CVE-2026-82720 Use-after-free in DoH stream cleanup code path
A use-after-free in Unbound's DoH cleanup allows remote attackers to cause a denial of service via crafted traffic when DoH is enabled.
- Exploited in the wild
- Not known
- Login needed
- Not needed
- Attackable
- Over the internet
- Victim action needed
- Not needed
- Fix
- Available
What is CVE-2026-82720?
CVE-2026-82720 is a medium-severity use after free vulnerability in Nlnetlabs Unbound, published on 16 Sep 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
Official description
NLnet Labs Unbound 1.12.0 up to and including 1.26.0 has a use-after-free vulnerability when compiled for DNS-over-HTTPs support with '--with-libnghttp2'. During failure code paths (i.e., RPZ drop query, jostle due to heavy traffic), a dropped DoH stream brings down the whole DoH session and does not account properly for other DoH streams in the same session. This leads to use-after-free in those code paths. If the prerequisites are satisfied (possible RPZ drop or heavy client traffic), a malicious actor can trigger the vulnerability with a single DoH connection and the appropriate traffic. Impact is limited as the reads are not user controlled and the use-after-free leads to early returns. However, a hardened allocator can catch the use-after-free and controllably terminate the process resulting to denial of service.
Weakness: CWE-416
Which versions are affected by CVE-2026-82720?
It affects 1 product.
Products include Nlnetlabs Unbound.
How do I fix CVE-2026-82720?
Apply the vendor's patch.
Upgrade to the fixed version. These bugs are found with fuzzing and sanitisers and avoided by memory-safe languages. More about Use after free.
Is CVE-2026-82720 being exploited?
There is no known exploitation of CVE-2026-82720 in the wild and no public exploit code.
Other vulnerabilities in Nlnetlabs Unbound
- CVE-2026-81642 Heap buffer overflow and possible Remote Code Execution when digesting DNSKEY
- CVE-2026-33278 Possible arbitrary code execution during DNSSEC validation
- CVE-2026-42944 Heap overflow with multiple NSID, COOKIE, PADDING EDNS options
- CVE-2026-42959 Crash during DNSSEC validation of malicious content
- CVE-2026-82717 CNAME synthesis could lead to heap corruption
Sources
Also known as AZL-101712, DEBIAN-CVE-2026-82720, ECHO-82dc-10c2-9d51, GHSA-9hp4-9wfp-2fh8, UBUNTU-CVE-2026-82720.