CVE-2026-118659 in Happy DOM
Summary
by MITRE • 10/28/2026
Happy DOM is a JavaScript implementation of a web browser without its graphical user interface. Prior to 21.0.2, the Happy DOM server-renderer package's ServerRendererServer accepts unauthenticated HTTP/2 requests and resolves attacker-controlled :path values against server.targetOrigin in #onIncomingRequest in packages/@happy-dom/server-renderer/src/ServerRendererServer.ts without confirming that the resolved origin remains trusted before calling fetch. A protocol-relative path can change the destination to an arbitrary HTTP or HTTPS endpoint reachable by the server, including local and internal services, and the proxy relays the fetched response headers and body to the requester. This issue is fixed in version 21.0.2.
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Analysis
by VulDB Data Team • 10/28/2026
The vulnerability identified within Happy DOM prior to version 21.0.2 represents a critical server-side request forgery flaw rooted in improper validation of HTTP/2 path values during server rendering operations. Happy DOM functions as a headless JavaScript implementation of a web browser, and its server-renderer package includes a component known as ServerRendererServer designed to handle incoming requests for dynamic content generation. The core technical deficiency lies within the onIncomingRequest method located in packages/@happy-dom/server-renderer/src/ServerRendererServer.ts. This function is responsible for processing HTTP/2 requests but fails to adequately sanitize or validate the :path header provided by the client before utilizing it to determine the target origin of a subsequent fetch operation.
The specific mechanism of exploitation involves an attacker sending unauthenticated HTTP/2 requests where the :path value is crafted to be protocol-relative, such as starting with double slashes without specifying http or https explicitly. When the ServerRendererServer processes this path, it resolves the destination against server.targetOrigin. Because the validation logic does not confirm that the resolved origin remains within a trusted list after resolving the protocol-relative path, an attacker can manipulate the resolution process to point towards arbitrary endpoints. This includes local services running on localhost or internal network resources that are reachable by the server but should not be accessible via this rendering interface.
The operational impact of this vulnerability is severe as it enables Server-Side Request Forgery attacks against infrastructure behind the Happy DOM server. By exploiting the lack of origin validation, an attacker can force the server to fetch arbitrary URLs and subsequently relay the response headers and body back to the requester. This capability allows for internal network reconnaissance, data exfiltration from unprotected local services, or potentially bypassing access controls on backend systems that rely solely on IP-based restrictions without verifying the source intent through proper origin checks. The ability to proxy responses means sensitive information returned by these internal services can be directly exposed to external attackers.
This flaw aligns with CWE-918, which describes Server-Side Request Forgery (SSRF), specifically highlighting the failure to validate user-supplied input before making server-side requests. It also relates to CWE-20, Improper Input Validation, as the application fails to ensure that the supplied path results in a safe and expected origin. From an ATT&CK perspective, this vulnerability facilitates T1598, Phishing for Information, or more accurately T1496, Resource Hijacking if used for crypto mining, but primarily it serves as a vector for T1071, Application Layer Protocol: Web Protocols, to access internal resources. The attack pattern is consistent with techniques that abuse legitimate application functionality to pivot into restricted network segments.
To mitigate this vulnerability, organizations must upgrade the Happy DOM server-renderer package to version 21.0.2 or later, where the issue has been resolved by implementing stricter validation of origin values after path resolution. In environments where immediate patching is not feasible, defensive measures should include configuring a reverse proxy in front of the Happy DOM service to filter and validate HTTP/2 headers before they reach the application server. Additionally, network-level controls such as firewall rules restricting outbound connections from the server to only known trusted domains can provide an additional layer of defense against SSRF exploitation attempts. Regular security audits focusing on input validation for any component that constructs URLs or performs fetch operations are essential to prevent similar flaws in other parts of the stack.