In NLnet Labs Unbound 1.20.0 up to and including 1.25.1, when Unbound is configured with serve-expired: yes and serve-expired-client-timeout > discard-timeout > 0 (contrary to the suggested values), the discard-timeout branch during the serve expired logic drops an aged client reply without performing the correct accounting for the number of reply addresses for the query. Other identical branches outside of serve expired perform the correct decrement. Since the counter is never decremented in such scenario, it can reach the maximum limit and new clients for duplicate in-flight queries are silently dropped resulting in degradation of resolution service. A malicious actor can exploit the vulnerability by querying the resolver for a client-controlled slow-on-demand authoritative zone that can drive the counter past the threshold. Shipped defaults for serve-expired-client-timeout: 1800 and discard-timeout: 1900 make the branch unreachable.
Weakness
The product does not release a resource after its effective lifetime has ended, i.e., after the resource is no longer needed.
Affected Software
| Name | Vendor | Start Version | End Version |
|---|---|---|---|
| Unbound | Nlnetlabs | 1.20.0 (including) | 1.25.2 (excluding) |
| Red Hat Hardened Images | RedHat | unbound-main-1.25.2-0.1.hum1 | * |
| Unbound | Ubuntu | upstream | * |
Potential Mitigations
- Use a language that does not allow this weakness to occur or provides constructs that make this weakness easier to avoid.
- For example, languages such as Java, Ruby, and Lisp perform automatic garbage collection that releases memory for objects that have been deallocated.
- Use resource-limiting settings provided by the operating system or environment. For example, when managing system resources in POSIX, setrlimit() can be used to set limits for certain types of resources, and getrlimit() can determine how many resources are available. However, these functions are not available on all operating systems.
- When the current levels get close to the maximum that is defined for the application (see CWE-770), then limit the allocation of further resources to privileged users; alternately, begin releasing resources for less-privileged users. While this mitigation may protect the system from attack, it will not necessarily stop attackers from adversely impacting other users.
- Ensure that the application performs the appropriate error checks and error handling in case resources become unavailable (CWE-703).