Buffer overflow in Libevent

CVE-2026-63387

Libevent is an event notification library. Prior to 2.1.13 and 2.2.2-alpha, libevent has an off-by-one stack buffer overflow in evdns.c when dnsname_to_labels formats a name-bearing DNS record at the end of the 64 KB stack buffer allocated by evdns_server_request_format_response. The final-label check permits j plus label_len plus one to equal buf_len, after which the terminating null byte is written to buf[buf_len]. A crafted DNS server response containing PTR, CNAME, MX, NS, or SOA data can trigger the one-byte out-of-bounds write and crash or corrupt the process. This issue is fixed in versions 2.1.13 and 2.2.2-alpha.

Vulnerability class: Buffer Overflow

Published · last modified .

CVSS v3 metric

CVSS v3 base score 7.0 (High). Vector: CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:L/I:L/A:H.

EPSS exploit prediction

EPSS: 0.004 (33.6th percentile), scored .

Very low probability of exploitation in the next 30 days; routine patching cadence is appropriate. 34th percentile — 33.6% of CVEs in the catalogue have a lower EPSS than this one. How to read EPSS.

EPSS trend (30 days)EPSS over the last 30 days for CVE-2026-63387: held from 0.003 to 0.004.

EPSS over last 30 days · oldest: 0.003 · newest: 0.004 · change: +0.001

Affected products

  • Libevent — versions < 2.1.13, >= 2.2.0-alpha, < 2.2.2-alpha

Weakness classification (CWE)

References

Frequently asked questions

What is CVE-2026-63387?
CVE-2026-63387 is a high-severity vulnerability in Libevent, classified under Stack-based Buffer Overflow. CVSS score: 7.0/10. Published 2026-08-20.
How severe is CVE-2026-63387?
High severity. CVSS v3 base score is 7.0 out of 10.