{"api_version":"1","generated_at":"2026-08-25T04:49:50+00:00","cve":"CVE-2026-54743","urls":{"html":"https://cve.report/CVE-2026-54743","api":"https://cve.report/api/cve/CVE-2026-54743.json","docs":"https://cve.report/api","cve_org":"https://www.cve.org/CVERecord?id=CVE-2026-54743","nvd":"https://nvd.nist.gov/vuln/detail/CVE-2026-54743"},"summary":{"title":"Lemmy: Stored XSS via markdown image alt-text in lemmy-ui html5-embed","description":"Lemmy is a link aggregator and forum for the fediverse. Prior to lemmy-ui 0.19.19-beta.1, LemmyNet/lemmy-ui renders Markdown in src/shared/markdown.ts for post bodies, comment bodies, private messages, and community and site sidebars through mdToHtml, which returns a raw __html object that Inferno injects without a sanitizer pass. setupMarkdown configures html as false but registers markdown-it-html5-embed@1.0.0 with useImageSyntax enabled, so an image targeting video media becomes a video element whose fallback incorporates the image alt text through unescaped string replacement. The html setting does not apply to plugin-generated output, allowing crafted alt text to reach the DOM as live HTML in contexts that do not use mdToHtmlNoImages. An approved member or a remote federated instance can store such content, and a viewer who renders it may execute JavaScript in the lemmy-ui origin, exposing the viewer's session and authenticated actions. The advisory notes that Content Security Policy prevents the described exploit in production, but also states that the tested default self-hosted deployment serves no Content-Security-Policy. This issue is fixed in lemmy-ui version 0.19.19-beta.1.","state":"PUBLISHED","assigner":"GitHub_M","published_at":"2026-08-19 21:16:58","updated_at":"2026-08-21 19:17:03"},"problem_types":["CWE-79","CWE-79 CWE-79: Improper Neutralization of Input During Web Page Generation ('Cross-site Scripting')"],"metrics":[{"version":"4.0","source":"security-advisories@github.com","type":"Secondary","score":"6.4","severity":"MEDIUM","vector":"CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:P/VC:N/VI:N/VA:N/SC:H/SI:H/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","data":{"version":"4.0","vectorString":"CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:P/VC:N/VI:N/VA:N/SC:H/SI:H/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","baseScore":6.4,"baseSeverity":"MEDIUM","attackVector":"NETWORK","attackComplexity":"LOW","attackRequirements":"NONE","privilegesRequired":"NONE","userInteraction":"PASSIVE","vulnConfidentialityImpact":"NONE","vulnIntegrityImpact":"NONE","vulnAvailabilityImpact":"NONE","subConfidentialityImpact":"HIGH","subIntegrityImpact":"HIGH","subAvailabilityImpact":"NONE","exploitMaturity":"NOT_DEFINED","confidentialityRequirement":"NOT_DEFINED","integrityRequirement":"NOT_DEFINED","availabilityRequirement":"NOT_DEFINED","modifiedAttackVector":"NOT_DEFINED","modifiedAttackComplexity":"NOT_DEFINED","modifiedAttackRequirements":"NOT_DEFINED","modifiedPrivilegesRequired":"NOT_DEFINED","modifiedUserInteraction":"NOT_DEFINED","modifiedVulnConfidentialityImpact":"NOT_DEFINED","modifiedVulnIntegrityImpact":"NOT_DEFINED","modifiedVulnAvailabilityImpact":"NOT_DEFINED","modifiedSubConfidentialityImpact":"NOT_DEFINED","modifiedSubIntegrityImpact":"NOT_DEFINED","modifiedSubAvailabilityImpact":"NOT_DEFINED","Safety":"NOT_DEFINED","Automatable":"NOT_DEFINED","Recovery":"NOT_DEFINED","valueDensity":"NOT_DEFINED","vulnerabilityResponseEffort":"NOT_DEFINED","providerUrgency":"NOT_DEFINED"}},{"version":"4.0","source":"CNA","type":"DECLARED","score":"6.4","severity":"MEDIUM","vector":"CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:P/VC:N/VI:N/VA:N/SC:H/SI:H/SA:N","data":{"attackComplexity":"LOW","attackRequirements":"NONE","attackVector":"NETWORK","baseScore":6.4,"baseSeverity":"MEDIUM","privilegesRequired":"NONE","subAvailabilityImpact":"NONE","subConfidentialityImpact":"HIGH","subIntegrityImpact":"HIGH","userInteraction":"PASSIVE","vectorString":"CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:P/VC:N/VI:N/VA:N/SC:H/SI:H/SA:N","version":"4.0","vulnAvailabilityImpact":"NONE","vulnConfidentialityImpact":"NONE","vulnIntegrityImpact":"NONE"}}],"references":[{"url":"https://github.com/LemmyNet/lemmy/security/advisories/GHSA-2g66-9fr3-ppwj","name":"https://github.com/LemmyNet/lemmy/security/advisories/GHSA-2g66-9fr3-ppwj","refsource":"134c704f-9b21-4f2e-91b3-4a467353bcc0","tags":[],"title":"","mime":"","httpstatus":"","archivestatus":"0"},{"url":"https://github.com/LemmyNet/lemmy/releases/tag/0.19.19-beta.1","name":"https://github.com/LemmyNet/lemmy/releases/tag/0.19.19-beta.1","refsource":"security-advisories@github.com","tags":[],"title":"","mime":"","httpstatus":"","archivestatus":"0"},{"url":"https://github.com/LemmyNet/lemmy-ui/commit/81a3fe07f2a5acc11188320534aa6bf937ebf7ff","name":"https://github.com/LemmyNet/lemmy-ui/commit/81a3fe07f2a5acc11188320534aa6bf937ebf7ff","refsource":"security-advisories@github.com","tags":[],"title":"","mime":"","httpstatus":"","archivestatus":"0"},{"url":"https://github.com/LemmyNet/lemmy-ui/pull/4213","name":"https://github.com/LemmyNet/lemmy-ui/pull/4213","refsource":"security-advisories@github.com","tags":[],"title":"","mime":"","httpstatus":"","archivestatus":"0"},{"url":"https://www.cve.org/CVERecord?id=CVE-2026-54743","name":"CVE Program record","refsource":"CVE.ORG","tags":["canonical"]},{"url":"https://nvd.nist.gov/vuln/detail/CVE-2026-54743","name":"NVD vulnerability detail","refsource":"NVD","tags":["canonical","analysis"]}],"affected":[{"source":"CNA","vendor":"LemmyNet","product":"lemmy","version":"affected < 0.19.19-beta.1","platforms":[]}],"timeline":[],"solutions":[],"workarounds":[],"exploits":[],"credits":[],"nvd_cpes":[],"vendor_comments":[],"enrichments":{"kev":null,"epss":{"cve_year":"2026","cve_id":"54743","cve":"CVE-2026-54743","epss":"0.004010000","percentile":"0.329610000","score_date":"2026-08-24","updated_at":"2026-08-25 00:11:33"},"legacy_qids":[]},"source_records":{"cve_program":{"containers":{"adp":[{"metrics":[{"other":{"content":{"id":"CVE-2026-54743","options":[{"Exploitation":"poc"},{"Automatable":"no"},{"Technical Impact":"partial"}],"role":"CISA Coordinator","timestamp":"2026-08-21T18:16:39.227018Z","version":"2.0.3"},"type":"ssvc"}}],"providerMetadata":{"dateUpdated":"2026-08-21T18:17:01.041Z","orgId":"134c704f-9b21-4f2e-91b3-4a467353bcc0","shortName":"CISA-ADP"},"references":[{"tags":["exploit"],"url":"https://github.com/LemmyNet/lemmy/security/advisories/GHSA-2g66-9fr3-ppwj"}],"title":"CISA ADP Vulnrichment"}],"cna":{"affected":[{"product":"lemmy","vendor":"LemmyNet","versions":[{"status":"affected","version":"< 0.19.19-beta.1"}]}],"descriptions":[{"lang":"en","value":"Lemmy is a link aggregator and forum for the fediverse. Prior to lemmy-ui 0.19.19-beta.1, LemmyNet/lemmy-ui renders Markdown in src/shared/markdown.ts for post bodies, comment bodies, private messages, and community and site sidebars through mdToHtml, which returns a raw __html object that Inferno injects without a sanitizer pass. setupMarkdown configures html as false but registers markdown-it-html5-embed@1.0.0 with useImageSyntax enabled, so an image targeting video media becomes a video element whose fallback incorporates the image alt text through unescaped string replacement. The html setting does not apply to plugin-generated output, allowing crafted alt text to reach the DOM as live HTML in contexts that do not use mdToHtmlNoImages. An approved member or a remote federated instance can store such content, and a viewer who renders it may execute JavaScript in the lemmy-ui origin, exposing the viewer's session and authenticated actions. The advisory notes that Content Security Policy prevents the described exploit in production, but also states that the tested default self-hosted deployment serves no Content-Security-Policy. This issue is fixed in lemmy-ui version 0.19.19-beta.1."}],"metrics":[{"cvssV4_0":{"attackComplexity":"LOW","attackRequirements":"NONE","attackVector":"NETWORK","baseScore":6.4,"baseSeverity":"MEDIUM","privilegesRequired":"NONE","subAvailabilityImpact":"NONE","subConfidentialityImpact":"HIGH","subIntegrityImpact":"HIGH","userInteraction":"PASSIVE","vectorString":"CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:P/VC:N/VI:N/VA:N/SC:H/SI:H/SA:N","version":"4.0","vulnAvailabilityImpact":"NONE","vulnConfidentialityImpact":"NONE","vulnIntegrityImpact":"NONE"}}],"problemTypes":[{"descriptions":[{"cweId":"CWE-79","description":"CWE-79: Improper Neutralization of Input During Web Page Generation ('Cross-site Scripting')","lang":"en","type":"CWE"}]}],"providerMetadata":{"dateUpdated":"2026-08-19T20:18:43.124Z","orgId":"a0819718-46f1-4df5-94e2-005712e83aaa","shortName":"GitHub_M"},"references":[{"name":"https://github.com/LemmyNet/lemmy/security/advisories/GHSA-2g66-9fr3-ppwj","tags":["x_refsource_CONFIRM"],"url":"https://github.com/LemmyNet/lemmy/security/advisories/GHSA-2g66-9fr3-ppwj"},{"name":"https://github.com/LemmyNet/lemmy-ui/pull/4213","tags":["x_refsource_MISC"],"url":"https://github.com/LemmyNet/lemmy-ui/pull/4213"},{"name":"https://github.com/LemmyNet/lemmy-ui/commit/81a3fe07f2a5acc11188320534aa6bf937ebf7ff","tags":["x_refsource_MISC"],"url":"https://github.com/LemmyNet/lemmy-ui/commit/81a3fe07f2a5acc11188320534aa6bf937ebf7ff"},{"name":"https://github.com/LemmyNet/lemmy/releases/tag/0.19.19-beta.1","tags":["x_refsource_MISC"],"url":"https://github.com/LemmyNet/lemmy/releases/tag/0.19.19-beta.1"}],"source":{"advisory":"GHSA-2g66-9fr3-ppwj","discovery":"UNKNOWN"},"title":"Lemmy: Stored XSS via markdown image alt-text in lemmy-ui html5-embed"}},"cveMetadata":{"assignerOrgId":"a0819718-46f1-4df5-94e2-005712e83aaa","assignerShortName":"GitHub_M","cveId":"CVE-2026-54743","datePublished":"2026-08-19T20:18:43.124Z","dateReserved":"2026-06-15T23:12:41.964Z","dateUpdated":"2026-08-21T18:17:01.041Z","state":"PUBLISHED"},"dataType":"CVE_RECORD","dataVersion":"5.2"},"nvd":{"publishedDate":"2026-08-19 21:16:58","lastModifiedDate":"2026-08-21 19:17:03","problem_types":["CWE-79","CWE-79 CWE-79: Improper Neutralization of Input During Web Page Generation ('Cross-site Scripting')"],"metrics":{"cvssMetricV40":[{"source":"security-advisories@github.com","type":"Secondary","cvssData":{"version":"4.0","vectorString":"CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:P/VC:N/VI:N/VA:N/SC:H/SI:H/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","baseScore":6.4,"baseSeverity":"MEDIUM","attackVector":"NETWORK","attackComplexity":"LOW","attackRequirements":"NONE","privilegesRequired":"NONE","userInteraction":"PASSIVE","vulnConfidentialityImpact":"NONE","vulnIntegrityImpact":"NONE","vulnAvailabilityImpact":"NONE","subConfidentialityImpact":"HIGH","subIntegrityImpact":"HIGH","subAvailabilityImpact":"NONE","exploitMaturity":"NOT_DEFINED","confidentialityRequirement":"NOT_DEFINED","integrityRequirement":"NOT_DEFINED","availabilityRequirement":"NOT_DEFINED","modifiedAttackVector":"NOT_DEFINED","modifiedAttackComplexity":"NOT_DEFINED","modifiedAttackRequirements":"NOT_DEFINED","modifiedPrivilegesRequired":"NOT_DEFINED","modifiedUserInteraction":"NOT_DEFINED","modifiedVulnConfidentialityImpact":"NOT_DEFINED","modifiedVulnIntegrityImpact":"NOT_DEFINED","modifiedVulnAvailabilityImpact":"NOT_DEFINED","modifiedSubConfidentialityImpact":"NOT_DEFINED","modifiedSubIntegrityImpact":"NOT_DEFINED","modifiedSubAvailabilityImpact":"NOT_DEFINED","Safety":"NOT_DEFINED","Automatable":"NOT_DEFINED","Recovery":"NOT_DEFINED","valueDensity":"NOT_DEFINED","vulnerabilityResponseEffort":"NOT_DEFINED","providerUrgency":"NOT_DEFINED"}}],"ssvcV203":[{"source":"134c704f-9b21-4f2e-91b3-4a467353bcc0","ssvcData":{"timestamp":"2026-08-21T18:16:39.227018Z","id":"CVE-2026-54743","options":[{"exploitation":"poc"},{"automatable":"no"},{"technicalImpact":"partial"}],"role":"CISA Coordinator","version":"2.0.3"}}]},"configurations":[]},"legacy_mitre":{"record":{"CveYear":"2026","CveId":"54743","Ordinal":"1","Title":"Lemmy: Stored XSS via markdown image alt-text in lemmy-ui html5-","CVE":"CVE-2026-54743","Year":"2026"},"notes":[{"CveYear":"2026","CveId":"54743","Ordinal":"1","NoteData":"Lemmy is a link aggregator and forum for the fediverse. Prior to lemmy-ui 0.19.19-beta.1, LemmyNet/lemmy-ui renders Markdown in src/shared/markdown.ts for post bodies, comment bodies, private messages, and community and site sidebars through mdToHtml, which returns a raw __html object that Inferno injects without a sanitizer pass. setupMarkdown configures html as false but registers markdown-it-html5-embed@1.0.0 with useImageSyntax enabled, so an image targeting video media becomes a video element whose fallback incorporates the image alt text through unescaped string replacement. The html setting does not apply to plugin-generated output, allowing crafted alt text to reach the DOM as live HTML in contexts that do not use mdToHtmlNoImages. An approved member or a remote federated instance can store such content, and a viewer who renders it may execute JavaScript in the lemmy-ui origin, exposing the viewer's session and authenticated actions. The advisory notes that Content Security Policy prevents the described exploit in production, but also states that the tested default self-hosted deployment serves no Content-Security-Policy. This issue is fixed in lemmy-ui version 0.19.19-beta.1.","Type":"Description","Title":"Lemmy: Stored XSS via markdown image alt-text in lemmy-ui html5-"}]}}}