CVE-2026-56434 in NGINX Plus
Summary
by MITRE • 07/15/2026
NGINX Plus and NGINX Open Source have a vulnerability in the ngx_http_ssi_module module. This vulnerability may exist when the Server-Side Includes (SSI), proxy_pass, and proxy_buffering off directives are configured. With this configuration, an unauthenticated attacker with man-in-the-middle (MITM) ability to control responses from an upstream server may be able to cause a heap buffer over-read in the NGINX worker process. This issue may lead to limited modification of memory or a restart of the NGINX worker process.
Impact: This vulnerability may allow remote attackers to have limited control to modify memory contents or restart the NGINX worker process. There is no control plane exposure; this is a data plane issue only.
Note: Software versions which have reached End of Technical Support (EoTS) are not evaluated.
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Analysis
by VulDB Data Team • 07/15/2026
The vulnerability exists within the ngx_http_ssi_module component of both NGINX Plus and NGINX Open Source software, representing a critical security flaw that arises from specific configuration combinations. This issue manifests when Server-Side Includes functionality is enabled alongside proxy_pass directives combined with proxy_buffering off settings. The flaw constitutes a heap buffer over-read condition that occurs within the NGINX worker process, creating potential for memory corruption and system instability. According to CWE-125, this vulnerability falls under the category of out-of-bounds read conditions where an attacker can access memory locations beyond the allocated buffer boundaries, potentially leading to information disclosure or denial of service scenarios.
The operational impact of this vulnerability is significant within environments where NGINX serves as a reverse proxy or load balancer with SSI functionality enabled. When an unauthenticated attacker possesses man-in-the-middle capabilities and can manipulate responses from upstream servers, they can exploit this weakness to cause heap buffer over-reads that may result in memory modification or worker process restarts. This represents a data plane vulnerability that does not require authentication access to the control plane, making it particularly dangerous for systems where network traffic interception is possible. The ATT&CK framework categorizes this as a privilege escalation technique through memory corruption, potentially allowing adversaries to gain further system control by destabilizing the NGINX process.
Security implications extend beyond simple service disruption as heap buffer over-reads can provide attackers with opportunities to execute arbitrary code within the NGINX worker process context. The vulnerability's exploitation requires specific network conditions and configuration settings, but when present, it creates a pathway for attackers to potentially modify memory contents or cause process restarts that could lead to service availability issues. Organizations using affected versions of NGINX must consider this vulnerability in their risk assessment frameworks, particularly those operating in environments where MITM capabilities might be available to adversaries. The absence of control plane exposure means the threat primarily affects data plane operations, but the potential for memory corruption and process instability still represents a substantial security concern requiring immediate attention.
Mitigation strategies should focus on implementing proper configuration controls and network security measures to prevent exploitation. Organizations should consider disabling SSI functionality when not required or ensuring that proxy_buffering is properly configured to avoid triggering the vulnerable code path. Network segmentation and traffic inspection controls can help reduce the likelihood of successful MITM attacks that would be necessary for exploitation. Additionally, implementing regular updates and patch management processes ensures that systems remain protected against known vulnerabilities. The recommended approach includes validating configuration settings through security audits and monitoring for unusual process behavior that might indicate memory corruption attempts. Organizations should also consider deploying intrusion detection systems to monitor for traffic patterns consistent with exploitation attempts targeting this specific vulnerability.