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An unbound security update has been released for Debian GNU/Linux 10 to address several security vulnerabilities.

DLA 3371-1: unbound security update

Debian LTS Advisory DLA-3371-1 Markus Koschany
March 29, 2023

Package : unbound
Version : 1.9.0-2+deb10u3
CVE ID : CVE-2020-28935 CVE-2022-3204 CVE-2022-30698 CVE-2022-30699
Debian Bug : 1016493 977165

Several security vulnerabilities have been discovered in unbound, a validating,
recursive, caching DNS resolver.


A vulnerability named 'Non-Responsive Delegation Attack' (NRDelegation
Attack) has been discovered in various DNS resolving software. The
NRDelegation Attack works by having a malicious delegation with a
considerable number of non responsive nameservers. The attack starts by
querying a resolver for a record that relies on those unresponsive
nameservers. The attack can cause a resolver to spend a lot of
time/resources resolving records under a malicious delegation point where a
considerable number of unresponsive NS records reside. It can trigger high
CPU usage in some resolver implementations that continually look in the
cache for resolved NS records in that delegation. This can lead to degraded
performance and eventually denial of service in orchestrated attacks.
Unbound does not suffer from high CPU usage, but resources are still needed
for resolving the malicious delegation. Unbound will keep trying to resolve
the record until hard limits are reached. Based on the nature of the attack
and the replies, different limits could be reached. From now on Unbound
introduces fixes for better performance when under load, by cutting
opportunistic queries for nameserver discovery and DNSKEY prefetching and
limiting the number of times a delegation point can issue a cache lookup
for missing records.

CVE-2022-30698 and CVE-2022-30699

Unbound is vulnerable to a novel type of the "ghost domain names" attack.
The vulnerability works by targeting an Unbound instance.
Unbound is queried for a rogue domain name when the cached delegation
information is about to expire. The rogue nameserver delays the response so
that the cached delegation information is expired. Upon receiving the
delayed answer containing the delegation information, Unbound overwrites
the now expired entries. This action can be repeated when the delegation
information is about to expire making the rogue delegation information
ever-updating. From now on Unbound stores the start time for a query and
uses that to decide if the cached delegation information can be


Unbound contains a local vulnerability that would allow for a local symlink

For Debian 10 buster, these problems have been fixed in version

We recommend that you upgrade your unbound packages.

For the detailed security status of unbound please refer to
its security tracker page at:

Further information about Debian LTS security advisories, how to apply
these updates to your system and frequently asked questions can be
found at: