CVE-2026-55717
Description
A flaw in Unbound allows a remote attacker controlling a delegated domain to trigger a NULL pointer dereference and crash the daemon. Exploitation occurs when serve-expired: yes and specific response-ip or RPZ rules are configured, resulting in a denial of service.
Statement
This Moderate severity denial of service flaw in Unbound requires specific configurations, including serve-expired: yes and either response-ip or RPZ CNAME override rules. Exploitation depends on a remote attacker controlling a delegated domain and delaying DNS responses, which limits the immediate impact on typical Red Hat Unbound deployments not utilizing these advanced features.
Mitigation
Ensure serve-expired remains disabled (default), or set serve-expired-client-timeout: 0 to bypass the vulnerable callback path. If serving expired answers is necessary, remove any response-ip CNAME redirect rules and RPZ rpz-cname-override entries. Additionally, use access-control to restrict query access, preventing untrusted clients from triggering the flaw via attacker-controlled names.
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 | 5.9 | N/A | 5.9 |
| Attack Vector | Network | N/A | Network |
| Attack Complexity | High | N/A | High |
| 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 | High | N/A | High |
Vector
Red Hat: CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H
cve.org: CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H
Understanding the Weakness (CWE)
Availability
Technical Impact: DoS: Crash, Exit, or Restart
NULL pointer dereferences usually result in the failure of the process unless exception handling (on some platforms) is available and implemented. Even when exception handling is being used, it can still be very difficult to return the software to a safe state of operation.
Integrity,Confidentiality
Technical Impact: Execute Unauthorized Code or Commands; Read Memory; Modify Memory
In rare circumstances, when NULL is equivalent to the 0x0 memory address and privileged code can access it, then writing or reading memory is possible, which may lead to code execution.
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