{"id":"CVE-2026-5751","state":"PUBLISHED","title":"justhtml before 1.14.0 Mutation XSS via custom sanitization policies","description":"justhtml versions 1.13.0 and earlier contain a parser-differential / mutation cross-site scripting (mXSS) vulnerability when using a custom SanitizationPolicy that preserves foreign namespaces (e.g., drop_foreign_namespaces=False with allowlisted SVG/MathML elements or raw-text containers such as <style>). Specially crafted input can sanitize into markup that appears safe but becomes unsafe when re-parsed by a browser or another HTML parser, allowing markup injection. The default safe configuration (sanitize=True) is not affected. Fixed in 1.14.0.","publishedAt":"2026-08-23T13:34:10.547Z","updatedAt":"2026-08-24T19:03:33.747Z","severity":{"score":5.1,"label":"Medium","version":"4.0","vector":"CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:A/VC:N/VI:N/VA:N/SC:L/SI:L/SA:N","source":"VulnCheck"},"assigner":"VulnCheck","products":["EmilStenstrom justhtml"],"affected":[{"vendor":"EmilStenstrom","product":"justhtml","packageURL":"pkg:pypi/justhtml","defaultStatus":"unaffected","versions":[{"version":"0","status":"affected","lessThan":"1.14.0","versionType":"semver"},{"version":"1.14.0","status":"unaffected","versionType":"semver"}]}],"references":["https://github.com/EmilStenstrom/justhtml/security/advisories/GHSA-r758-8hxw-4845","https://www.vulncheck.com/advisories/justhtml-before-mutation-xss-via-custom-sanitization-policies"],"weaknesses":["CWE-79"],"ingestedAt":"2026-10-29T20:12:32.778Z","source":{"repository":"CVEProject/cvelistV5","path":"cves/2026/5xxx/CVE-2026-5751.json","commit":"556c9d3a8cd17e218bb8e452ba7cd619a384ff47"},"raw":{"dataType":"CVE_RECORD","dataVersion":"5.2","cveMetadata":{"cveId":"CVE-2026-5751","state":"PUBLISHED","datePublished":"2026-08-23T13:34:10.547Z","dateUpdated":"2026-08-24T19:03:33.747Z","assignerShortName":"VulnCheck","assignerOrgId":"83251b91-4cc7-4094-a5c7-464a1b83ea10","dateReserved":"2026-04-07T15:54:18.686Z"},"containers":{"cna":{"title":"justhtml before 1.14.0 Mutation XSS via custom sanitization policies","descriptions":[{"lang":"en","value":"justhtml versions 1.13.0 and earlier contain a parser-differential / mutation cross-site scripting (mXSS) vulnerability when using a custom SanitizationPolicy that preserves foreign namespaces (e.g., drop_foreign_namespaces=False with allowlisted SVG/MathML elements or raw-text containers such as <style>). Specially crafted input can sanitize into markup that appears safe but becomes unsafe when re-parsed by a browser or another HTML parser, allowing markup injection. The default safe configuration (sanitize=True) is not affected. Fixed in 1.14.0."}],"affected":[{"vendor":"EmilStenstrom","product":"justhtml","packageURL":"pkg:pypi/justhtml","defaultStatus":"unaffected","versions":[{"version":"0","status":"affected","lessThan":"1.14.0","versionType":"semver"},{"version":"1.14.0","status":"unaffected","versionType":"semver"}]}],"references":[{"url":"https://github.com/EmilStenstrom/justhtml/security/advisories/GHSA-r758-8hxw-4845","tags":["vendor-advisory"],"name":"GitHub Security Advisory (GHSA-r758-8hxw-4845)"},{"url":"https://www.vulncheck.com/advisories/justhtml-before-mutation-xss-via-custom-sanitization-policies","name":"VulnCheck Advisory: justhtml before 1.14.0 Mutation XSS via custom sanitization policies","tags":["third-party-advisory"]}],"metrics":[{"cvssV4_0":{"Automatable":"NOT_DEFINED","Recovery":"NOT_DEFINED","Safety":"NOT_DEFINED","attackComplexity":"LOW","attackRequirements":"NONE","attackVector":"NETWORK","baseScore":5.1,"baseSeverity":"MEDIUM","exploitMaturity":"NOT_DEFINED","privilegesRequired":"NONE","providerUrgency":"NOT_DEFINED","subAvailabilityImpact":"NONE","subConfidentialityImpact":"LOW","subIntegrityImpact":"LOW","userInteraction":"ACTIVE","valueDensity":"NOT_DEFINED","vectorString":"CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:A/VC:N/VI:N/VA:N/SC:L/SI:L/SA:N","version":"4.0","vulnAvailabilityImpact":"NONE","vulnConfidentialityImpact":"NONE","vulnIntegrityImpact":"NONE","vulnerabilityResponseEffort":"NOT_DEFINED"},"format":"CVSS"},{"cvssV3_1":{"attackComplexity":"LOW","attackVector":"NETWORK","availabilityImpact":"NONE","baseScore":6.1,"baseSeverity":"MEDIUM","confidentialityImpact":"LOW","integrityImpact":"LOW","privilegesRequired":"NONE","scope":"CHANGED","userInteraction":"REQUIRED","vectorString":"CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:N","version":"3.1"},"format":"CVSS"}],"problemTypes":[{"descriptions":[{"cweId":"CWE-79","lang":"en","description":"Improper Neutralization of Input During Web Page Generation ('Cross-site Scripting')","type":"CWE"}]}],"providerMetadata":{"shortName":"VulnCheck","orgId":"83251b91-4cc7-4094-a5c7-464a1b83ea10","dateUpdated":"2026-08-23T13:34:10.547Z"},"datePublic":"2026-04-05T00:00:00.000Z","credits":[{"lang":"en","value":"EmilStenstrom","type":"coordinator"}],"x_generator":{"engine":"vulncheck-endgame"}},"adp":[{"title":"CISA ADP Vulnrichment","metrics":[{"other":{"type":"ssvc","content":{"timestamp":"2026-08-24T19:03:25.322496Z","id":"CVE-2026-5751","options":[{"Exploitation":"none"},{"Automatable":"no"},{"Technical Impact":"partial"}],"role":"CISA Coordinator","version":"2.0.3"}}}],"providerMetadata":{"shortName":"CISA-ADP","orgId":"134c704f-9b21-4f2e-91b3-4a467353bcc0","dateUpdated":"2026-08-24T19:03:33.747Z"}}]}}}