{"id":"CVE-2021-27516","aliases":["EUVD-2021-0686","GHSA-p6j9-7xhc-rhwp"],"status":"published","title":"URIjs Hostname spoofing via backslashes in URL","description":"URI.js (aka urijs) before 1.19.6 mishandles certain uses of backslash such as http:\\/ and interprets the URI as a relative path.","severity":{"version":"3.1","score":7.5,"vector":"CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:N/A:N","level":"high","assessor":"primary","from":"nvd"},"epss":{"day":"2026-11-12","score":0.02483,"percentile":0.8483,"model":"v2026.06.15"},"facts":{"in_kev":false,"known_exploited":false,"public_exploit":false,"fix_available":true},"cwes":["CWE-20"],"published":"2021-02-21T23:29:56Z","modified":"2026-10-20T10:47:52Z","fixes":[{"ecosystem":"npm","package":"urijs","fixed_in":["1.19.6"],"sources":["ghsa","osv"]}],"affected":[{"source":"ghsa","ecosystem":"npm","package":"urijs","ranges":[{"type":"ECOSYSTEM","events":[{"introduced":"0"},{"fixed":"1.19.6"}]}]},{"source":"osv","ecosystem":"npm","package":"urijs","ranges":[{"type":"SEMVER","events":[{"introduced":"0"},{"fixed":"1.19.6"}]}]}],"products":[{"vendor":"uri.js_project","product":"uri.js"}],"references":[{"url":"https://github.com/medialize/URI.js/commit/a1ad8bcbc39a4d136d7e252e76e957f3ece70839","tags":["x_refsource_MISC","x_transferred","Patch","Third Party Advisory","fix","web"]},{"url":"https://github.com/medialize/URI.js/releases/tag/v1.19.6","tags":["x_refsource_MISC","x_transferred","Third Party Advisory","advisory","web"]},{"url":"https://advisory.checkmarx.net/advisory/CX-2021-4305","tags":["x_refsource_MISC","x_transferred","Exploit","Third Party Advisory","evidence","web"]},{"url":"https://github.com/medialize/URI.js/security/advisories/GHSA-p6j9-7xhc-rhwp","tags":["web"]},{"url":"https://nvd.nist.gov/vuln/detail/CVE-2021-27516","tags":["advisory"]},{"url":"https://github.com/advisories/GHSA-p6j9-7xhc-rhwp"},{"url":"https://github.com/medialize/URI.js","tags":["package"]}],"sources":{"cve":[{"id":"CVE-2021-27516","description":"URI.js (aka urijs) before 1.19.6 mishandles certain uses of backslash such as http:\\/ and interprets the URI as a relative path.","status":"published","published":"2021-02-21T23:29:56Z","modified":"2024-08-03T21:26:09.101Z","updated_at":"2026-11-02T14:33:04.360272Z"}],"euvd":[{"id":"EUVD-2021-0686","description":"URI.js (aka urijs) before 1.19.6 mishandles certain uses of backslash such as http:\\/ and interprets the URI as a relative path.","status":"published","published":"2021-02-21T23:29:56Z","modified":"2024-08-03T21:26:09Z","updated_at":"2026-11-03T03:24:48.919164Z"}],"ghsa":[{"id":"GHSA-p6j9-7xhc-rhwp","title":"URIjs Hostname spoofing via backslashes in URL","description":"### Impact\nIf using affected versions to determine a URL's hostname, the hostname can be spoofed by using a backslash (`\\`) character as part of the scheme delimiter, e.g. `scheme:/\\hostname`. If the hostname is used in security decisions, the decision may be incorrect.\n\nDepending on library usage and attacker intent, impacts may include allow/block list bypasses, SSRF attacks, open redirects, or other undesired behavior.\n\nExample URL: `https:/\\expected-example.com/path`\nEscaped string: `https:/\\\\expected-example.com/path` (JavaScript strings must escape backslash)\n\nAffected versions incorrectly return no hostname. Patched versions correctly return `expected-example.com`. Patched versions match the behavior of other parsers which implement the [WHATWG URL specification](https://url.spec.whatwg.org/), including web browsers and [Node's built-in URL class](https://nodejs.org/api/url.html).\n\n### Patches\nVersion 1.19.6 is patched against all known payload variants.\n\n### References\nhttps://github.com/medialize/URI.js/releases/tag/v1.19.6 (fix for this particular bypass)\nhttps://github.com/medialize/URI.js/releases/tag/v1.19.4 (fix for related bypass)\nhttps://github.com/medialize/URI.js/releases/tag/v1.19.3 (fix for related bypass)\n[PR #233](https://github.com/medialize/URI.js/pull/233) (initial fix for backslash handling)\n\n### For more information\nIf you have any questions or comments about this advisory, open an issue in https://github.com/medialize/URI.js\n\n### Reporter credit\n[Yaniv Nizry](https://twitter.com/ynizry) from the CxSCA AppSec team at Checkmarx","status":"published","published":"2021-03-01T20:03:53Z","modified":"2023-12-08T05:05:18Z","cvss":[{"version":"3.1","score":7.5,"vector":"CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:N/A:N","level":"high"}],"cwes":["CWE-20"],"updated_at":"2026-11-02T14:37:24.593982Z"}],"nvd":[{"id":"CVE-2021-27516","description":"URI.js (aka urijs) before 1.19.6 mishandles certain uses of backslash such as http:\\/ and interprets the URI as a relative path.","status":"published","published":"2021-02-22T00:15:12.62Z","modified":"2026-06-17T03:45:00.637Z","cvss":[{"version":"3.1","score":7.5,"vector":"CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:N/A:N","level":"high","assessor":"primary"},{"version":"2.0","score":5,"vector":"AV:N/AC:L/Au:N/C:P/I:N/A:N","level":"medium","assessor":"primary"}],"updated_at":"2026-11-02T14:06:50.031227Z"}],"osv":[{"id":"CVE-2021-27516","description":"URI.js (aka urijs) before 1.19.6 mishandles certain uses of backslash such as http:\\/ and interprets the URI as a relative path.","status":"published","published":"2021-02-22T00:15:12.62Z","modified":"2026-10-20T10:47:52.406771441Z","cvss":[{"version":"3.1","score":7.5,"vector":"CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:N/A:N","level":"high"}],"updated_at":"2026-11-02T15:44:19.129331Z"},{"id":"GHSA-p6j9-7xhc-rhwp","title":"URIjs Hostname spoofing via backslashes in URL","description":"### Impact\nIf using affected versions to determine a URL's hostname, the hostname can be spoofed by using a backslash (`\\`) character as part of the scheme delimiter, e.g. `scheme:/\\hostname`. If the hostname is used in security decisions, the decision may be incorrect.\n\nDepending on library usage and attacker intent, impacts may include allow/block list bypasses, SSRF attacks, open redirects, or other undesired behavior.\n\nExample URL: `https:/\\expected-example.com/path`\nEscaped string: `https:/\\\\expected-example.com/path` (JavaScript strings must escape backslash)\n\nAffected versions incorrectly return no hostname. Patched versions correctly return `expected-example.com`. Patched versions match the behavior of other parsers which implement the [WHATWG URL specification](https://url.spec.whatwg.org/), including web browsers and [Node's built-in URL class](https://nodejs.org/api/url.html).\n\n### Patches\nVersion 1.19.6 is patched against all known payload variants.\n\n### References\nhttps://github.com/medialize/URI.js/releases/tag/v1.19.6 (fix for this particular bypass)\nhttps://github.com/medialize/URI.js/releases/tag/v1.19.4 (fix for related bypass)\nhttps://github.com/medialize/URI.js/releases/tag/v1.19.3 (fix for related bypass)\n[PR #233](https://github.com/medialize/URI.js/pull/233) (initial fix for backslash handling)\n\n### For more information\nIf you have any questions or comments about this advisory, open an issue in https://github.com/medialize/URI.js\n\n### Reporter credit\n[Yaniv Nizry](https://twitter.com/ynizry) from the CxSCA AppSec team at Checkmarx","status":"published","published":"2021-03-01T20:03:53Z","modified":"2023-11-08T04:05:24.794143Z","cvss":[{"version":"3.1","score":7.5,"vector":"CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:N/A:N","level":"high"}],"cwes":["CWE-20"],"updated_at":"2026-11-02T15:46:52.891208Z"}]}}