CVE-2026-66035: libssh2 Heap Buffer Overflow via ETM Cipher Negotiation

highCVSS 7.5CVE-2026-66035

CSIRTS triage

vendor: libssh2 product: libssh2 Memory corruption

What: A heap buffer overflow vulnerability exists via ETM cipher negotiation in libssh2.
Who is affected: Users of libssh2 are affected by this vulnerability.
Urgency: Remediation is urgent due to the high severity rating and potential for exploitation.
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: high — CVSS 7.5
Published: 2026-07-14
Exploitation: Not in CISA KEV at last sync

Original advisory: https://msrc.microsoft.com/update-guide/vulnerability/CVE-2026-66035

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-2026-660350.32% 30-day exploitation probability — currently an unlikely target, but scores change as exploit code circulates. Riskier than 24% of all scored CVEs.

Referenced CVEs

CVE	CSIRTS overview	External
CVE-2026-66035	coverage & exploitation status	NVD · CVE.org

Same CVEs, other sources

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

Recent advisories for libssh2 Heap Buffer

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.

highCVE-2026-66035: libssh2 through 1.11.1, fixed in commit 42e33d8, contains a pre-authentication heap buffer ove…nvd · 2026-07-24
mediumCVE-2025-15661: libssh2 - Heap Buffer Over-read via sftp_symlink() in sftp.cmsrc · 2026-06-09