CVE-2026-55717
'serve-expired-client-timeout' and 'response-ip' CNAME redirect could lead to a crash
Description
In NLnet Labs Unbound 1.10.0 up to and including 1.25.1, when 'serve-expired: yes' is set together with a 'response-ip: <net> redirect' /'response-ip-data: <net> CNAME <target>' rule (or the RPZ 'rpz-cname-override' equivalent), a remote client who controls any delegated domain can crash the daemon. The serve-expired-client-timeout callback runs a two-pass loop to chase the respip-generated CNAME alias; on the second pass it resets 'alias_rrset' but not 'partial_rep'. Later, this inconsistency leads to a NULL pointer dereference and an eventual crash. A malicious actor can exploit the vulnerability by controlling any zone that replies with an A/AAAA record that falls inside the configured response-ip/rpz subnet. By delaying the answer when the previous record has expired, the vulnerable path of 'serve-expired-client-timeout' is taken leading to denial of service via the server crash.
INFO
Published Date :
July 22, 2026, 2:17 p.m.
Last Modified :
July 22, 2026, 2:17 p.m.
Remotely Exploit :
No
Source :
[email protected]
CVSS Scores
| Score | Version | Severity | Vector | Exploitability Score | Impact Score | Source |
|---|---|---|---|---|---|---|
| CVSS 3.1 | MEDIUM | 206fc3a0-e175-490b-9eaa-a5738056c9f6 | ||||
| CVSS 3.1 | MEDIUM | [email protected] |
Solution
- Update Unbound to version 1.26.0 or later.
- Review and adjust 'serve-expired' and 'response-ip' configurations.
- Consider disabling 'serve-expired' if updates are not possible.
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-2026-55717.
| URL | Resource |
|---|---|
| https://www.nlnetlabs.nl/downloads/unbound/CVE-2026-55717.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-2026-55717 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-2026-55717
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-2026-55717 vulnerability anywhere in the article.
The following table lists the changes that have been made to the
CVE-2026-55717 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.
-
New CVE Received by [email protected]
Jul. 22, 2026
Action Type Old Value New Value Added Affected [{'vendor': 'NLnet Labs', 'product': 'Unbound', 'versions': [{'status': 'affected', 'version': '1.10.0', 'lessThan': '1.25.2', 'versionType': 'semver'}], 'defaultStatus': 'unaffected'}] Added Description In NLnet Labs Unbound 1.10.0 up to and including 1.25.1, when 'serve-expired: yes' is set together with a 'response-ip: <net> redirect' /'response-ip-data: <net> CNAME <target>' rule (or the RPZ 'rpz-cname-override' equivalent), a remote client who controls any delegated domain can crash the daemon. The serve-expired-client-timeout callback runs a two-pass loop to chase the respip-generated CNAME alias; on the second pass it resets 'alias_rrset' but not 'partial_rep'. Later, this inconsistency leads to a NULL pointer dereference and an eventual crash. A malicious actor can exploit the vulnerability by controlling any zone that replies with an A/AAAA record that falls inside the configured response-ip/rpz subnet. By delaying the answer when the previous record has expired, the vulnerable path of 'serve-expired-client-timeout' is taken leading to denial of service via the server crash. Added CVSS V3.1 AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H Added CWE CWE-476 Added Reference https://www.nlnetlabs.nl/downloads/unbound/CVE-2026-55717.txt