{"id":"CVE-2026-86205","aliases":["EUVD-2026-72105","GHSA-fp4x-ggrf-wmc6","GHSA-vhx4-2r84-67h4"],"status":"published","title":"h3 before 2.0.1-rc.18 Open Redirect via redirectBack()","description":"h3 versions before 2.0.1-rc.18 contain an open redirect vulnerability in the redirectBack() utility that fails to sanitize protocol-relative paths in the Referer header pathname. Attackers can craft a same-origin URL with a double-slash path segment that passes origin validation but produces a Location header interpreted by browsers as a protocol-relative redirect to an external domain.","severity":{"version":"4.0","score":5.3,"vector":"CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:P/VC:L/VI:L/VA:N/SC:N/SI:N/SA:N","level":"medium","assessor":"cna","from":"cve"},"epss":{"day":"2026-11-09","score":0.00271,"percentile":0.19436,"model":"v2026.06.15"},"facts":{"in_kev":false,"known_exploited":false,"public_exploit":false,"fix_available":true},"cwes":["CWE-601"],"published":"2026-03-23T21:48:24Z","modified":"2026-10-28T02:30:29Z","fixes":[{"ecosystem":"npm","package":"h3","fixed_in":["2.0.1-rc.18"],"sources":["ghsa","osv"]}],"affected":[{"source":"ghsa","ecosystem":"npm","package":"h3","ranges":[{"type":"ECOSYSTEM","events":[{"introduced":"2.0.1-rc.17"},{"last_affected":"2.0.1-rc.17"},{"fixed":"2.0.1-rc.18"}]}],"versions":["2.0.1-rc.17"]},{"source":"osv","ecosystem":"npm","package":"h3","ranges":[{"type":"SEMVER","events":[{"introduced":"2.0.1-rc.17"},{"fixed":"2.0.1-rc.18"}]}],"versions":["2.0.1-rc.17"]}],"products":[{"vendor":"h3js","product":"h3"}],"references":[{"url":"https://github.com/h3js/h3/security/advisories/GHSA-fp4x-ggrf-wmc6","tags":["vendor-advisory","exploit","advisory","web"]},{"url":"https://www.vulncheck.com/advisories/h3-before-2.0.1-rc.18-open-redirect-via-redirectback","tags":["third-party-advisory","advisory"]},{"url":"https://github.com/h3js/h3/commit/459a1c6593365b0810e9c502df7c3e82837321d7","tags":["web"]},{"url":"https://github.com/h3js/h3/releases/tag/v2.0.1-rc.18","tags":["web"]},{"url":"https://github.com/advisories/GHSA-fp4x-ggrf-wmc6"},{"url":"https://nvd.nist.gov/vuln/detail/CVE-2026-86205","tags":["advisory"]},{"url":"https://github.com/advisories/GHSA-vhx4-2r84-67h4"},{"url":"https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/86xxx/CVE-2026-86205.json","tags":["advisory"]},{"url":"https://github.com/h3js/h3","tags":["package"]}],"sources":{"cve":[{"id":"CVE-2026-86205","title":"h3 before 2.0.1-rc.18 Open Redirect via redirectBack()","description":"h3 versions before 2.0.1-rc.18 contain an open redirect vulnerability in the redirectBack() utility that fails to sanitize protocol-relative paths in the Referer header pathname. Attackers can craft a same-origin URL with a double-slash path segment that passes origin validation but produces a Location header interpreted by browsers as a protocol-relative redirect to an external domain.","status":"published","published":"2026-09-06T12:00:25.53Z","modified":"2026-09-08T13:28:55.791Z","cvss":[{"version":"4.0","score":5.3,"vector":"CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:P/VC:L/VI:L/VA:N/SC:N/SI:N/SA:N","level":"medium","assessor":"cna"},{"version":"3.1","score":5.4,"vector":"CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:L/I:L/A:N","level":"medium","assessor":"cna"}],"cwes":["CWE-601"],"updated_at":"2026-11-02T13:49:31.546614Z"}],"euvd":[{"id":"EUVD-2026-72105","description":"h3 versions before 2.0.1-rc.18 contain an open redirect vulnerability in the redirectBack() utility that fails to sanitize protocol-relative paths in the Referer header pathname. Attackers can craft a same-origin URL with a double-slash path segment that passes origin validation but produces a Location header interpreted by browsers as a protocol-relative redirect to an external domain.","status":"published","published":"2026-09-06T12:00:25Z","modified":"2026-09-08T13:28:55Z","cvss":[{"version":"4.0","score":5.3,"vector":"CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:P/VC:L/VI:L/VA:N/SC:N/SI:N/SA:N","level":"medium"}],"updated_at":"2026-11-02T17:28:29.861344Z"}],"ghsa":[{"id":"GHSA-fp4x-ggrf-wmc6","title":"H3 has an Open Redirect via Protocol-Relative Path in redirectBack() Referer Validation","description":"## Summary\n\nThe `redirectBack()` utility in h3 validates that the `Referer` header shares the same origin as the request before using its pathname as the redirect `Location`. However, the pathname is not sanitized for protocol-relative paths (starting with `//`). An attacker can craft a same-origin URL with a double-slash path segment that passes the origin check but produces a `Location` header interpreted by browsers as a protocol-relative redirect to an external domain.\n\n## Details\n\nThe vulnerable code is in `src/utils/response.ts:89-97`:\n\n```typescript\nexport function redirectBack(\n  event: H3Event,\n  opts: { fallback?: string; status?: number; allowQuery?: boolean } = {},\n): HTTPResponse {\n  const referer = event.req.headers.get(\"referer\");\n  let location = opts.fallback ?? \"/\";\n  if (referer \u0026\u0026 URL.canParse(referer)) {\n    const refererURL = new URL(referer);\n    if (refererURL.origin === event.url.origin) {\n      // BUG: pathname can be \"//evil.com/path\" which browsers interpret\n      // as a protocol-relative URL\n      location = refererURL.pathname + (opts.allowQuery ? refererURL.search : \"\");\n    }\n  }\n  return redirect(location, opts.status);\n}\n```\n\nThe root cause is a discrepancy between how the WHATWG URL parser and browsers handle double-slash paths:\n\n1. `new URL(\"http://target.com//evil.com/path\").origin` → `\"http://target.com\"` — origin check **passes**\n2. `new URL(\"http://target.com//evil.com/path\").pathname` → `\"//evil.com/path\"` — extracted as redirect location\n3. Browser receives `Location: //evil.com/path` → interprets as protocol-relative URL → **redirects to `evil.com`**\n\n**Attack scenario:** The attacker shares a link like `http://target.com//evil.com/page`. If the target application has catch-all routes (common in SPAs built with h3/Nitro), the app serves its page at that URL. When the user navigates to an endpoint calling `redirectBack()`, the browser sends `Referer: http://target.com//evil.com/page`. The origin check passes, and the user is redirected to `evil.com`, which can host a phishing page mimicking the target.\n\n## PoC\n\n```bash\n# 1. Create a minimal h3 app with redirectBack\ncat \u003e /tmp/h3-redirect-poc.ts \u003c\u003c 'SCRIPT'\nimport { H3, redirectBack } from \"h3\";\n\nconst app = new H3();\napp.post(\"/submit\", (event) =\u003e redirectBack(event));\n\nconst res = await app.fetch(new Request(\"http://localhost/submit\", {\n  method: \"POST\",\n  headers: { referer: \"http://localhost//evil.com/steal\" }\n}));\n\nconsole.log(\"Status:\", res.status);\nconsole.log(\"Location:\", res.headers.get(\"location\"));\n// Expected: a same-origin path\n// Actual: \"//evil.com/steal\" — protocol-relative redirect to evil.com\nSCRIPT\n\n# 2. Verify URL parsing behavior\nnode -e \"\nconst u = new URL('http://localhost//evil.com/steal');\nconsole.log('origin:', u.origin);         // http://localhost\nconsole.log('pathname:', u.pathname);     // //evil.com/steal\nconsole.log('origin matches localhost:', u.origin === 'http://localhost');  // true\n\"\n# Output:\n# origin: http://localhost\n# pathname: //evil.com/steal\n# origin matches localhost: true\n```\n\n## Impact\n\nAn attacker can redirect users from a trusted application to an attacker-controlled domain. This enables:\n\n- **Credential phishing**: Redirect to a lookalike login page to harvest credentials\n- **OAuth token theft**: In OAuth flows using `redirectBack()`, steal authorization codes by redirecting to an attacker's callback\n- **Trust exploitation**: Users see the initial link points to the trusted domain, lowering suspicion\n\nThe vulnerability requires no authentication and affects any endpoint using `redirectBack()`.\n\n## Recommended Fix\n\nSanitize the extracted pathname to prevent protocol-relative URLs. In `src/utils/response.ts`, after extracting the pathname from the referer:\n\n```typescript\nexport function redirectBack(\n  event: H3Event,\n  opts: { fallback?: string; status?: number; allowQuery?: boolean } = {},\n): HTTPResponse {\n  const referer = event.req.headers.get(\"referer\");\n  let location = opts.fallback ?? \"/\";\n  if (referer \u0026\u0026 URL.canParse(referer)) {\n    const refererURL = new URL(referer);\n    if (refererURL.origin === event.url.origin) {\n      let pathname = refererURL.pathname;\n      // Prevent protocol-relative open redirect (e.g., \"//evil.com\")\n      if (pathname.startsWith(\"//\")) {\n        pathname = \"/\" + pathname.replace(/^\\/+/, \"\");\n      }\n      location = pathname + (opts.allowQuery ? refererURL.search : \"\");\n    }\n  }\n  return redirect(location, opts.status);\n}\n```","status":"published","published":"2026-03-23T21:48:24Z","modified":"2026-03-23T21:48:24Z","cvss":[{"version":"3.1","score":5.4,"vector":"CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:L/I:L/A:N","level":"medium"}],"cwes":["CWE-601"],"updated_at":"2026-11-02T14:39:40.633038Z"},{"id":"GHSA-vhx4-2r84-67h4","title":"h3 versions before 2.0.1-rc.18 contain an open redirect vulnerability in the redirectBack()...","description":"h3 versions before 2.0.1-rc.18 contain an open redirect vulnerability in the redirectBack() utility that fails to sanitize protocol-relative paths in the Referer header pathname. Attackers can craft a same-origin URL with a double-slash path segment that passes origin validation but produces a Location header interpreted by browsers as a protocol-relative redirect to an external domain.","status":"published","published":"2026-09-06T12:30:23Z","modified":"2026-09-06T12:30:30Z","cvss":[{"version":"4.0","score":5.3,"vector":"CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:P/VC:L/VI:L/VA:N/SC:N/SI:N/SA:N/E:X/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/MVI:X/MVA:X/MSC:X/MSI:X/MSA:X/S:X/AU:X/R:X/V:X/RE:X/U:X","level":"medium"},{"version":"3.1","score":5.4,"vector":"CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:L/I:L/A:N","level":"medium"}],"cwes":["CWE-601"],"updated_at":"2026-11-02T15:11:25.773648Z"}],"nvd":[{"id":"CVE-2026-86205","description":"h3 versions before 2.0.1-rc.18 contain an open redirect vulnerability in the redirectBack() utility that fails to sanitize protocol-relative paths in the Referer header pathname. Attackers can craft a same-origin URL with a double-slash path segment that passes origin validation but produces a Location header interpreted by browsers as a protocol-relative redirect to an external domain.","status":"published","published":"2026-09-06T12:17:15.277Z","modified":"2026-09-08T20:00:18.87Z","cvss":[{"version":"4.0","score":5.3,"vector":"CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:P/VC:L/VI:L/VA:N/SC:N/SI:N/SA:N/E:X/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/MVI:X/MVA:X/MSC:X/MSI:X/MSA:X/S:X/AU:X/R:X/V:X/RE:X/U:X","level":"medium","assessor":"secondary"},{"version":"3.1","score":5.4,"vector":"CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:L/I:L/A:N","level":"medium","assessor":"secondary"}],"cwes":["CWE-601"],"updated_at":"2026-11-02T14:12:44.082631Z"}],"osv":[{"id":"CVE-2026-86205","title":"h3 before 2.0.1-rc.18 Open Redirect via redirectBack()","description":"h3 versions before 2.0.1-rc.18 contain an open redirect vulnerability in the redirectBack() utility that fails to sanitize protocol-relative paths in the Referer header pathname. Attackers can craft a same-origin URL with a double-slash path segment that passes origin validation but produces a Location header interpreted by browsers as a protocol-relative redirect to an external domain.","status":"published","published":"2026-09-06T12:00:25.53Z","modified":"2026-10-28T02:30:29.280312274Z","cvss":[{"version":"4.0","score":5.3,"vector":"CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:P/VC:L/VI:L/VA:N/SC:N/SI:N/SA:N","level":"medium"}],"cwes":["CWE-601"],"updated_at":"2026-11-02T15:45:18.825071Z"},{"id":"GHSA-fp4x-ggrf-wmc6","title":"H3 has an Open Redirect via Protocol-Relative Path in redirectBack() Referer Validation","description":"## Summary\n\nThe `redirectBack()` utility in h3 validates that the `Referer` header shares the same origin as the request before using its pathname as the redirect `Location`. However, the pathname is not sanitized for protocol-relative paths (starting with `//`). An attacker can craft a same-origin URL with a double-slash path segment that passes the origin check but produces a `Location` header interpreted by browsers as a protocol-relative redirect to an external domain.\n\n## Details\n\nThe vulnerable code is in `src/utils/response.ts:89-97`:\n\n```typescript\nexport function redirectBack(\n  event: H3Event,\n  opts: { fallback?: string; status?: number; allowQuery?: boolean } = {},\n): HTTPResponse {\n  const referer = event.req.headers.get(\"referer\");\n  let location = opts.fallback ?? \"/\";\n  if (referer \u0026\u0026 URL.canParse(referer)) {\n    const refererURL = new URL(referer);\n    if (refererURL.origin === event.url.origin) {\n      // BUG: pathname can be \"//evil.com/path\" which browsers interpret\n      // as a protocol-relative URL\n      location = refererURL.pathname + (opts.allowQuery ? refererURL.search : \"\");\n    }\n  }\n  return redirect(location, opts.status);\n}\n```\n\nThe root cause is a discrepancy between how the WHATWG URL parser and browsers handle double-slash paths:\n\n1. `new URL(\"http://target.com//evil.com/path\").origin` → `\"http://target.com\"` — origin check **passes**\n2. `new URL(\"http://target.com//evil.com/path\").pathname` → `\"//evil.com/path\"` — extracted as redirect location\n3. Browser receives `Location: //evil.com/path` → interprets as protocol-relative URL → **redirects to `evil.com`**\n\n**Attack scenario:** The attacker shares a link like `http://target.com//evil.com/page`. If the target application has catch-all routes (common in SPAs built with h3/Nitro), the app serves its page at that URL. When the user navigates to an endpoint calling `redirectBack()`, the browser sends `Referer: http://target.com//evil.com/page`. The origin check passes, and the user is redirected to `evil.com`, which can host a phishing page mimicking the target.\n\n## PoC\n\n```bash\n# 1. Create a minimal h3 app with redirectBack\ncat \u003e /tmp/h3-redirect-poc.ts \u003c\u003c 'SCRIPT'\nimport { H3, redirectBack } from \"h3\";\n\nconst app = new H3();\napp.post(\"/submit\", (event) =\u003e redirectBack(event));\n\nconst res = await app.fetch(new Request(\"http://localhost/submit\", {\n  method: \"POST\",\n  headers: { referer: \"http://localhost//evil.com/steal\" }\n}));\n\nconsole.log(\"Status:\", res.status);\nconsole.log(\"Location:\", res.headers.get(\"location\"));\n// Expected: a same-origin path\n// Actual: \"//evil.com/steal\" — protocol-relative redirect to evil.com\nSCRIPT\n\n# 2. Verify URL parsing behavior\nnode -e \"\nconst u = new URL('http://localhost//evil.com/steal');\nconsole.log('origin:', u.origin);         // http://localhost\nconsole.log('pathname:', u.pathname);     // //evil.com/steal\nconsole.log('origin matches localhost:', u.origin === 'http://localhost');  // true\n\"\n# Output:\n# origin: http://localhost\n# pathname: //evil.com/steal\n# origin matches localhost: true\n```\n\n## Impact\n\nAn attacker can redirect users from a trusted application to an attacker-controlled domain. This enables:\n\n- **Credential phishing**: Redirect to a lookalike login page to harvest credentials\n- **OAuth token theft**: In OAuth flows using `redirectBack()`, steal authorization codes by redirecting to an attacker's callback\n- **Trust exploitation**: Users see the initial link points to the trusted domain, lowering suspicion\n\nThe vulnerability requires no authentication and affects any endpoint using `redirectBack()`.\n\n## Recommended Fix\n\nSanitize the extracted pathname to prevent protocol-relative URLs. In `src/utils/response.ts`, after extracting the pathname from the referer:\n\n```typescript\nexport function redirectBack(\n  event: H3Event,\n  opts: { fallback?: string; status?: number; allowQuery?: boolean } = {},\n): HTTPResponse {\n  const referer = event.req.headers.get(\"referer\");\n  let location = opts.fallback ?? \"/\";\n  if (referer \u0026\u0026 URL.canParse(referer)) {\n    const refererURL = new URL(referer);\n    if (refererURL.origin === event.url.origin) {\n      let pathname = refererURL.pathname;\n      // Prevent protocol-relative open redirect (e.g., \"//evil.com\")\n      if (pathname.startsWith(\"//\")) {\n        pathname = \"/\" + pathname.replace(/^\\/+/, \"\");\n      }\n      location = pathname + (opts.allowQuery ? refererURL.search : \"\");\n    }\n  }\n  return redirect(location, opts.status);\n}\n```","status":"published","published":"2026-03-23T21:48:24Z","modified":"2026-09-10T03:51:00.303376066Z","cvss":[{"version":"3.1","score":5.4,"vector":"CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:L/I:L/A:N","level":"medium"}],"cwes":["CWE-601"],"updated_at":"2026-11-02T15:46:46.760785Z"}]}}