Unbound 1.19.1 released

We are pleased to announce the release of version 1.19.1 of the Unbound recursive DNS resolver.

This security release fixes two DNSSEC validation vulnerabilities: CVE-2023-50387 (referred here as the KeyTrap vulnerability) and CVE-2023-50868 (referred here as the NSEC3 vulnerability).

The KeyTrap vulnerability works by using a combination of Keys (also colliding Keys), Signatures and number of RRSETs on a malicious zone. Answers from that zone can force a DNSSEC validator down a very CPU intensive and time costly validation path.

The NSEC3 vulnerability uses specially crafted responses on a malicious zone with multiple NSEC3 RRSETs to force a DNSSEC validator down a very CPU intensive and time costly NSEC3 hash calculation path.

Both can force Unbound to spend an enormous time (comparative to regular traffic) validating a single specially crafted DNSSEC response while everything else is on hold for that thread. A trivially orchestrated attack could render all threads busy with such responses leading to denial of service.

From version 1.19.1 on, Unbound introduces suspension on DNSSEC response validations that seem to require more attempts than Unbound is willing to make per response validation run. Suspension means that Unbound will continue with other work before resuming a suspended validation offering CPU time between validation resumptions to other tasks. There is a backoff timer when suspending which is further influenced by the number of suspends already used and the amount of work currently in Unbound.

The introduced builtin limits in Unbound are:

Max 4 DNSSEC key collissions are allowed when building chain of trust. More than that without a secure key treats the delegation as bogus.
8 validation attempts per RRSET (combination of keys + signatures). If more are needed and Unbound has yet to find a valid signature the RRSET is treated as bogus.
More than 8 validation attempts per answer will suspend validation.
8 NSEC3 hash calculations are allowed before suspension. More than that will suspend validation.
The limit of total suspensions is 16 after which the query will error out. Any completed RRSET validations populate the cache for use in future queries.

While under attack Unbound could show higher CPU load because of the needed validations but the suspend strategy would guarantee the CPU is not locked on any particular validation task.

We would like to thank Elias Heftrig, Haya Schulmann, Niklas Vogel, and Michael Waidner from the German National Research Center for Applied Cybersecurity ATHENE for discovering and responsibly disclosing the KeyTrap vulnerability.

We would like to thank Petr Špaček from ISC for discovering and responsibly disclosing the NSEC3 vulnerability.

To learn further about the vulnerabilities and the coordination process, we encourage you to read ISC's blog post titled BIND 9 Security Release and Multi-Vendor Vulnerability Handling.

For a full list of changes and binary and source packages, see the download page.