CVE-2026-78227
Use-after-free in DoQ stream output buffer on reset re-transmission
Description
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.
INFO
Published Date :
Sept. 16, 2026, 9:17 a.m.
Last Modified :
Sept. 16, 2026, 7:41 p.m.
Remotely Exploit :
No
Source :
[email protected]
CVSS Scores
| Score | Version | Severity | Vector | Exploitability Score | Impact Score | Source |
|---|---|---|---|---|---|---|
| CVSS | 134c704f-9b21-4f2e-91b3-4a467353bcc0 | |||||
| CVSS 3.1 | MEDIUM | 206fc3a0-e175-490b-9eaa-a5738056c9f6 | ||||
| CVSS 3.1 | MEDIUM | MITRE-CVE | ||||
| CVSS 3.1 | MEDIUM | [email protected] |
Solution
- Update Unbound to a version that fixes the use-after-free vulnerability.
- Ensure DNS-over-QUIC support and ngtcp2 are configured correctly.
- Apply vendor-provided patches for the identified memory corruption issue.
References to Advisories, Solutions, and Tools
Here, you will find a curated list of external links that provide in-depth
information, practical solutions, and valuable tools related to
CVE-2026-78227.
| URL | Resource |
|---|---|
| https://www.nlnetlabs.nl/downloads/unbound/CVE-2026-78227.txt |
CWE - Common Weakness Enumeration
While CVE identifies
specific instances of vulnerabilities, CWE categorizes the common flaws or
weaknesses that can lead to vulnerabilities. CVE-2026-78227 is
associated with the following CWEs:
Common Attack Pattern Enumeration and Classification (CAPEC)
Common Attack Pattern Enumeration and Classification
(CAPEC)
stores attack patterns, which are descriptions of the common attributes and
approaches employed by adversaries to exploit the CVE-2026-78227
weaknesses.
We scan GitHub repositories to detect new proof-of-concept exploits. Following list is a collection of public exploits and proof-of-concepts, which have been published on GitHub (sorted by the most recently updated).
Results are limited to the first 15 repositories due to potential performance issues.
The following list is the news that have been mention
CVE-2026-78227 vulnerability anywhere in the article.
-
The Hacker News
Critical Unbound DNSSEC Validator Flaw Could Allow RCE via a Malicious DNS Zone
Every release of the Unbound DNS resolver before 1.26.1 has a critical heap overflow in its DNSSEC validator, maintainer NLnet Labs said in an advisory on Wednesday. An attacker who controls a malicio ... Read more
The following table lists the changes that have been made to the
CVE-2026-78227 vulnerability over time.
Vulnerability history details can be useful for understanding the evolution of a vulnerability, and for identifying the most recent changes that may impact the vulnerability's severity, exploitability, or other characteristics.
-
CVE Modified by 134c704f-9b21-4f2e-91b3-4a467353bcc0
Sep. 16, 2026
Action Type Old Value New Value Added SSVC {'id': 'CVE-2026-78227', 'role': 'CISA Coordinator', 'options': [{'exploitation': 'none'}, {'automatable': 'no'}, {'technicalImpact': 'partial'}], 'version': '2.0.3', 'timestamp': '2026-09-16T15:33:40.171223Z'} -
New CVE Received by [email protected]
Sep. 16, 2026
Action Type Old Value New Value Added Description 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. Added CVSS V3.1 AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:N/A:L Added CWE CWE-416 Added Affected New affected value received. <a href="https://github.com/CVEProject/cvelistV5/blob/main/cves/2026/78xxx/CVE-2026-78227.json">CVE-2026-78227</a> Added Reference https://www.nlnetlabs.nl/downloads/unbound/CVE-2026-78227.txt