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.

A critical off-by-one stack buffer overflow vulnerability exists in libevent versions prior to 2.1.13 and 2.2.2-alpha. The flaw resides in evdns.c within the dnsname_to_labels function, which incorrectly permits a terminating null byte to be written one position beyond the end of a 64 KB stack buffer. The vulnerability can be triggered by a crafted DNS server response containing PTR, CNAME, MX, NS, or SOA record types. Successful exploitation can lead to process crash or memory corruption. The root cause is a faulty boundary check in the final-label validation logic inside evdns_server_request_format_response. The issue affects any application or system relying on libevent for DNS resolution. Fixes are available in libevent versions 2.1.13-stable and 2.2.2-alpha. Users are strongly advised to upgrade to patched versions immediately.