c-ares is a C library for asynchronous DNS requests. `ares__read_line()` is used to parse local configuration files such as `/etc/resolv.conf`, `/etc/nsswitch.conf`, the `HOSTALIASES` file, and if using a c-ares version prior to 1.27.0, the `/etc/hosts` file. If any of these configuration files has an embedded `NULL` character as the first character in a new line, it can lead to attempting to read memory prior to the start of the given buffer which may result in a crash. This issue is fixed in c-ares 1.27.0. No known workarounds exist.

c-ares is an asynchronous resolver library. c-ares is vulnerable to denial of service. If a target resolver sends a query, the attacker forges a malformed UDP packet with a length of 0 and returns them to the target resolver. The target resolver erroneously interprets the 0 length as a graceful shutdown of the connection. This issue has been patched in version 1.19.1.

c-ares is an asynchronous resolver library. When /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. This issue has been fixed in version 1.19.1.

c-ares is an asynchronous resolver library. 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. This issue has been fixed in 1.19.1.

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.