.5 (Important)
- EPSS: 0.0092
- CISA KEV: not listed
- Exploited (vendor report): no
- Publicly disclosed: no
- Published: Jun 17, 2026
- Weakness: Loop with Unreachable Exit Condition ('Infinite Loop')
- Vendor advisory: https://msrc.microsoft.com/update-guide/vulnerability/CVE-2026-55199

## Description

libssh2 through 1.11.1, fixed in commit 1762685, contains a pre-authentication denial of service vulnerability in the SSH_MSG_EXT_INFO handler in src/packet.c that allows a malicious SSH server to cause a client CPU exhaustion loop by sending a crafted extension count value. A malicious server can set nr_extensions to 0xFFFFFFFF during key exchange, causing the client to spin in a tight CPU loop for over 60 seconds because return values from _libssh2_get_string() are unchecked and the session timeout does not apply to CPU-bound loops.

## Where it's fixed

| Platform | Product | Fixed in | Fix | Released |
|---|---|---|---|---|
| Linux | libssh2 Debian 13.5 (trixie) | 1.11.1-1+deb13u1 | [DEBIAN-CVE-2026-55199](https://patchblox.com/security/data/fix/linux/debian-cve-2026-55199.md) | Jun 26, 2026 |
| Linux | libssh2 Debian 14 | 1.11.1-4 | [DEBIAN-CVE-2026-55199](https://patchblox.com/security/data/fix/linux/debian-cve-2026-55199.md) | Jun 26, 2026 |
| Linux | libssh2 Fedora 43 | 1.11.1-9.fc43 | [FEDORA-2026-EED9E67393](https://patchblox.com/security/data/fix/linux/fedora-2026-eed9e67393.md) | Jun 24, 2026 |
| Linux | libssh2-1t64 Ubuntu 24.04.5 LTS (noble) | 1.11.0-4.1ubuntu0.24.04.2 | [libssh2-1t64 1.11.0-4.1ubuntu0.24.04.2](https://patchblox.com/security/data/fix/linux/ubuntu-noble-libssh2-1t64-1.11.0-4.1ubuntu0.24.04.2.md) | Jul 3, 2026 |
| Linux | libssh2-1t64 Ubuntu 25.10 (questing) | 1.11.1-1ubuntu0.25.10.2 | [libssh2-1t64 1.11.1-1ubuntu0.25.10.2](https://patchblox.com/security/data/fix/linux/ubuntu-questing-libssh2-1t64-1.11.1-1ubuntu0.25.10.2.md) | Jul 3, 2026 |
| Linux | libssh2-1t64 Ubuntu 26.04.1 LTS (resolute) | 1.11.1-1ubuntu0.26.04.2 | [libssh2-1t64 1.11.1-1ubuntu0.26.04.2](https://patchblox.com/security/data/fix/linux/ubuntu-resolute-libssh2-1t64-1.11.1-1ubuntu0.26.04.2.md) | Jun 18, 2026 |
| Linux | libssh2-debuginfo Fedora 43 | 1.11.1-9.fc43 | [FEDORA-2026-EED9E67393](https://patchblox.com/security/data/fix/linux/fedora-2026-eed9e67393.md) | Jun 24, 2026 |
| Linux | libssh2-debugsource Fedora 43 | 1.11.1-9.fc43 | [FEDORA-2026-EED9E67393](https://patchblox.com/security/data/fix/linux/fedora-2026-eed9e67393.md) | Jun 24, 2026 |
| Linux | libssh2-devel Fedora 43 | 1.11.1-9.fc43 | [FEDORA-2026-EED9E67393](https://patchblox.com/security/data/fix/linux/fedora-2026-eed9e67393.md) | Jun 24, 2026 |
| Linux | libssh2-docs Fedora 43 | 1.11.1-9.fc43 | [FEDORA-2026-EED9E67393](https://patchblox.com/security/data/fix/linux/fedora-2026-eed9e67393.md) | Jun 24, 2026 |
| Linux | libssh2_org opensuse-leap 16.0 | 1.11.1-160000.4.1 | [OPENSUSE-SU-2026:21057-1](https://patchblox.com/security/data/fix/linux/opensuse-su-2026-21057-1.md) | Jun 25, 2026 |
| Linux | libssh2_org sles 15.6 | 1.11.0-150600.20.3.1 | [SUSE-SU-2026:3076-1](https://patchblox.com/security/data/fix/linux/suse-su-2026-3076-1.md) | Jul 16, 2026 |
| Linux | libssh2_org sles 15.7 | 1.11.0-150600.20.3.1 | [SUSE-SU-2026:3076-1](https://patchblox.com/security/data/fix/linux/suse-su-2026-3076-1.md) | Jul 16, 2026 |
| Linux | libssh2_org sles 16.0 | 1.11.1-160000.4.1 | [SUSE-SU-2026:22364-1](https://patchblox.com/security/data/fix/linux/suse-su-2026-22364-1.md) | Jun 25, 2026 |

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Data as of 2026-11-08T06:59:42Z UTC. Source: Patchblox Patch Radar (https://patchblox.com/security/patch-radar). Patchblox finds out-of-date software on Windows, macOS and Linux and updates it through release rings: https://patchblox.com/demo

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