High [CVE-2026-63387] Off-by-one stack buffer overflow leading to denial of service or data corruption

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.

A remote attacker could send a specially crafted DNS server response, which may lead to a crash or corruption of the process, resulting in a denial of service or potential information disclosure and integrity impact.

This Important vulnerability in the libevent library's DNS parsing component can lead to a denial of service or data corruption due to an off-by-one stack buffer overflow. A remote attacker can exploit this flaw with low attack complexity by providing a specially crafted DNS server response to services utilizing libevent for DNS resolution.

Red Hat severity: Important — CVSS 8.6 (CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:L/A:H). Weakness: CWE-787.