CVE-2026-55199

Publication date 17 June 2026

Last updated 4 September 2026

Ubuntu priority

Medium

Why this priority?

Cvss 3 Severity Score

5.9 · Medium

Score breakdown

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.

Read the notes from the security team

Status

Package Ubuntu Release Status
libssh2 26.04 LTS resolute
Fixed 1.11.1-1ubuntu0.26.04.2
25.10 questing
Fixed 1.11.1-1ubuntu0.25.10.2
24.04 LTS noble
Fixed 1.11.0-4.1ubuntu0.24.04.2
22.04 LTS jammy
Not affected
20.04 LTS focal
Not affected
18.04 LTS bionic
Not affected
16.04 LTS xenial
Not affected
14.04 LTS trusty
Not affected

Notes

leosilva

trusty (1.4.3), xenial (1.5.0), bionic (1.8.0), focal (1.8.0), jammy (1.10.0), esm-apps/jammy (1.10.0): not-affected because the SSH_MSG_EXT_INFO handler in src/packet.c was introduced in libssh2 1.11.0; releases shipping versions < 1.11.0 do not contain the vulnerable code.

Patch details

For informational purposes only. We recommend not to cherry-pick updates. How can I get the fixes?

Package Patch details
libssh2

Severity score breakdown

CVSS version:

Base score 8.2 · High

Vector: CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N

Base score 5.9 · Medium

Vector: CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H

References

Related Ubuntu Security Notices (USN)

Other references

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