{"id":"CVE-2026-5388","aliases":["GHSA-c9vm-hv86-f23r"],"title":"justhtml includes multiple security fixes","summary":"justhtml includes multiple security fixes","severity":"medium","vendor":"justhtml","product":"justhtml","ecosystem":"pip","affected":["justhtml < 1.15.0"],"patched":["justhtml 1.15.0"],"published":"2026-04-10","updated":"2026-08-24","source":"OSV","sourceUrl":"https://osv.dev/vulnerability/GHSA-c9vm-hv86-f23r","references":[{"url":"https://github.com/EmilStenstrom/justhtml/security/advisories/GHSA-c9vm-hv86-f23r"},{"url":"https://github.com/EmilStenstrom/justhtml"},{"url":"https://github.com/EmilStenstrom/justhtml/compare/v1.14.0...v1.15.0"},{"url":"https://github.com/EmilStenstrom/justhtml/releases/tag/v1.15.0"}],"tags":["osv","pip"],"epss":0.00345,"epssPercentile":0.2803,"ingestedAt":"2026-08-24T19:25:44.153Z","slug":"CVE-2026-5388","body":"## Overview\n\n## Summary\n\n`justhtml` `1.15.0` includes multiple security fixes affecting URL sanitization helpers, HTML serialization, Markdown passthrough, and several custom sanitization-policy edge cases.\n\nThese issues have different impact levels and do not all affect the default configuration in the same way.\n\n## Affected versions\n\n- `justhtml` `<= 1.14.0`\n\n## Fixed version\n\n- `justhtml` `1.15.0` released on April 9, 2026\n\n## Impact overview\n\n### Helper and serialization issues\nThese issues could affect applications using JustHTML helpers or programmatic DOM construction, even outside the default HTML sanitization path.\n\n- `JustHTML.clean_url_value(...)` and `clean_url_in_js_string(...)` could accept URL values such as `javascript&#58...`, which became active `javascript:` URLs after HTML attribute parsing.\n- URL sanitization could treat values like `\\\\evil.example/x` or `/\\\\evil.example/x` as safe relative URLs even though browsers could resolve them as remote requests.\n- Malformed bracketed hosts such as `https://[evil.example]/x` could raise exceptions and crash sanitization when host allowlists were used.\n- Programmatic element or attribute names containing markup-breaking characters could be serialized into active HTML.\n- Programmatic HTML comments containing `-->` could break out of the comment and inject live markup.\n\n### Markdown passthrough issue\n- `to_markdown(html_passthrough=True)` could reintroduce active HTML from sanitized `<textarea>` content by emitting a raw closing `</textarea>` sequence.\n\n### Custom policy issues\nThese issues affected custom policies more than the default safe configuration.\n\n- `a[ping]` was handled as a single URL even though browsers interpret it as a space-separated URL list.\n- `attributionsrc` was not treated as URL-bearing and could preserve attacker-controlled reporting endpoints.\n- `link[imagesrcset]` was not treated as URL-bearing and could preserve attacker-controlled image candidates.\n- Preserved `<meta http-equiv=\"refresh\">` tags could keep redirect targets without URL-policy enforcement.\n- Preserved `<base href>` tags could rewrite how later relative URLs resolved in the browser.\n- Preserved `<style>` blocks could keep resource-loading CSS such as `@import`, `url(...)`, or `image-set(...)`.\n- Mixed-case attribute names in custom transform pipelines could bypass or confuse security-related transforms such as `DropAttrs(...)`, `DropUrlAttrs(...)`, `AllowStyleAttrs(...)`, and `MergeAttrs(...)`.\n\n## Default configuration\n\nMost of the custom-policy issues above did **not** affect the default `JustHTML(..., sanitize=True)` behavior.\n\nThe main exceptions were:\n- helper APIs such as `clean_url_value(...)`\n- programmatic DOM / serializer usage\n- applications explicitly using `html_passthrough=True`\n- applications using custom policies or custom transform pipelines\n\n## Recommended action\n\nUpgrade to `justhtml` `1.15.0`.\n\nIf you cannot upgrade immediately:\n\n- avoid `html_passthrough=True` for untrusted content\n- avoid preserving `<style>`, `<meta http-equiv=\"refresh\">`, and `<base href>` in custom policies\n- avoid allowing `ping`, `attributionsrc`, or `imagesrcset` unless you explicitly validate them\n- avoid serializing untrusted programmatic node names, attribute names, or comment data\n\n## Affected packages\n\n- `justhtml < 1.15.0`\n\n## Remediation\n\nUpgrade to a patched release:\n\n- `justhtml 1.15.0`","depth":"sunlit","depthScore":28,"depthScoreParts":{"impact":27.5,"likelihood":0.1,"exploitation":0,"ransomware":0},"changes":[]}