CVE-2026-104871

A Path Traversal vulnerability exists in the prerendered (SSG) page retrieval logic of `CommonEngine` in `@angular/ssr/node` (and `@angular/ssr` in earlier versions). When deployed on Windows, an attacker can craft a request path with backslash directory traversal sequences that causes `CommonEngine` to serve prerendered pages from sibling output directories. The vulnerability occurs due to how relative URLs and Windows file paths are resolved and validated: 1. A request URL containing a backslash parent traversal segment (e.g., `/..\app-admin`) is passed to `CommonEngine.render({ url })`. 2. The engine parses the URL using `new URL(url, 'resolve://')`. Because `resolve://` is a non-special scheme under the WHATWG URL standard, backslashes are not normalized to forward slashes, leaving the `pathname` unnormalized as `/..\app-admin`. 3. The engine constructs the candidate file path using `join(publicPath, pathname, 'index.html')`. On Windows, `path.join` treats `\` as a path delimiter, resolving the parent segment (`..\`) out of `publicPath` (e.g., `dist\app`) into a sibling directory (e.g., `dist\app-admin\index.html`). 4. The containment check (`pagePath.startsWith(normalize(publicPath))`) performs a prefix match without a trailing path delimiter. Because the sibling folder name starts with the configured public folder name (e.g., `dist\app-admin` starts with `dist\app`), the check erroneously succeeds. 5. If the target file exists and contains the Angular SSG marker (`ng-server-context="...ssg..."`), `CommonEngine` reads and serves the sibling page instead of rendering the requested route. ### Impact This vulnerability allows unauthorized access to prerendered static pages from adjacent applications or build outputs: - **Information Disclosure:** Prerendered HTML pages located in sibling directories sharing the same name prefix as the public directory (e.g., an internal administration app `app-admin` adjacent to `app`) can be accessed by unauthorized users. - **Limited Scope:** Only HTML files matching the Angular SSG marker regex are served. Arbitrary non-Angular files, secrets, or configuration files without the SSG context attribute cannot be read through this mechanism. ### Attack Preconditions - The application must use `CommonEngine` from `@angular/ssr/node` (or `@angular/ssr` in v17–v18). - The application must be hosted on Windows (where Node's `path.join` resolves `\` as a path separator). - The application passes a relative request URL (such as `req.url` or `req.originalUrl` without origin) into `CommonEngine.render({ url })`. - A sibling directory exists whose name starts with the same prefix as the configured `publicPath` directory (e.g., `dist\app-admin` alongside `dist\app`). - The targeted sibling directory contains prerendered HTML output containing the Angular SSG marker. ### Affected Versions - `>= 22.0.0, < 22.1.7` - `>= 21.0.0, < 21.2.23` - `>= 20.0.0, < 20.3.36` - `<= 19.2.27` ### Patches - `22.1.7` - `21.2.23` - `20.3.36` > Versions `<= 19.2.27` have reached End of Support / LTS expiration and will not be patched. ### Workarounds Until a patch is applied, developers using `CommonEngine` on Windows can sanitize request URLs in `server.ts` before passing them to `CommonEngine.render`, ensuring backslashes are replaced or that an absolute HTTP URL is constructed: ```ts server.use((req, res, next) => { const { protocol, originalUrl, headers } = req; // Normalize backslashes or construct a full HTTP URL (which forces WHATWG URL normalization) const sanitizedUrl = `${protocol}://${headers.host}${originalUrl.replace(/\\/g, '/')}`; commonEngine .render({ bootstrap, documentFilePath: indexHtml, url: sanitizedUrl, publicPath: distFolder, }) .then((html) => res.send(html)) .catch((err) => next(err)); }); ``` Alternatively, migrate from the deprecated `CommonEngine` to `AngularNodeAppEngine`, which relies on manifest-based route matching rather than direct filesystem path resolution.