{
    "dataType": "CVE_RECORD",
    "dataVersion": "5.2",
    "cveMetadata": {
        "cveId": "CVE-2026-54743",
        "assignerOrgId": "a0819718-46f1-4df5-94e2-005712e83aaa",
        "state": "PUBLISHED",
        "assignerShortName": "GitHub_M",
        "dateReserved": "2026-06-15T23:12:41.964Z",
        "datePublished": "2026-08-19T20:18:43.124Z",
        "dateUpdated": "2026-08-19T20:18:43.124Z"
    },
    "containers": {
        "cna": {
            "title": "Lemmy: Stored XSS via markdown image alt-text in lemmy-ui html5-embed",
            "problemTypes": [
                {
                    "descriptions": [
                        {
                            "cweId": "CWE-79",
                            "lang": "en",
                            "description": "CWE-79: Improper Neutralization of Input During Web Page Generation ('Cross-site Scripting')",
                            "type": "CWE"
                        }
                    ]
                }
            ],
            "metrics": [
                {
                    "cvssV4_0": {
                        "attackVector": "NETWORK",
                        "attackComplexity": "LOW",
                        "attackRequirements": "NONE",
                        "privilegesRequired": "NONE",
                        "userInteraction": "PASSIVE",
                        "vulnConfidentialityImpact": "NONE",
                        "vulnIntegrityImpact": "NONE",
                        "vulnAvailabilityImpact": "NONE",
                        "subConfidentialityImpact": "HIGH",
                        "subIntegrityImpact": "HIGH",
                        "subAvailabilityImpact": "NONE",
                        "baseScore": 6.4,
                        "baseSeverity": "MEDIUM",
                        "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"
                    }
                }
            ],
            "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"
                }
            ],
            "affected": [
                {
                    "vendor": "LemmyNet",
                    "product": "lemmy",
                    "versions": [
                        {
                            "version": "< 0.19.19-beta.1",
                            "status": "affected"
                        }
                    ]
                }
            ],
            "providerMetadata": {
                "orgId": "a0819718-46f1-4df5-94e2-005712e83aaa",
                "shortName": "GitHub_M",
                "dateUpdated": "2026-08-19T20:18:43.124Z"
            },
            "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."
                }
            ],
            "source": {
                "advisory": "GHSA-2g66-9fr3-ppwj",
                "discovery": "UNKNOWN"
            }
        }
    }
}