{"id":"CVE-2021-27516","aliases":["GHSA-p6j9-7xhc-rhwp"],"url":"https://o3.security/vulnerability/CVE-2021-27516","summary":"URIjs Hostname spoofing via backslashes in URL","details":"### 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","published":"2021-02-22T00:15:12.620Z","modified":"2026-10-20T10:47:52.406771441Z","cvss":{"score":7.5,"severity":"HIGH","vector":"CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:N/A:N"},"epss":null,"cisaKev":null,"exploitsKnown":1,"affectedPackages":[{"ecosystem":"npm","name":"urijs","fixedVersion":"1.19.6"}],"fix":{"url":"https://github.com/medialize/URI.js/commit/a1ad8bcbc39a4d136d7e252e76e957f3ece70839","label":"medialize/URI.js@a1ad8bc"},"references":[{"type":"ADVISORY","url":"https://github.com/medialize/URI.js/releases/tag/v1.19.6"},{"type":"FIX","url":"https://github.com/medialize/URI.js/commit/a1ad8bcbc39a4d136d7e252e76e957f3ece70839"},{"type":"EVIDENCE","url":"https://advisory.checkmarx.net/advisory/CX-2021-4305"},{"type":"WEB","url":"https://github.com/medialize/URI.js/security/advisories/GHSA-p6j9-7xhc-rhwp"},{"type":"ADVISORY","url":"https://nvd.nist.gov/vuln/detail/CVE-2021-27516"},{"type":"PACKAGE","url":"https://github.com/medialize/URI.js"}],"provenance":{"sources":["OSV.dev","FIRST.org (EPSS)"],"lastVerified":"2026-10-20T10:47:52.406771441Z"}}