CVE-2026-63387: Libevent: Off-by-one stack buffer overflow in dnsname_to_labels via crafted DNS server response

highCVSS 7CVE-2026-63387

CSIRTS triage

vendor: Libevent product: Libevent Memory corruption

What: Libevent contains an off-by-one stack buffer overflow in dnsname_to_labels when processing a crafted DNS server response.
Who is affected: Applications using Libevent for DNS resolution that accept responses from untrusted or compromised DNS servers.
Urgency: High severity (CVSS 7.0) and not yet exploited; promptly update DNS-dependent infrastructure.
Action: Upgrade Libevent to a patched version with corrected bounds checking in DNS name label parsing.

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
Published: 2026-08-11
Exploitation: Not in CISA KEV at last sync

Original advisory: https://msrc.microsoft.com/update-guide/vulnerability/CVE-2026-63387

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-2026-633870.35% 30-day exploitation probability — currently an unlikely target, but scores change as exploit code circulates. Riskier than 28% of all EPSS-scored CVEs.

Referenced CVEs

CVE	CSIRTS overview	External
CVE-2026-63387	coverage & exploitation status	NVD · CVE.org

Same CVEs, other sources

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

highCVE-2026-63387: Libevent is an event notification library. Prior to 2.1.13 and 2.2.2-alpha, libevent has an of…nvd

Recent advisories for Libevent

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-2026-63495: Libevent is an event notification library. From 2.2.0-alpha-dev until 2.2.2-alpha, the libeven…nvd · 2026-08-20
highCVE-2026-63388: Libevent is an event notification library. Prior to 2.1.13 and 2.2.2-alpha, libevent has a hea…nvd · 2026-08-20
highCVE-2026-63387: Libevent is an event notification library. Prior to 2.1.13 and 2.2.2-alpha, libevent has an of…nvd · 2026-08-20
unknownCVE-2026-63385: Libevent is an event notification library. Prior to 2.1.13 and 2.2.2-alpha, libevent has two H…nvd · 2026-08-20
unknownCVE-2026-63384: Libevent is an event notification library. Prior to 2.1.13 and 2.2.2-alpha, libevent has an in…nvd · 2026-08-20
unknownCVE-2026-63383: Libevent is an event notification library. Prior to 2.1.13 and 2.2.2-alpha, libevent can read …nvd · 2026-08-20