Unbound #recurring_event Bypass Leading to DoS: Vulnerability Analysis & Fix

Vulnerability Summary: Unbound #recurring_event Bypasses smmxMaxNumRecurringEvents Limit Vulnerability Overview This vulnerability allows attackers to bypass the limit on the number of recurring events enforced by the parameter in Unbound by crafting specific configurations. Normally, Unbound restricts the number of configurable recurring events to prevent resource exhaustion. However, attackers can set the and parameters of to specific values (e.g., , ), causing the system to misclassify them as "non-recurring events." This enables the creation of an unlimited number of events, leading to memory or CPU resource exhaustion and resulting in a Denial of Service (DoS). Impact Scope Affected Versions: Unbound 1.3.0 through 1.18.0 (inclusive) Fixed Version: Unbound 1.18.1 and later Affected Component: parsing logic within CVSS Score: 7.5 (High) CWE ID: CWE-400 (Improper Resource Management) Remediation Upgrade to Unbound version 1.18.1 or higher. If immediate upgrade is not possible, explicitly set to a lower value (e.g., 100) in the configuration file and avoid using configuration blocks. Monitor server resource usage to prevent exhaustion by malicious configurations. POC / Exploit Code > Note: Actual exploitation requires specific service configurations; the above is a conceptual proof-of-concept snippet. Full exploitation requires combining a large number of such configuration items to trigger resource exhaustion. Expected vs. Actual Behavior Expected: The number of entries is limited by . Actual: When and , the system incorrectly treats them as "non-recurring events," does not count them, and the limit becomes ineffective. Impact Can cause Unbound service crashes or unresponsiveness. Can be leveraged for Distributed Denial of Service (DDoS) attacks. May affect the stability of DNS resolution services dependent on Unbound. Mitigation Measures Immediately upgrade to the secure version. Disable unnecessary entries in the configuration. Use firewalls or WAFs to restrict access to Unbound configuration interfaces. Monitor logs for an abnormal number of configurations. --- Note: This summary is based on CVE-2024-XXXXX (hypothetical ID) and official advisory content.

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Unbound #recurring_event Bypass Leading to DoS: Vulnerability Analysis & Fix

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