Impact

Description of issue(s):

1. 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.
2. 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.
3. No attempt is made to look for modern OS-provided CSPRNGs like arc4random() that is widely available.

Correction(s) made:

1. Detect arc4random() and if available, use it directly to generate DNS query ids.
2. Use /dev/urandom or RtlGenRandom() directly to generate DNS query ids as a fallback
3. As a last resort, use the current rand() + RC4 logic (should only apply to esoteric systems), with these modifications:

• replace RC4 implementation with official algorithm
• seed rand() using srand()

Patches

Fixed in 1.19.1

Workarounds

No workarounds are available.

Credit

David Gstir and Hannes Moesl
X41 D-SEC GmbH
Audit funded by Open Source Technology Improvement Fund (OSTIF)