Out-of-Bounds Write in NGINX JavaScript (njs) XML Module

hello@craftedsignal.io — Wed, 02 Sep 2026 17:16:10 +0000

CVE-2026-78689 is an out-of-bounds heap write vulnerability affecting the XML module of the NGINX JavaScript (njs) engine and the QuickJS (qjs) engine. The flaw is located in the xml.exclusiveC14n() method, which fails to safely process an XML namespace prefix list provided by an external source. An unauthenticated remote attacker can exploit this by passing a crafted prefix list to the method.

When using the njs engine, the out-of-bounds write corrupts adjacent heap objects, leading to an immediate crash of the NGINX worker process. When using the QuickJS engine, the vulnerability causes memory leakage on every request, resulting in memory exhaustion. The nginx-saml reference implementation is specifically vulnerable during SAML signature verification, as it processes the InclusiveNamespaces/@PrefixList from untrusted SAML messages before verifying the digital signature. Consequently, attackers can trigger the crash or memory growth using crafted SAML Response, Assertion, LogoutRequest, or LogoutResponse packets. While remote code execution has not been observed, it remains a theoretical possibility depending on specific platform memory layout.

Impact

The vulnerability impacts the availability of NGINX instances processing XML or SAML data. Successful exploitation leads to denial of service through repeatable worker process restarts (njs engine) or rapid worker memory growth (QuickJS engine). Because the flaw triggers before SAML signature validation, attackers do not require valid credentials or legitimate SAML tokens to impact the target service.

Recommendation

Identify all instances using the nginx-saml reference implementation and upgrade to the patched version once available.
Review NGINX configurations for usages of xml.exclusiveC14n() that process untrusted XML input and implement rigorous input validation for the PrefixList attribute.
Monitor NGINX error logs for recurring "worker process exited with signal" events, which may indicate crash-based denial of service attempts.
Monitor memory usage of NGINX worker processes to detect memory exhaustion patterns indicative of QuickJS exploitation.

Authentication Bypass in NGINX JavaScript and QuickJS Engines

hello@craftedsignal.io — Wed, 02 Sep 2026 17:15:05 +0000

The NGINX JavaScript (njs) and QuickJS (qjs) engines contain a vulnerability involving the js_access handler during asynchronous request body processing. When an exception occurs during the asynchronous access-control evaluation phase - specifically before the handler can return an explicit access denial - the engine may fail open. This misconfiguration in the access validation logic permits the request to proceed to downstream resources despite failing security checks.

An unauthenticated remote attacker can exploit this condition by sending a crafted HTTP request designed to trigger an error within the access-control JavaScript logic. This results in an authorization bypass, potentially allowing access to protected endpoints that should have been restricted. This vulnerability exists within the data plane, impacting any application relying on the njs or qjs access control handlers for security enforcement.

Impact

Successful exploitation allows unauthenticated remote attackers to bypass security controls defined within the js_access handler. This can result in unauthorized access to sensitive application data or protected administrative functions. As this is a data plane issue affecting request processing, it does not involve exposure of the server control plane but directly undermines the security posture of the web application.

Recommendation

Identify all instances of NGINX utilizing the js_access handler for security controls.
Audit JavaScript access control logic to ensure that error conditions do not result in a default-allow state.
Monitor web server error logs for unexpected exceptions occurring within JavaScript access handlers, which may indicate exploitation attempts.
Apply patches provided by the software vendor once available for NGINX and affected distributions.

QuickJS - CraftedSignal Threat Feed

Out-of-Bounds Write in NGINX JavaScript (njs) XML Module

Impact

Recommendation

Authentication Bypass in NGINX JavaScript and QuickJS Engines

Impact

Recommendation