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Fencer Vulns
medium 5.9

CVE-2026-55199 json

libssh2 - Pre-Authentication DoS via SSH_MSG_EXT_INFO Handler

published 2026-06-17 4 mo ago modified 2026-11-08 20 h ago also EUVD-2026-37782 , GHSA-3cfq-4xx4-rmpg , JLSEC-2026-660

cvss
5.9
version 3.1, CNA
cisa kev
Not listed
not in CISA's catalog
epss
0.9%
EPSS 0.9% chance of exploitation in 30 days, percentile 60%
public exploits
0
none known

Summary

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.

Patch / fix

  • julia LibSSH2_jll 1.11.102+0
  • libssh2 Debian 13 1.11.1-1+deb13u1
  • libssh2 Debian 14 1.11.1-4

debian advisories DEBIAN-CVE-2026-55199

  • libssh2 Ubuntu 24.04 LTS 1.11.0-4.1ubuntu0.24.04.2
  • libssh2 Ubuntu 25.10 1.11.1-1ubuntu0.25.10.2
  • libssh2 Ubuntu 26.04 LTS 1.11.1-1ubuntu0.26.04.2

ubuntu advisories UBUNTU-CVE-2026-55199

Ubuntu security data © Canonical Ltd., from https://github.com/canonical/ubuntu-security-notices via OSV; changed: normalised to fixed package versions per release. CC BY-SA 4.0

  • libssh2 hummingbird 1 1.11.1-8.hum1

red hat advisories RHSA-2026:29950

  • libssh2_org Linux Micro 6.1 1.11.0-slfo.1.1_2.1
  • libssh2_org SLES 15 SP4-LTSS, SLE High Performance Computing 15 SP4-LTSS, SLE High Performance Computing 15 SP4-ESPOS 1.11.0-150000.4.32.1
  • libssh2_org SLES for SAP 15 SP6, SLES 15 SP6-LTSS, SLE Module for Basesystem 15 SP7 1.11.0-150600.20.3.1
  • libssh2_org SLES for SAP 15 SP5, SLES for SAP 15 SP4, SLES 15 SP5-LTSS +8 more 1.11.0-150200.9.5.1
  • libssh2_org Linux Micro 6.0 1.11.0-3.1
  • libssh2_org SLES 16.0, Linux Micro 6.2, SLES for SAP applications 16.0 1.11.1-160000.4.1
  • libssh2_org SLES LTSS Extended Security 12 SP5, SLES 12 SP5-LTSS 1.11.0-29.18.1

suse advisories SUSE-SU-2026:23187-1SUSE-SU-2026:3525-1SUSE-SU-2026:3082-1SUSE-SU-2026:3076-1SUSE-SU-2026:3074-1SUSE-SU-2026:22438-1

  • libssh2_org openSUSE Leap 16.0 1.11.1-160000.4.1
  • libssh2_org openSUSE Tumbleweed 1.11.1-3.1

opensuse advisories openSUSE-SU-2026:21057-1openSUSE-SU-2026:11109-1

  • rust Alpine 3.24 1.96.1-r0
  • libssh2 Alpine 3.22, Alpine 3.21 1.11.1-r2
  • libssh2 Alpine 3.24, Alpine 3.23 1.11.1-r3
  • chromium-rust Alpine 3.24 1.96.1-r0

alpine advisories ALPINE-CVE-2026-55199

Alpine secdb © Alpine Linux, from https://secdb.alpinelinux.org via OSV; changed: normalised to fixed package versions per release. CC BY-SA 4.0

  • libssh2 Azure Linux 3.0 1.11.1-4

azure linux advisories AZL-90933

Azure Linux vulnerability data, Copyright (c) Microsoft Corporation, from https://github.com/microsoft/AzureLinuxVulnerabilityData via OSV, under the MIT licence; changed: normalised to fixed package versions per release. MIT

Vendor advisories

What the vendors themselves say about their products: affected, fixed or not affected, and what to do.

red hat product security vex CVE-2026-55199 updated cvss 5.9

libssh2: libssh2: Denial of Service via crafted SSH_MSG_EXT_INFO message

  • Red Hat Enterprise Linux 6 libssh2 Red Hat Enterprise Linux 7 libssh2
    affected: workaround, no fix planned “Out of support scope”
  • Red Hat Hardened Images libssh2 1.11.1-8.hum1
    fixed: vendor fix RHSA-2026:29950

Red Hat CSAF VEX © Red Hat, Inc., from https://security.access.redhat.com/data/csaf/v2/vex/; changed: reduced to product statuses and remediations. CC BY 4.0 · csaf

CISA SSVC

Exploitation
none
Automatable
no
Technical impact
partial
Assessed
4 mo ago

Exploit prediction

0.92% chance of exploitation in the next 30 days, higher than 60% of all scored CVEs (FIRST EPSS, 2026-11-08).

CVSS scores

assigned by version score vector
CNA 4.0 8.2 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
CNA 3.1 5.9 CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H
CNA 4.0 8.2 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/E:X/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/MVI:X/MVA:X/MSC:X/MSI:X/MSA:X/S:X/AU:X/R:X/V:X/RE:X/U:X
NVD 3.1 7.5 CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
OSV 3.1 7.5 CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H

Affected packages

ecosystem package affected fixed in
julia LibSSH2_jll
<1.11.102+0
1.11.102+0

Affected products

  • libssh2 libssh2

Classification

Weakness
  • CWE-835 Loop with Unreachable Exit Condition

References

Sources

Fencer Vulns keeps one entry per vulnerability: the CVE id wins, then GHSA, and records that alias each other are merged, except that two CVEs always keep separate entries. The same data is available as JSON.