# CVE-2026-63387: Libevent: Off-by-one stack buffer overflow in dnsname_to_labels via crafted DNS server response CVE-2026-63387 is fixed in 8 products on 1 platform, first by libevent-2.1-7t64 2.1.12-stable-10ubuntu0.2, released August 20, 2026. It is not reported as exploited. Patch Radar priority: High. - Web page: https://patchblox.com/security/cve/CVE-2026-63387 - Priority: High - CVSS: 8.6 (Moderate) - EPSS: 0.0042 - CISA KEV: not listed - Exploited (vendor report): no - Publicly disclosed: no - Published: Aug 20, 2026 - Weakness: Stack-based Buffer Overflow - Vendor advisory: https://msrc.microsoft.com/update-guide/vulnerability/CVE-2026-63387 ## Description 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. ## Where it's fixed | Platform | Product | Fixed in | Fix | Released | |---|---|---|---|---| | Linux | libevent Debian 12.15 (bookworm) | 2.1.12-stable-8+deb12u1 | [DEBIAN-CVE-2026-63387](https://patchblox.com/security/data/fix/linux/debian-cve-2026-63387.md) | Aug 21, 2026 | | Linux | libevent Debian 13.5 (trixie) | 2.1.13-stable-1~deb13u1 | [DEBIAN-CVE-2026-63387](https://patchblox.com/security/data/fix/linux/debian-cve-2026-63387.md) | Aug 21, 2026 | | Linux | libevent Debian 14 | 2.1.13-stable-1 | [DEBIAN-CVE-2026-63387](https://patchblox.com/security/data/fix/linux/debian-cve-2026-63387.md) | Aug 21, 2026 | | Linux | libevent RHEL 10 | 2.1.13-1.el10_2 | [RHSA-2026:67909](https://patchblox.com/security/data/fix/linux/rhsa-2026-67909.md) | Sep 17, 2026 | | Linux | libevent RHEL 8 | 0:2.1.8-11.el8_10 | [RHSA-2026:67908](https://patchblox.com/security/data/fix/linux/rhsa-2026-67908.md) | Sep 19, 2026 | | Linux | libevent RHEL 9 | 0:2.1.13-1.el9_8 | [RHSA-2026:67910](https://patchblox.com/security/data/fix/linux/rhsa-2026-67910.md) | Sep 19, 2026 | | Linux | libevent almalinux 10 | 2.1.13-1.el10_2 | [ALSA-2026:67909](https://patchblox.com/security/data/fix/linux/alsa-2026-67909.md) | Sep 16, 2026 | | Linux | libevent almalinux 8 | 2.1.8-11.el8_10 | [ALSA-2026:67908](https://patchblox.com/security/data/fix/linux/alsa-2026-67908.md) | Sep 16, 2026 | | Linux | libevent almalinux 9 | 2.1.13-1.el9_8 | [ALSA-2026:67910](https://patchblox.com/security/data/fix/linux/alsa-2026-67910.md) | Sep 16, 2026 | | Linux | libevent ol 10 | 0:2.1.13-1.el10_2 | [ELSA-2026-67909](https://patchblox.com/security/data/fix/linux/elsa-2026-67909.md) | Sep 16, 2026 | | Linux | libevent ol 8 | 0:2.1.8-11.el8_10 | [ELSA-2026-67908](https://patchblox.com/security/data/fix/linux/elsa-2026-67908.md) | Sep 17, 2026 | | Linux | libevent ol 9 | 0:2.1.13-1.el9_8 | [ELSA-2026-67910](https://patchblox.com/security/data/fix/linux/elsa-2026-67910.md) | Sep 16, 2026 | | Linux | libevent rocky 10 | 0:2.1.13-1.el10_2 | [RLSA-2026:67909](https://patchblox.com/security/data/fix/linux/rlsa-2026-67909.md) | Sep 17, 2026 | | Linux | libevent rocky 8 | 0:2.1.8-11.el8_10 | [RLSA-2026:67908](https://patchblox.com/security/data/fix/linux/rlsa-2026-67908.md) | Sep 17, 2026 | | Linux | libevent rocky 9 | 0:2.1.13-1.el9_8 | [RLSA-2026:67910](https://patchblox.com/security/data/fix/linux/rlsa-2026-67910.md) | Sep 17, 2026 | | Linux | libevent-2.1-7t64 Ubuntu 26.04.1 LTS (resolute) | 2.1.12-stable-10ubuntu0.2 | [libevent-2.1-7t64 2.1.12-stable-10ubuntu0.2](https://patchblox.com/security/data/fix/linux/ubuntu-resolute-libevent-2.1-7t64-2.1.12-stable-10ubuntu0.2.md) | Aug 20, 2026 | | Linux | libevent-core-2.1-7t64 Ubuntu 26.04.1 LTS (resolute) | 2.1.12-stable-10ubuntu0.2 | [libevent-core-2.1-7t64 2.1.12-stable-10ubuntu0.2](https://patchblox.com/security/data/fix/linux/ubuntu-resolute-libevent-core-2.1-7t64-2.1.12-stable-10ubuntu0.2.md) | Aug 20, 2026 | | Linux | libevent-devel RHEL 8 | 0:2.1.8-11.el8_10 | [RHSA-2026:67908](https://patchblox.com/security/data/fix/linux/rhsa-2026-67908.md) | Sep 19, 2026 | | Linux | libevent-devel RHEL 9 | 0:2.1.13-1.el9_8 | [RHSA-2026:67910](https://patchblox.com/security/data/fix/linux/rhsa-2026-67910.md) | Sep 19, 2026 | | Linux | libevent-devel almalinux 10 | 2.1.13-1.el10_2 | [ALSA-2026:67909](https://patchblox.com/security/data/fix/linux/alsa-2026-67909.md) | Sep 16, 2026 | | Linux | libevent-devel almalinux 8 | 2.1.8-11.el8_10 | [ALSA-2026:67908](https://patchblox.com/security/data/fix/linux/alsa-2026-67908.md) | Sep 16, 2026 | | Linux | libevent-devel almalinux 9 | 2.1.13-1.el9_8 | [ALSA-2026:67910](https://patchblox.com/security/data/fix/linux/alsa-2026-67910.md) | Sep 16, 2026 | | Linux | libevent-devel ol 10 | 0:2.1.13-1.el10_2 | [ELSA-2026-67909](https://patchblox.com/security/data/fix/linux/elsa-2026-67909.md) | Sep 16, 2026 | | Linux | libevent-devel ol 8 | 0:2.1.8-11.el8_10 | [ELSA-2026-67908](https://patchblox.com/security/data/fix/linux/elsa-2026-67908.md) | Sep 17, 2026 | | Linux | libevent-devel ol 9 | 0:2.1.13-1.el9_8 | [ELSA-2026-67910](https://patchblox.com/security/data/fix/linux/elsa-2026-67910.md) | Sep 16, 2026 | | Linux | libevent-doc RHEL 8 | 0:2.1.8-11.el8_10 | [RHSA-2026:67908](https://patchblox.com/security/data/fix/linux/rhsa-2026-67908.md) | Sep 19, 2026 | | Linux | libevent-doc RHEL 9 | 0:2.1.13-1.el9_8 | [RHSA-2026:67910](https://patchblox.com/security/data/fix/linux/rhsa-2026-67910.md) | Sep 19, 2026 | | Linux | libevent-doc almalinux 10 | 2.1.13-1.el10_2 | [ALSA-2026:67909](https://patchblox.com/security/data/fix/linux/alsa-2026-67909.md) | Sep 16, 2026 | | Linux | libevent-doc almalinux 8 | 2.1.8-11.el8_10 | [ALSA-2026:67908](https://patchblox.com/security/data/fix/linux/alsa-2026-67908.md) | Sep 16, 2026 | | Linux | libevent-doc almalinux 9 | 2.1.13-1.el9_8 | [ALSA-2026:67910](https://patchblox.com/security/data/fix/linux/alsa-2026-67910.md) | Sep 16, 2026 | | Linux | libevent-doc ol 10 | 0:2.1.13-1.el10_2 | [ELSA-2026-67909](https://patchblox.com/security/data/fix/linux/elsa-2026-67909.md) | Sep 16, 2026 | | Linux | libevent-doc ol 8 | 0:2.1.8-11.el8_10 | [ELSA-2026-67908](https://patchblox.com/security/data/fix/linux/elsa-2026-67908.md) | Sep 17, 2026 | | Linux | libevent-doc ol 9 | 0:2.1.13-1.el9_8 | [ELSA-2026-67910](https://patchblox.com/security/data/fix/linux/elsa-2026-67910.md) | Sep 16, 2026 | | Linux | libevent-extra-2.1-7t64 Ubuntu 26.04.1 LTS (resolute) | 2.1.12-stable-10ubuntu0.2 | [libevent-extra-2.1-7t64 2.1.12-stable-10ubuntu0.2](https://patchblox.com/security/data/fix/linux/ubuntu-resolute-libevent-extra-2.1-7t64-2.1.12-stable-10ubuntu0.2.md) | Aug 20, 2026 | | Linux | libevent-openssl-2.1-7t64 Ubuntu 26.04.1 LTS (resolute) | 2.1.12-stable-10ubuntu0.2 | [libevent-openssl-2.1-7t64 2.1.12-stable-10ubuntu0.2](https://patchblox.com/security/data/fix/linux/ubuntu-resolute-libevent-openssl-2.1-7t64-2.1.12-stable-10ubuntu0.2.md) | Aug 20, 2026 | | Linux | libevent-pthreads-2.1-7t64 Ubuntu 26.04.1 LTS (resolute) | 2.1.12-stable-10ubuntu0.2 | [libevent-pthreads-2.1-7t64 2.1.12-stable-10ubuntu0.2](https://patchblox.com/security/data/fix/linux/ubuntu-resolute-libevent-pthreads-2.1-7t64-2.1.12-stable-10ubuntu0.2.md) | Aug 20, 2026 | --- Data as of 2026-11-09T13:31:49Z UTC. 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