CVE-2026-63387 Libevent: Off-by-one stack buffer overflow in dnsname_to_labels via crafted DNS server response
A crafted DNS response triggers a stack buffer overflow in Libevent, allowing attackers to crash the application or potentially execute arbitrary code.
- Exploited in the wild
- Not known
- Login needed
- Not needed
- Attackable
- Over the internet
- Victim action needed
- Not needed
- Fix
- Available
What is CVE-2026-63387?
CVE-2026-63387 is a high-severity memory corruption vulnerability in libevent, published on 20 Aug 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
Official 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.
Which versions are affected by CVE-2026-63387?
How do I fix CVE-2026-63387?
Upgrade to a fixed version.
Upgrade to the fixed version. Memory-safe languages, compiler hardening and fuzzing prevent most new cases. More about Memory corruption.
Is CVE-2026-63387 being exploited?
It is not confirmed as exploited in the wild, but public exploit code for CVE-2026-63387 exists.
Sources
Also known as AZL-96918, AZL-96950, AZL-97530, AZL-97548, BELL-CVE-2026-63387, CLSA-2026-1791302296 and 5 more.