A flaw was found in the c-ares package. The ares_set_sortlist is missing checks about the validity of the input string, which allows a possible arbitrary length stack overflow. This issue may cause a denial of service or a limited impact on confidentiality and integrity. When cross-compiling c-ares and using the autotools build system, CARES_RANDOM_FILE will not be set, as seen when cross compiling aarch64 android. This will downgrade to using rand as a fallback which could allow an attacker to take advantage of the lack of entropy by not using a CSPRNG. ares_inet_net_pton is vulnerable to a buffer underflow for certain ipv6 addresses, in particular "0::00:00:00/2" was found to cause an issue. C-ares only uses this function internally for configuration purposes which would require an administrator to configure such an address via ares_set_sortlist.However, users may externally use ares_inet_net_pton for other purposes and thus be vulnerable to more severe issues. Insufficient randomness in generation of DNS query IDsWhen /dev/urandom or RtlGenRandom are unavailable, c-ares uses rand to generate random numbers used for DNS query ids. This is not a CSPRNG, and it is also not seeded by srand so will generate predictable output.Input from the random number generator is fed into a non-compilant RC4 implementation and may not be as strong as the original RC4 implementation.No attempt is made to look for modern OS-provided CSPRNGs like arc4random that is widely available. Denial of Service.Attack Steps:The target resolver sends a queryThe attacker forges a malformed UDP packet with a length of 0 and returns them to the target resolverThe target resolver erroneously interprets the 0 length as a graceful shutdown of the connection. Current resolution fails, DoS attack is achieved