CVE-2026-55199Network attack vectorHinoki check available

Client hang via unchecked SSH extension parsing

Published Jun 17, 2026 · Updated Jul 14, 2026

DoS in libssh2 1.11.1 and earlier allows remote attackers to hang clients through crafted SSH_MSG_EXT_INFO messages. The handler in src/packet.c ignores _libssh2_get_string() failures and continues decrementing a server-supplied extension count after the packet buffer is exhausted. A client must connect to a malicious SSH server; before authentication, an oversized count causes CPU exhaustion beyond the session timeout, without reported data access or modification.

CVSS severity8.2
High
EPSS probability0.92%
Next 30 days · Oct 19, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkAvailable
Coverage for this vulnerability

See if you're affected

Explore vulnerability checks for your environment with Hinoki.

Get started

Summary

What happened

DoS in libssh2 1.11.1 and earlier allows remote attackers to hang clients through crafted SSH_MSG_EXT_INFO messages. The handler in src/packet.c ignores _libssh2_get_string() failures and continues decrementing a server-supplied extension count after the packet buffer is exhausted. A client must connect to a malicious SSH server; before authentication, an oversized count causes CPU exhaustion beyond the session timeout, without reported data access or modification.

The record

CVE
CVE-2026-55199
Published
Jun 17, 2026
Updated
Jul 14, 2026
Vendor
The libssh2 Project
Product
libssh2
Classifications
CWE-835, T1499.004
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. Jun 17, 2026CVE publishedPublication date reported by the CVE source.
  2. Jul 14, 2026Record updatedLatest update available in the CVE record.

Exploitability

Present is not the same as exploitable

Compare your product and version with the public record. A matching version still requires validation against your environment.

Is a vulnerable build present?

Compare these published version ranges with your installed build and any vendor patches.

  1. Affected versionversion=0 <=1.11.1

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

The libssh2 Project · libssh2version=0 <=1.11.1

Published CVSS scores

8.2VulnCheckCVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N
5.9VulnCheckCVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H
5.9SUSECVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H
7.5NVDCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
5.9Amazon LinuxCVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H

CVSS describes severity. EPSS estimates exploitation probability.

Attacks

What attackers are doing with it

Daily unique IPs observed by Shadowserver honeypots for known exploited vulnerabilities (KEVs). Missing observations do not establish an absence of attacks.

Activity data is delayed. The latest report is Sep 28, 2026.

Daily unique IPsNo honeypot observations are available for this CVE in the selected window.

No observations available

Sep 22, 2026Sep 28, 2026
Latest reporting daySep 28, 2026
Latest daily unique IPsUnavailable
Prior 30-day averageUnavailable
SourceShadowserver honeypots (KEV)
Vectornetwork
Privilegesunauthenticated
Known exploitationUnconfirmed
Public exploitUnconfirmed

Weakness, pattern, technique

CWE-835Loop with Unreachable Exit Condition ('Infinite Loop')
T1499.004Application or System Exploitation

Public exploit references

No public exploit references are available in this record.

Labels summarize the accepted research assessment. They do not indicate a test against your environment.

Technologies

Your stack

See the directory against your own environment.

Your stack

Check the software in your environment

Get started to see how Hinoki identifies affected software and validates exploitability in your environment.

Get started