CVE-2026-42534
Description
A flaw was found in Unbound. An adversary who can query a vulnerable Unbound instance and control a slow or malicious domain name server can exploit a vulnerability in the jostle logic. This flaw allows retransmitted queries to renew the age of slow-running queries, preventing them from being identified as aged and replaced. This can degrade Unbound's resolution performance, potentially leading to a denial of resolution service through coordinated attacks.
Statement
This flaw is rated as Important. An attacker controlling a malicious DNS server could exploit a vulnerability in Unbound's jostle logic, leading to degraded resolution performance and a potential denial of service. This occurs because retransmitted queries can incorrectly renew the age of slow-running queries, preventing their proper replacement.
Mitigation
To mitigate the risk of denial of resolution service, restrict Unbound's network exposure. Configure Unbound to only accept queries from trusted internal networks and to forward its own queries exclusively to trusted, well-maintained upstream DNS servers. This reduces the opportunity for an adversary to interact with or act as a malicious upstream for the vulnerable Unbound instance. After modifying Unbound's configuration, a service restart is required for changes to take effect. This may temporarily interrupt DNS resolution services.
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 | 7.5 | 5.3 | 7.5 |
| Attack Vector | Network | Network | Network |
| Attack Complexity | Low | Low | Low |
| Privileges Required | None | None | None |
| User Interaction | None | None | None |
| Scope | Unchanged | Unchanged | Unchanged |
| Confidentiality | None | None | None |
| Integrity Impact | None | None | None |
| Availability Impact | High | Low | High |
Vector
Red Hat: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
NVD: 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:H
Understanding the Weakness (CWE)
Availability
Technical Impact: DoS: Resource Consumption (Memory); DoS: Resource Consumption (Other)
An adversary that can cause a resource counter to become inaccurate may be able to create situations where resources are not accounted for and not released, thus causing resources to become scarce for future needs.
Availability
Technical Impact: DoS: Crash, Exit, or Restart
An adversary that can cause a resource counter to become inaccurate may be able to force an error that causes the product to crash or exit out of its current operation.
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