c-ares before 1.4.0 uses a predictable seed for the random number generator for the DNS Transaction ID field, which might allow remote attackers to spoof DNS responses by guessing the field value.

The ares_init:randomize_key function in c-ares, on platforms other than Windows, uses a weak facility for producing a random number sequence (Unix rand), which makes it easier for remote attackers to spoof DNS responses by guessing certain values.