{"id":"CVE-2026-33397","aliases":["GHSA-vfx2-hv2g-xj5f"],"url":"https://o3.security/vulnerability/CVE-2026-33397","summary":"Angular SSR Vulnerable to Protocol-Relative URL Injection via Single Backslash Bypass","details":"An Open Redirect vulnerability exists in `@angular/ssr` due to an incomplete fix for CVE-2026-27738. While the original fix successfully blocked multiple leading slashes (e.g., `///`), the internal validation logic fails to account for a single backslash (`\\`) bypass.\n\nWhen an Angular SSR application is deployed behind a proxy that passes the `X-Forwarded-Prefix` header:\n\n- An attacker provides a value starting with a single backslash (e.g., `\\evil.com`).\n- The internal validation failed to flag the single backslash as invalid.\n- The application prepends a leading forward slash, resulting in a `Location` header containing `/\\evil.com`.\n- Modern browsers interpret the `/\\` sequence as `//`, treating it as a protocol-relative URL and redirecting the user to the attacker-controlled domain.\n\nFurthermore, the response lacks the `Vary: X-Forwarded-Prefix` header, allowing the malicious redirect to be stored in intermediate caches (Web Cache Poisoning).\n\n### Impact\nThis vulnerability allows attackers to conduct large-scale phishing and SEO hijacking:\n\n- **Scale**: A single request can poison a high-traffic route, impacting all users until the cache expires.\n- **SEO Poisoning**: Search engine crawlers may follow and index these malicious redirects, causing the legitimate site to be delisted or associated with malicious domains.\n- **Trust**: Because the initial URL belongs to the trusted domain, users and security tools are less likely to flag the redirect as malicious.\n\n### Patches\n\n- 22.0.0-next.2\n- 21.2.3\n- 20.3.21\n\n### Workarounds\nUntil the patch is applied, developers should sanitize the `X-Forwarded-Prefix` header in their `server.ts` before the Angular engine processes the request:\n\n```ts\napp.use((req, res, next) => {\n  const prefix = req.headers['x-forwarded-prefix'];\n  if (typeof prefix === 'string') {\n    // Sanitize by removing all leading forward and backward slashes\n    req.headers['x-forwarded-prefix'] = prefix.trim().replace(/^[/\\\\]+/, '/');\n  }\n  next();\n});\n```\n\n\n### References\n\n- Fix: https://github.com/angular/angular-cli/pull/32771\n- Original CVE: CVE-2026-27738","published":"2026-03-26T13:46:16.145Z","modified":"2026-10-28T02:31:08.207768306Z","cvss":null,"epss":{"score":0.00255,"percentile":0.16786,"asOf":"2026-08-24"},"cisaKev":null,"exploitsKnown":0,"affectedPackages":[{"ecosystem":"npm","name":"@angular/ssr","fixedVersion":"22.0.0-next.2"},{"ecosystem":"npm","name":"@angular/ssr","fixedVersion":"21.2.3"},{"ecosystem":"npm","name":"@angular/ssr","fixedVersion":"20.3.21"}],"fix":{"url":"https://github.com/angular/angular-cli/pull/32771","label":"angular/angular-cli#32771"},"references":[{"type":"ADVISORY","url":"https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/33xxx/CVE-2026-33397.json"},{"type":"ADVISORY","url":"https://github.com/advisories/GHSA-xh43-g2fq-wjrj"},{"type":"ADVISORY","url":"https://github.com/angular/angular-cli/security/advisories/GHSA-vfx2-hv2g-xj5f"},{"type":"ADVISORY","url":"https://nvd.nist.gov/vuln/detail/CVE-2026-33397"},{"type":"FIX","url":"https://github.com/angular/angular-cli/pull/32771"},{"type":"PACKAGE","url":"https://github.com/angular/angular-cli"}],"provenance":{"sources":["OSV.dev","FIRST.org (EPSS)"],"lastVerified":"2026-10-28T02:31:08.207768306Z"}}