Being an incubating CNCF project makes us eligible for nice things like a security assessment (cue ominous music).
The CNCF asked Cure53 to perform such an assessment.
TL;DR: CoreDNS is in good shape, but Cure53 did find one critical issue (which we’ve fixed with the CoreDNS 1.1.1 release):
DNS-01-003 Cache: DNS Cache poisoning via malicious Response (Critical)
The CoreDNS application allows to configure the caching of the DNS responses via the cache plugin. It was discovered that CoreDNS only verifies the transaction IDs but fails to check whether the domain in a request matches the response. This can be abused to inject malicious A records in the cache of the DNS server. As the CoreDNS application has a different cache for each domain
The other three issues found will be tracked via GitHub issues, like plugin/rewrite: log bypass, and [plugin/secondary: Denial-of-Service via endless Zone Transfer](plugin/secondary: Denial-of-Service via endless Zone Transfer). Third one was a generic DDoS.
On a positive note the final report includes quotes like these:
The CoreDNS software tested by Cure53 during this March 2018 assessment has made a clearly positive impression.
To conclude, even though four issues were found during this Cure53 assessment, they were generally - with a single exception - minor, miscellaneous and manageable. Despite Cure53 testers’ considerable efforts, the software was found to be hard to corrupt. Therefore, the CoreDNS project stands out as secure, robust and legitimately security-aware.
The full report can be found here. As for future improvements in CoreDNS: we will increase the use of fuzzing, increase test coverage and look closer at DNS DoS mitigations, such as DNS Cookies (described in RFC 7873).