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No exploitation reported

CVE-2026-55199

No exploitation reported by the sources we read.

  • Published2026-06-17
  • WeaknessCWE-835

Am I affected?

With a domain, we read the version its public headers and pages expose. A distribution build can carry the fix under an older number.

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.

Timeline

dates in UTC
  1. 2026-06-1720:17CVE publishedNVD

Observed activity

isMalicious corpus · updated 2026-10-23 15:37 UTC
Indicators tagged with this CVE, per day0 indicators over 7 days
Block these indicators
  • Firewall list (EDL, text): https://api.ismalicious.com/edl/cve/CVE-2026-55199?format=txt
  • CSV export: https://api.ismalicious.com/edl/cve/CVE-2026-55199?format=csv
  • TAXII collection of exploited CVEs
  • The EDL and CSV take an API key on the Pro plan.

Versions per branch

NVD
Libssh2
BranchAffectedFixed in
1.11≤ 1.11.1no fix published

as publishedRanges as NVD and OSV publish them; a distribution’s build can be fixed under an older number.

Distribution builds and vendor advisories

Packages from OSV.dev and products from the vendors' CSAF advisories.

version not verifiedVersions not verified: read as OSV.dev and the vendors publish them, not checked against each vendor's release list.

Vendor advisories

  • CVE-2026-55199 Red Hat Product Security4 products fixed · 8 known affected

    libssh2: libssh2: Denial of Service via crafted SSH_MSG_EXT_INFO message

    libssh2 as a component of Red Hat Enterprise Linux 6 · libssh2-devel as a component of Red Hat Enterprise Linux 6 · libssh2-docs as a component of Red Hat Enterprise Linux 6 · +5 more

  • RHSA-2026:29950 Red Hat Product Security7 products fixed

    Red Hat Security Advisory: Red Hat Hardened Images RPMs bug fix and enhancement update

Full record on NVD

Fix

  1. Vendor fix Red Hat Product Security RHSA-2026:29950 · 7 productsFor details on how to apply this update, which includes the changes described in this advisory, refer to: https://images.redhat.com/
  2. Workaround from the vendor Red Hat Product Security RHSA-2026:29950 · 7 productsTo mitigate this issue, ensure your libssh2 clients only connect to trusted SSH servers. You can enforce this by implementing outbound network access controls (egress filtering) to block applications from initiating connections to unknown or untrusted external hosts.
  3. No fix planned, says the vendor Red Hat Product Security CVE-2026-55199 · 8 productsOut of support scope

Advisories and patches

Detection

Nuclei template
none so far
ProjectDiscovery
Exploit-DB
no entry
Exploit-DB
Proof-of-concept repositories
none listed
PoC-in-GitHub

Sources

Technical details

CVSS v3 Vector
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
CVSS 4.0 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/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
Published
2026-06-17
Last Modified
2026-07-14
NVD status
Analyzed
MSRC Title
libssh2 - Pre-Authentication DoS via SSH_MSG_EXT_INFO Handler

Frequently asked questions

What is CVE-2026-55199?

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.

Is CVE-2026-55199 actively exploited?

Active exploitation of CVE-2026-55199 has not been confirmed. Its EPSS score was 0.9% on 2026-10-22, the estimated probability of exploitation in the next 30 days.

What is the CVSS score for CVE-2026-55199?

CVE-2026-55199 has a CVSS v3 base score of 7.5 (HIGH severity), with vector string CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H.

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