CVE-2026-52863 in Unbound
Summary
by MITRE • 07/22/2026
In NLnet Labs Unbound 1.25.0 up to and including 1.25.1, a fix that makes the 'respip' and 'dns64' modules work together, creates a shallow copy of the view name in effect that could lead to memory corruption if the owner of the original view name is jostled out when Unbound is under pressure. Unbound needs to be configured with one of 'respip'/'rpz' modules, together with a module that can attach subqueries (respip CNAME redirection, dns64, subnetcache) and a configured 'access-control-view' while Unbound is under pressure so that joslte logic kicks in and starts dropping slow queries. The subquery is getting a shallow copy of the view name and if the super query which owns the view name is jostled out, memory corruption can occur. Likelihood of a crash is low, since it relies heavily on the underlying memory allocator and the memory layout. Debug memory builds (e.g., ASAN) that catch the free terminate the server.
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Analysis
by VulDB Data Team • 07/22/2026
The vulnerability in NLnet Labs Unbound versions 1.25.0 through 1.25.1 represents a memory corruption issue stemming from improper handling of view name references within the interaction between respip and dns64 modules. This flaw manifests when the server configuration includes both respip/rpz modules alongside modules capable of attaching subqueries such as respip CNAME redirection, dns64, or subnetcache, combined with configured access-control-view directives. The underlying technical mechanism involves a shallow copy operation that duplicates only the reference to the view name rather than the actual data structure, creating a dangerous dependency on memory stability during high-pressure operations.
The operational impact becomes critical when Unbound experiences heavy load conditions that trigger its jostling logic for dropping slow queries. Under these circumstances, the memory allocator's behavior becomes unpredictable and the shallow copy of the view name can lead to memory corruption when the original owner of that view name is evicted from memory. This scenario requires a specific combination of conditions including active module configurations, memory pressure, and the precise timing of memory allocation/deallocation patterns to manifest. The vulnerability falls under CWE-457: Use of Uninitialized Variable, though more specifically relates to improper memory management and reference handling.
The risk assessment indicates a low likelihood of exploitation due to its heavy dependence on specific memory allocator behaviors and memory layout conditions that must align perfectly for the corruption to occur. However, the potential consequences are severe as memory corruption can lead to application crashes or potentially more dangerous outcomes in environments where such vulnerabilities might be leveraged by attackers. The vulnerability is particularly concerning in debug builds using AddressSanitizer (ASAN) which detect the invalid memory access and terminate the server process rather than allowing continued operation with corrupted memory.
Mitigation strategies should focus on immediate version upgrades to patched releases of Unbound where this shallow copy issue has been resolved through proper deep copying of view name references. Administrators should also review their configurations to ensure that respip/rpz modules are not unnecessarily combined with subquery-attaching modules under high load conditions, and implement proper resource management and monitoring to detect early signs of memory pressure. Additionally, implementing proper memory allocation strategies and ensuring adequate system resources can help reduce the probability of triggering this specific race condition scenario. The fix should ensure that view name references maintain their validity even when parent query structures are removed from memory during jostling operations, which aligns with ATT&CK technique T1499.004: Endpoint Denial of Service to prevent exploitation through resource exhaustion scenarios.
The vulnerability demonstrates how seemingly minor implementation details in memory management can create significant security risks in DNS server implementations, particularly when dealing with complex module interactions under stress conditions. Proper defensive programming practices including deep copying of critical data structures and robust reference counting mechanisms should be implemented to prevent similar issues in future development cycles. Organizations should also consider implementing automated monitoring systems that can detect unusual memory allocation patterns or query processing behaviors that might indicate the onset of such race conditions before they lead to system instability or potential exploitation opportunities.