CVE-2026-93011 in dnsmasq
Summary
by MITRE • 10/29/2026
A flaw was found in dnsmasq. After a reduction of internal domain-name buffer sizes, the DNSSEC validator can write a few bytes past a name buffer when converting names to and from DNS wire format. An attacker who can cause the server to validate a crafted DNSSEC response may overwrite a small amount of adjacent heap metadata. DNSSEC support is not compiled in by default and must also be enabled at runtime.
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Analysis
by VulDB Data Team • 10/29/2026
The vulnerability identified in dnsmasq represents a critical memory safety issue within its Domain Name System Security Extensions (DNSSEC) validation module, specifically stemming from an improper handling of buffer boundaries during name conversion operations. This flaw was introduced following a recent optimization that reduced the size of internal domain-name buffers to improve efficiency and reduce memory footprint. However, this reduction inadvertently created a scenario where the DNSSEC validator fails to adequately check bounds when converting names between their human-readable format and the binary wire format used in network packets. When processing specific crafted inputs, the application writes data beyond the allocated limits of these smaller buffers, resulting in an out-of-bounds write condition that affects adjacent memory regions on the heap.
From a technical perspective, this vulnerability is classified as CWE-120 Buffer Overflow without explicit size check or CWE-787 Out-of-bounds Write depending on the specific implementation details of the boundary violation. The core issue lies in the logic responsible for translating DNS names into their compressed wire format and vice versa during the validation process. Because the buffer sizes were reduced, the calculation for required space did not account for edge cases or worst-case scenarios where name compression might require more bytes than anticipated under certain conditions. Consequently, when a crafted DNSSEC response is processed, the validator writes several bytes past the end of the designated name buffer. This overflow does not merely corrupt application data but targets adjacent heap metadata structures managed by the memory allocator.
The operational impact of this vulnerability is severe due to its potential for remote code execution and denial of service. By overwriting small amounts of adjacent heap metadata, an attacker can manipulate how the system allocates or frees memory in subsequent operations. This manipulation can lead to arbitrary write primitives, which are foundational techniques used by attackers to achieve full control over the target process. In a DNS server context, this means that if an adversary can trigger the validation of maliciously crafted DNSSEC responses, they may execute arbitrary code with the privileges of the dnsmasq service user or potentially escalate privileges depending on the deployment environment and system configuration. Additionally, even without successful exploitation for code execution, such memory corruption can cause immediate crashes or instability in the DNS resolution service, leading to a denial of service condition that disrupts network connectivity for clients relying on this resolver.
It is crucial to note that exploiting this vulnerability requires specific conditions regarding both compilation and runtime configuration. By default, dnsmasq does not compile with DNSSEC support enabled; it must be explicitly configured during the build process. Furthermore, even if compiled with support, the feature must also be activated at runtime through configuration directives. This means that a significant portion of deployments may remain unaffected unless they have specifically opted into using DNSSEC validation features. However, for organizations that rely on DNSSEC to ensure the authenticity and integrity of DNS responses, this vulnerability poses a direct threat to their security posture. The requirement for an attacker to cause the server to validate a crafted response implies that active interaction with the vulnerable service is necessary, typically through standard UDP or TCP queries directed at the open port where dnsmasq listens.
Mitigation strategies should focus on immediate patching and configuration review. Administrators running versions of dnsmasq susceptible to this flaw must upgrade to a patched version as soon as it becomes available from official distribution channels. The fix typically involves restoring proper bounds checking logic within the name conversion routines or reverting the buffer size reduction until safer allocation strategies are implemented. For environments where DNSSEC validation is not strictly required, disabling this feature at runtime provides an effective immediate mitigation by removing the vulnerable code path entirely. This can be achieved by adjusting configuration files to turn off DNSSEC validation options. Additionally, implementing network-level controls such as rate limiting or ingress filtering can help mitigate the risk of exploitation attempts by restricting the volume of queries processed from untrusted sources, although this is a secondary defense rather than a primary fix.
In terms of threat intelligence and detection, this vulnerability aligns with ATT&CK techniques related to Initial Access via Exploitation for Remote Code Execution or Denial of Service through Resource Exhaustion if exploited for crashes. Security teams should monitor logs for unusual patterns in DNS query volumes or error messages indicating memory allocation failures within the dnsmasq process. While direct detection of heap metadata corruption is difficult without advanced runtime application self-protection tools, observing anomalies in service stability after updates to configuration settings involving DNSSEC can serve as an indicator of compromise attempts. Continuous monitoring and adherence to vendor security advisories are essential for maintaining resilience against such memory safety vulnerabilities that persist even in widely used open-source infrastructure software.