CVE-2026-55991: Remote DNS-over-QUIC (DoQ) flow-control assertion failure in libngtcp2

mediumCVSS 5.9CVE-2026-55991

CSIRTS triage

Other

What: A flow-control assertion failure in libngtcp2 affects remote DNS-over-QUIC.
Who is affected: Deployments using libngtcp2 for DNS-over-QUIC are affected.
Urgency: Remediation is medium priority due to the medium severity of the vulnerability.
Action: Apply patches or mitigations as soon as they are available.

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

Details

Source: Microsoft Security Response Center (INTL · vendor-psirt · site)
Severity: medium — CVSS 5.9
Published: 2026-07-14
Exploitation: Not in CISA KEV at last sync

Original advisory: https://msrc.microsoft.com/update-guide/vulnerability/CVE-2026-55991

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-2026-559910.24% 30-day exploitation probability — currently an unlikely target, but scores change as exploit code circulates. Riskier than 15% of all scored CVEs.

Referenced CVEs

CVE	CSIRTS overview	External
CVE-2026-55991	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 Remote DNS-over-QUIC

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.

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