{"id":"CVE-2026-104871","aliases":["GHSA-7g7c-h8rr-7p6q"],"url":"https://o3.security/vulnerability/CVE-2026-104871","summary":"Angular SSR: Path Traversal to Sibling Directories in CommonEngine on Windows","details":"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.\n\nThe vulnerability occurs due to how relative URLs and Windows file paths are resolved and validated:\n\n1. A request URL containing a backslash parent traversal segment (e.g., `/..\\app-admin`) is passed to `CommonEngine.render({ url })`.\n2. 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`.\n3. 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`).\n4. 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.\n5. 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.\n\n### Impact\n\nThis vulnerability allows unauthorized access to prerendered static pages from adjacent applications or build outputs:\n\n- **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.\n- **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.\n\n### Attack Preconditions\n\n- The application must use `CommonEngine` from `@angular/ssr/node` (or `@angular/ssr` in v17–v18).\n- The application must be hosted on Windows (where Node's `path.join` resolves `\\` as a path separator).\n- The application passes a relative request URL (such as `req.url` or `req.originalUrl` without origin) into `CommonEngine.render({ url })`.\n- A sibling directory exists whose name starts with the same prefix as the configured `publicPath` directory (e.g., `dist\\app-admin` alongside `dist\\app`).\n- The targeted sibling directory contains prerendered HTML output containing the Angular SSG marker.\n\n### Affected Versions\n\n- `>= 22.0.0, < 22.1.7`\n- `>= 21.0.0, < 21.2.23`\n- `>= 20.0.0, < 20.3.36`\n- `<= 19.2.27`\n\n### Patches\n\n- `22.1.7`\n- `21.2.23`\n- `20.3.36`\n\n> Versions `<= 19.2.27` have reached End of Support / LTS expiration and will not be patched.\n\n### Workarounds\n\nUntil 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:\n\n```ts\nserver.use((req, res, next) => {\n  const { protocol, originalUrl, headers } = req;\n\n  // Normalize backslashes or construct a full HTTP URL (which forces WHATWG URL normalization)\n  const sanitizedUrl = `${protocol}://${headers.host}${originalUrl.replace(/\\\\/g, '/')}`;\n\n  commonEngine\n    .render({\n      bootstrap,\n      documentFilePath: indexHtml,\n      url: sanitizedUrl,\n      publicPath: distFolder,\n    })\n    .then((html) => res.send(html))\n    .catch((err) => next(err));\n});\n```\n\nAlternatively, migrate from the deprecated `CommonEngine` to `AngularNodeAppEngine`, which relies on manifest-based route matching rather than direct filesystem path resolution.","published":"2026-10-02T19:43:50.475Z","modified":"2026-10-21T02:54:32.323505063Z","cvss":null,"epss":null,"cisaKev":null,"exploitsKnown":null,"affectedPackages":[{"ecosystem":"npm","name":"@angular/ssr","fixedVersion":"22.1.7"},{"ecosystem":"npm","name":"@angular/ssr","fixedVersion":"21.2.23"},{"ecosystem":"npm","name":"@angular/ssr","fixedVersion":"20.3.36"},{"ecosystem":"npm","name":"@angular/ssr","fixedVersion":null}],"fix":{"url":"https://github.com/angular/angular-cli/commit/645e41a47d21b7651837a4250de99af0509626e2","label":"angular/angular-cli@645e41a"},"references":[{"type":"WEB","url":"https://github.com/angular/angular-cli/releases/tag/v20.3.36"},{"type":"WEB","url":"https://github.com/angular/angular-cli/releases/tag/v21.2.23"},{"type":"WEB","url":"https://github.com/angular/angular-cli/releases/tag/v22.1.7"},{"type":"ADVISORY","url":"https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/104xxx/CVE-2026-104871.json"},{"type":"ADVISORY","url":"https://github.com/angular/angular-cli/security/advisories/GHSA-7g7c-h8rr-7p6q"},{"type":"ADVISORY","url":"https://nvd.nist.gov/vuln/detail/CVE-2026-104871"},{"type":"FIX","url":"https://github.com/angular/angular-cli/commit/645e41a47d21b7651837a4250de99af0509626e2"},{"type":"FIX","url":"https://github.com/angular/angular-cli/commit/70748ca8e76fe4b42798719410336119d943e755"},{"type":"FIX","url":"https://github.com/angular/angular-cli/commit/bb72145f9ab45aee29f523236b3a25cd0813a841"},{"type":"FIX","url":"https://github.com/angular/angular-cli/commit/c3e5982e49705f0f6a913cb94622b4c555d2d914"},{"type":"PACKAGE","url":"https://github.com/angular/angular-cli"}],"provenance":{"sources":["OSV.dev","FIRST.org (EPSS)"],"lastVerified":"2026-10-21T02:54:32.323505063Z"}}