An issue was discovered in FNET through 4.6.4. The code for IPv6 fragment reassembly tries to access a previous fragment starting from a network incoming fragment that still doesn't have a reference to the previous one (which supposedly resides in the reassembly list). When faced with an incoming fragment that belongs to a non-empty fragment list, IPv6 reassembly must check that there are no empty holes between the fragments: this leads to an uninitialized pointer dereference in _fnet_ip6_reassembly in fnet_ip6.c, and causes Denial-of-Service.

References

• https://nvd.nist.gov/vuln/detail/CVE-2020-17469
• https://us-cert.cisa.gov/ics/advisories/icsa-20-343-01
• https://www.kb.cert.org/vuls/id/815128
• http://fnet.sourceforge.net/manual/fnet_history.html