NLnet Labs Unbound up to 1.26.1 DNS-over-QUIC use after free

Summaryinfo

A vulnerability was found in NLnet Labs Unbound up to 1.26.1. It has been declared as critical. Impacted is an unknown function of the component DNS-over-QUIC. Executing a manipulation can lead to use after free. This vulnerability is handled as CVE-2026-78227. The attack can be executed remotely. There is not any exploit available.

Detailsinfo

A vulnerability classified as critical was found in NLnet Labs Unbound up to 1.26.1. This vulnerability affects some unknown processing of the component DNS-over-QUIC. The manipulation with an unknown input leads to a use after free vulnerability. The CWE definition for the vulnerability is CWE-416. Referencing memory after it has been freed can cause a program to crash, use unexpected values, or execute code. As an impact it is known to affect confidentiality, integrity, and availability. CVE summarizes:

NLnet Labs Unbound 1.22.0 up to and including 1.26.1, has a use-after-free vulnerability when compiled for DNS-over-QUIC support with '--with-libngtcp2'. Each DoQ stream owns an output buffer that holds the DNS response. ngtcp2's retransmission buffer keeps a shallow pointer into the output buffer for as long as a STREAM frame may be resent. On a client RESET_STREAM, the output buffer is freed but ngtcp2 still holds the matching retransmission entries. The next PTO timeout makes ngtcp2 re-encode the STREAM frame and copy from the freed buffer. A malicious actor that can query Unbound over DoQ and that withholds ACKs, sends RESET_STREAM, and waits for PTO, reaches this use-after-free with no privilege. This leads to retransmissions against freed memory and eventually an abnormal server exit under a 20-query spray.

The advisory is available at nlnetlabs.nl. This vulnerability was named CVE-2026-78227 since 09/07/2026. The exploitation appears to be easy. The attack can be initiated remotely. No form of authentication is required for a successful exploitation. The technical details are unknown and an exploit is not available. The structure of the vulnerability defines a possible price range of USD $0-$5k at the moment (estimation calculated on 09/16/2026).

There is no information about possible countermeasures known. It may be suggested to replace the affected object with an alternative product.

If you want to get best quality of vulnerability data, you may have to visit VulDB.

Productinfo

Vendor

• NLnet Labs

Name

• Unbound

Version

• 1.26.0
• 1.26.1

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If you want to get best quality of vulnerability data, you may have to visit VulDB.

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