CVE-2026-33397: @angular/ssr Protocol-Relative URL Injection Vulnerability

{
  "schema_version" : "1.4.0",
  "id" : "GHSA-vfx2-hv2g-xj5f",
  "modified" : "2026-05-06T20:32:54Z",
  "published" : "2026-03-19T21:22:52Z",
  "aliases" : [ "CVE-2026-33397" ],
  "summary" : "Protocol-Relative URL Injection via Single Backslash Bypass in Angular SSR",
  "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",
  "severity" : [ {
    "type" : "CVSS_V3",
    "score" : "CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:N"
  }, {
    "type" : "CVSS_V4",
    "score" : "CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:N/SC:L/SI:L/SA:N"
  } ],
  "affected" : [ {
    "package" : {
      "ecosystem" : "npm",
      "name" : "@angular/ssr"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "22.0.0-next.0"
      }, {
        "fixed" : "22.0.0-next.2"
      } ]
    } ]
  }, {
    "package" : {
      "ecosystem" : "npm",
      "name" : "@angular/ssr"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "21.0.0-next.0"
      }, {
        "fixed" : "21.2.3"
      } ]
    } ]
  }, {
    "package" : {
      "ecosystem" : "npm",
      "name" : "@angular/ssr"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "20.0.0-next.0"
      }, {
        "fixed" : "20.3.21"
      } ]
    } ]
  } ],
  "references" : [ {
    "type" : "WEB",
    "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" : "WEB",
    "url" : "https://github.com/angular/angular-cli/pull/32771"
  }, {
    "type" : "ADVISORY",
    "url" : "https://github.com/advisories/GHSA-xh43-g2fq-wjrj"
  }, {
    "type" : "PACKAGE",
    "url" : "https://github.com/angular/angular-cli"
  } ],
  "database_specific" : {
    "cwe_ids" : [ "CWE-601" ],
    "severity" : "MODERATE",
    "github_reviewed" : true,
    "github_reviewed_at" : "2026-03-19T21:22:52Z",
    "nvd_published_at" : "2026-03-26T15:16:38Z"
  }
}