Cloudflare’s handling of a bug in interpreting IPv4-mapped IPv6 addresses

In November 2022, our bug bounty program received a critical and very interesting report. The report stated that certain types of DNS records could be used to bypass some of our network policies and connect to ports on the loopback address (e.g. 127.0.0.1) of our servers. This post will explain how we dealt with the report, how we fixed the bug, and the outcome of our internal investigation to see if the vulnerability had been previously exploited.

RFC 4291 defines ways to embed an IPv4 address into IPv6 addresses. One of the methods defined in the RFC is to use IPv4-mapped IPv6 addresses, that have the following format:

   |                80 bits               | 16 |      32 bits        |
   +--------------------------------------+--------------------------+
   |0000..............................0000|FFFF|    IPv4 address     |
   +--------------------------------------+----+---------------------+

In IPv6 notation, the corresponding mapping for 127.0.0.1 is ::ffff:127.0.0.1 (RFC 4038)

The researcher was able to use DNS entries based on mapped addresses to bypass some of our controls and access ports on the loopback address or non-routable IPs.

This vulnerability was reported on November 27 to our bug bounty program. Our Security Incident Response Team (SIRT) was contacted, and incident response activities Continue reading