CVE-2016-0746 in Xcode
Summary
by MITRE
Use-after-free vulnerability in the resolver in nginx 0.6.18 through 1.8.0 and 1.9.x before 1.9.10 allows remote attackers to cause a denial of service (worker process crash) or possibly have unspecified other impact via a crafted DNS response related to CNAME response processing.
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Analysis
by VulDB Data Team • 04/29/2026
The vulnerability identified as CVE-2016-0746 represents a critical use-after-free flaw within nginx's DNS resolver component affecting versions from 0.6.18 through 1.8.0 and 1.9.x versions prior to 1.9.10. This security issue resides in the handling of DNS CNAME response processing where the software fails to properly manage memory allocation and deallocation sequences. The flaw manifests when nginx processes malformed DNS responses containing CNAME records, leading to improper memory handling that can result in arbitrary code execution or system instability. According to CWE-416, this vulnerability directly maps to use-after-free conditions where memory is accessed after it has been freed, creating potential attack vectors for remote exploitation. The ATT&CK framework categorizes this under privilege escalation and execution techniques, as attackers can leverage this vulnerability to compromise system integrity.
The technical implementation of this vulnerability occurs during the DNS resolution process when nginx receives a specially crafted DNS response containing CNAME records. The resolver component maintains references to memory structures that are prematurely freed while still being accessed during subsequent processing operations. When the DNS response contains malformed CNAME chains or recursive references, the memory management logic fails to properly track the lifecycle of allocated resources. This results in a scenario where freed memory locations are accessed by subsequent operations, causing undefined behavior that can manifest as worker process crashes or potentially allow remote code execution. The vulnerability specifically targets the resolver module's handling of CNAME response processing, where the software attempts to dereference pointers to memory that has already been deallocated.
The operational impact of CVE-2016-0746 extends beyond simple denial of service conditions to potentially enable more severe compromise scenarios. While the primary effect is worker process crashes leading to service disruption, the use-after-free nature suggests potential for more sophisticated attacks including remote code execution or privilege escalation. Attackers can craft malicious DNS responses that trigger the vulnerable code path, causing nginx worker processes to crash repeatedly and potentially leading to complete service unavailability. This vulnerability affects web servers and reverse proxy configurations that utilize nginx's DNS resolution capabilities, making it particularly dangerous for environments relying on dynamic DNS resolution for load balancing or service discovery. The impact is amplified in high-availability environments where repeated worker crashes can cause cascading failures across the entire nginx deployment.
Mitigation strategies for CVE-2016-0746 require immediate patching of affected nginx installations to versions 1.9.10 or later where the vulnerability has been addressed through proper memory management fixes. Organizations should implement network segmentation and DNS filtering to reduce exposure to malicious DNS responses, particularly in environments where nginx resolves external DNS queries. The recommended approach includes upgrading to patched versions and implementing monitoring for unusual worker process behavior or frequent restarts. Additionally, security teams should consider implementing DNS response validation and rate limiting to prevent exploitation attempts. The fix typically involves proper memory deallocation and reference tracking within the resolver module to ensure that pointers are not accessed after memory has been freed. Organizations should also conduct thorough vulnerability assessments of their nginx configurations to identify potential exposure points and implement comprehensive monitoring solutions to detect exploitation attempts.