CVE-2026-50251
Description
A flaw was found in Unbound. A remote malicious actor can exploit a vulnerability by controlling a DNS delegation that returns specific glue records. This can cause Unbound to repeatedly clear its DNS caches, leading to a continuous Denial of Service (DoS) for DNS resolution.
Statement
This Moderate flaw in Unbound allows a remote attacker to trigger a denial of service by controlling a DNS delegation. When the unwanted-reply-threshold option is enabled, specially crafted glue records can cause Unbound to repeatedly clear its DNS caches, disrupting DNS resolution. This occurs due to Unbound's internal handling of locally routed 0.0.0.0/::0 glue records, which are incorrectly flagged as unwanted replies.
Mitigation
To mitigate this issue, configure Unbound to disable the `unwanted-reply-threshold` option by setting it to `0` in the `unbound.conf` file. This prevents the cache clearing behavior triggered by malicious glue records. After modifying the configuration, restart the Unbound service for the changes to take effect.
Common Vulnerability Scoring System (CVSS) Score Details
Info alert:Important note
CVSS scores for open source components depend on vendor-specific factors (e.g. version or build chain). Therefore, Red Hat's score and impact rating can be different from NVD and other vendors. Red Hat remains the authoritative CVE Naming Authority (CNA) source for its products and services (see Red Hat classifications).
CVSS v3 Score Breakdown
| Red Hat | NVD | cve.org | |
|---|---|---|---|
| Base Score | 5.3 | N/A | 5.3 |
| Attack Vector | Network | N/A | Network |
| Attack Complexity | Low | N/A | Low |
| Privileges Required | None | N/A | None |
| User Interaction | None | N/A | None |
| Scope | Unchanged | N/A | Unchanged |
| Confidentiality | None | N/A | None |
| Integrity Impact | None | N/A | None |
| Availability Impact | Low | N/A | Low |
Vector
Red Hat: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L
cve.org: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L
Understanding the Weakness (CWE)
Availability
Technical Impact: DoS: Resource Consumption (CPU); DoS: Resource Consumption (Memory); DoS: Amplification
An infinite loop will cause unexpected consumption of resources, such as CPU cycles or memory. The software's operation may slow down, or cause a long time to respond.
Frequently Asked Questions
Not sure what something means? Check out our Security Glossary.
Want to get errata notifications? Sign up here.
