CVE-2016-0742 in Xcode
Summary
by MITRE
The resolver in nginx before 1.8.1 and 1.9.x before 1.9.10 allows remote attackers to cause a denial of service (invalid pointer dereference and worker process crash) via a crafted UDP DNS response.
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Analysis
by VulDB Data Team • 04/29/2026
The vulnerability identified as CVE-2016-0742 affects the nginx web server software and represents a critical denial of service weakness that could be exploited by remote attackers to crash worker processes. This issue specifically resides within nginx's DNS resolver functionality and impacts versions prior to 1.8.1 and 1.9.10, making it a significant concern for organizations relying on nginx as their primary web server. The vulnerability is classified under CWE-476 which denotes a null pointer dereference, a common class of software flaws that can lead to system instability and service disruption. The attack vector involves sending a specially crafted UDP DNS response to a system running an affected nginx version, which then triggers an invalid pointer dereference in the resolver component.
The technical flaw manifests when nginx processes DNS responses from external servers without proper validation of response structure and content. When a malicious DNS response is received that contains malformed or unexpected data structures, the resolver code fails to properly handle the invalid input and attempts to dereference a null or invalid pointer, resulting in an immediate crash of the worker process handling the request. This behavior aligns with the ATT&CK technique T1499.004 which covers network denial of service attacks, specifically targeting the availability of services through process crashes. The vulnerability is particularly dangerous because it requires minimal effort from an attacker to exploit, as they only need to send a single malformed DNS response to potentially disrupt service availability.
The operational impact of this vulnerability extends beyond simple service disruption to potentially affect entire infrastructure availability, especially in environments where nginx serves as a critical component for web traffic management and load balancing. When worker processes crash, nginx automatically restarts them, but this process can lead to temporary service degradation or complete unavailability during the restart cycle. Organizations may experience cascading failures if multiple worker processes crash simultaneously, particularly in high-traffic environments where nginx is handling thousands of concurrent connections. The vulnerability also poses challenges for incident response teams as the crash pattern can be difficult to distinguish from other system failures, potentially delaying detection and remediation efforts. Additionally, the impact on system resources can be significant, as each crash requires system overhead for process restart and resource re-allocation, potentially leading to increased CPU and memory consumption during the recovery phase.
Mitigation strategies for CVE-2016-0742 should prioritize immediate patching of affected nginx installations to versions 1.8.1 or 1.9.10 and later, which contain the necessary code fixes to properly validate DNS response structures before processing. Organizations should implement network-level controls such as DNS response validation and filtering to prevent malicious DNS responses from reaching nginx servers, particularly in environments where external DNS servers cannot be fully trusted. Configuration hardening measures should include implementing proper DNS resolver timeouts and limiting the number of concurrent DNS requests to reduce the window of opportunity for exploitation. Security monitoring should be enhanced to detect worker process crashes and unusual DNS traffic patterns that may indicate exploitation attempts. The implementation of redundant nginx instances with proper load balancing can provide resilience against single-point failures, while automated monitoring and alerting systems can help detect and respond to service disruptions more rapidly. Regular security audits and vulnerability assessments should be conducted to ensure all nginx installations remain up-to-date with security patches and that appropriate network segmentation controls are in place to limit the potential impact of such vulnerabilities.