{"id":"CVE-2021-4138","url":"https://www.cyber-defence.io/tools/cve/CVE-2021-4138","generatedAt":"2026-10-27T17:45:02.348Z","title":"Improved Host header checks to reject requests not sent to a well-known local hostname or IP, or the server-specified hostname.","description":"Improved Host header checks to reject requests not sent to a well-known local hostname or IP, or the server-specified hostname.","published":"2022-05-02T23:15:08.000Z","lastModified":"2026-06-17T04:19:06.000Z","status":"Modified","sourceIdentifier":"security@mozilla.org","cvss":{"version":"3.1","score":5.3,"severity":"MEDIUM","vector":"CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:L/A:N"},"cwe":[],"affected":["mozilla/geckodriver"],"epss":{"score":0.00847,"percentile":0.57818,"date":"2026-10-26","history":[]},"kev":{"listed":false},"exploits":{"count":0,"verified":false,"firstPublished":null,"source":"Exploit-DB (https://gitlab.com/exploit-database/exploitdb)","entries":[]},"verdict":{"level":"low","text":"Lower severity and a low EPSS score (0.85%). Track it; it rarely justifies an emergency change on its own."},"changes":[],"references":[{"url":"https://bugzilla.mozilla.org/show_bug.cgi?id=1652612","tags":["Issue Tracking","Permissions Required","Third Party Advisory"],"source":"security@mozilla.org"},{"url":"https://github.com/mozilla/geckodriver/releases/tag/v0.30.0","tags":["Release Notes","Third Party Advisory"],"source":"security@mozilla.org"},{"url":"https://bugzilla.mozilla.org/show_bug.cgi?id=1652612","tags":["Issue Tracking","Permissions Required","Third Party Advisory"],"source":"af854a3a-2127-422b-91ae-364da2661108"},{"url":"https://github.com/mozilla/geckodriver/releases/tag/v0.30.0","tags":["Release Notes","Third Party Advisory"],"source":"af854a3a-2127-422b-91ae-364da2661108"}],"sources":{"nvd":"https://nvd.nist.gov/vuln/detail/CVE-2021-4138","epss":"https://www.first.org/epss/","kev":"https://www.cisa.gov/known-exploited-vulnerabilities-catalog","exploitdb":"https://www.exploit-db.com/"},"licence":"CC BY 4.0 — link back to the CVE Explorer if you publish the results; upstream data remains subject to its own terms."}