CVE-2026-55199: libssh2 - Pre-Authentication DoS via SSH_MSG_EXT_INFO Handler

mediumCVSS 5.9CVE-2026-55199

CSIRTS triage

vendor: libssh2 product: libssh2 Denial of service

What: There is a pre-authentication denial of service vulnerability via the SSH_MSG_EXT_INFO handler.
Who is affected: Deployments of libssh2 are affected.
Urgency: Remediation is necessary due to the medium severity of the vulnerability.
Action: Update to the latest version of libssh2.

AI-assisted analysis generated from the source advisory — verify against the original.

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Details

Source: Microsoft Security Response Center (INTL · vendor-psirt · site)
Severity: medium — CVSS 5.9
Published: 2026-06-09
Exploitation: Not in CISA KEV at last sync

Original advisory: https://msrc.microsoft.com/update-guide/vulnerability/CVE-2026-55199

Exploitation outlook

EPSS (FIRST.org) estimates each CVE’s probability of exploitation in the next 30 days — here is the CSIRTS.com read on those numbers.

Low exploitation riskCVE-2026-551990.41% 30-day exploitation probability — currently an unlikely target, but scores change as exploit code circulates. Riskier than 33% of all scored CVEs.

Referenced CVEs

CVE	CSIRTS overview	External
CVE-2026-55199	coverage & exploitation status	NVD · CVE.org

Same CVEs, other sources

How other CERTs, PSIRTs and databases cover the vulnerabilities in this advisory.

unknownMultiple vulnerabilities in Microsoft Azure Linux (July 7, 2026)cert-fr-avis
high[UPDATE] [high] libssh2: Multiple vulnerabilitiescert-bund
unknownDSA-6365-1 libssh2 - security updatedebian
unknownNCSC-2026-0210 [1.01] [M/H] Vulnerabilities fixed in libssh2 by libsshncsc-nl

Recent advisories for libssh2 - Pre-Authentication

A cluster of recent advisories against the same product widens the attack surface — attackers routinely chain freshly published CVEs on one product, so review these together.

high[UPDATE] [high] libssh2: Multiple vulnerabilitiescert-bund · 2026-07-03
mediumCVE-2026-58051: libssh2 through 1.11.1 grows its publickey list with SSH2_REALLOC but does not zero-initialize…nvd · 2026-06-28
highCVE-2026-58050: libssh2 through 1.11.1 reads an attacker-controlled 32-bit attribute count from a publickey-su…nvd · 2026-06-28
unknownDSA-6365-1 libssh2 - security updatedebian · 2026-06-25
unknownNCSC-2026-0210 [1.01] [M/H] Vulnerabilities fixed in libssh2 by libsshncsc-nl · 2026-06-24
highCVE-2026-55200: libssh2 - Out-of-Bounds Write via Unchecked packet_length in transport.cmsrc · 2026-06-09