CVE-2026-55200

libssh2 through 1.11.1, fixed in commit 7acf3df contains an out-of-bounds write vulnerability in ssh2_transport_read() that fails to enforce upper bounds on packet_length field. Remote attackers can send crafted SSH packets with excessively large packet_length values to corrupt heap memory and achieve remote code execution.

What this means for your business

It affects Libssh2. It matters if your company, or a supplier that handles your data, runs it.
Code showing how to exploit it has been published, but real attacks have not been confirmed.
An attacker can use it remotely, over a network, without a login, and without anyone at your company clicking anything.
FIRST's prediction model gives it a 4% chance of attack attempts being seen in the next 30 days, ranking above 90% of all known flaws.

What to do

1Check whether your company or your suppliers run Libssh2, and which version. The affected versions are listed further down this page.
2If you do, apply the vendor's fix. A patch or vendor advisory has been published.

Not sure if your company is exposed?

Fastnexa’s certified penetration testers can check whether attackers could use this flaw, or others like it, against your websites, apps and network. The full test is free for our first 10 founding clients until 31 December 2026.

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Scoring

CVSS: 8.1 (v3.1)
Vector: CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:H
CVSS v4: 9.2
Weakness: CWE-680
Assigned by: disclosure@vulncheck.com

Dates

Published: 2026-06-17
Last modified: 2026-09-17
Sources: NVD

Affected products

Libssh2 Libssh21.11.1

As listed in the NVD configuration data. Not a statement about your estate.