CVE-2026-55717: 'serve-expired-client-timeout' and 'response-ip' CNAME redirect could lead to a crash

CSIRTS triage

What

A CNAME redirect vulnerability could lead to a crash.

Who is affected

Deployments using the affected configuration are at risk.

Urgency

Remediation is important due to the potential for service disruption, though exploitation is not currently observed.

Action

Implement configuration changes to mitigate the issue.

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-2026-55717

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-2026-557170.24% 30-day exploitation probability — currently an unlikely target, but scores change as exploit code circulates. Riskier than 15% of all scored CVEs.

Referenced CVEs

Same CVEs, other sources

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

• medium[NEW] [medium] Unbound: Multiple vulnerabilitiescert-bund
• mediumCVE-2026-55717: In NLnet Labs Unbound 1.10.0 up to and including 1.25.1, when 'serve-expired: yes' is set toge…nvd

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