An off-by-one error in ngx_resolver_copy() while processing DNS responses allows a network attacker to write a dot character ('.', 0x2E) out of bounds in a heap allocated buffer. The vulnerability can be triggered by a DNS response in reply to a DNS request from nginx when the resolver primitive is configured. A specially crafted packet allows overwriting the least significant byte of next heap chunk metadata with 0x2E. A network attacker capable of providing DNS responses to a nginx server can achieve Denial-of-Service and likely remote code execution. Due to the lack of DNS spoofing mitigations in nginx and the fact that the vulnerable function is called before checking the DNS Transaction ID, remote attackers might be able to exploit this vulnerability by flooding the victim server with poisoned DNS responses in a feasible amount of time.

CVE-2021-23017