GHSA-w22m-hvvm-xmwx Fabric.js improper escaping in fabric.Gradient colorStops leads to XSS in SVG serialization

• npm fabric: < 7.4.0 → fixed in 7.4.0

Summary A potential Cross-Site Scripting (XSS) vulnerability exists in Fabric.js due to improper escaping of user-controlled input during SVG serialization via the `toSVG()` method. Specifically, the `color` field within the `colorStops` array of a `fabric.Gradient` object is not properly escaped when converted into SVG `` elements. If an application renders the generated SVG string into the DOM (e.g., via `innerHTML`), this may allow an attacker to inject arbitrary HTML/SVG and execute JavaScript in the victim's browser. Details During SVG export, Fabric.js serializes gradient color stops into `` elements like: ```xml ``` However, the `color` value is inserted into the `stop-color` attribute without proper escaping of special characters such as `"`, ``. This allows crafted input to break out of the attribute context and inject arbitrary markup. For example: ```js color: 'red">' ``` may result in: ```xml ``` This breaks the intended SVG structure and introduces executable HTML. PoC (Proof of Concept) Successfully verified on v7.2.0 (current latest version). The following HTML and JavaScript code reproduces the vulnerability. The code constructs a rectangle with a maliciously crafted gradient color stop and exports it to SVG: ```html Fabric.js SVG Export XSS Bypass Test Fabric.js SVG Export XSS Bypass Test (Gradient Color) SVG Output Rendering: setTimeout(() => { const canvas = new fabric.Canvas('c'); // Construct a malicious gradient object const maliciousGradient = new fabric.Gradient({ type: 'linear', coords: { x1: 0, y1: 0, x2: 100, y2: 0 }, colorStops: [ { offset: 0, // Inject XSS payload to prematurely close the attribute/tag color: 'red">' }, { offset: 1, color: 'blue' } ] }); const rect = new fabric.Rect({ left: 50, top: 50, width: 300, height: 100, fill: maliciousGradient }); canvas.add(rect); // Export to SVG string containing the malicious code const svgOutput = canvas.toSVG(); // Render on the page to trigger the XSS document.getElementById('svg-output').innerHTML = svgOutput; }, 100); ``` Impact This issue can lead to XSS in applications that: 1. Allow user-controlled input in gradient definitions (e.g., color values) 2. Use `canvas.toSVG()` to export content 3. Insert the resulting SVG string into the DOM without sanitization (e.g., via `innerHTML`) Successful exploitation may result in the execution of arbitrary JavaScript in the victim's browser, theft of sensitive data, or unauthorized actions on behalf of the user. Suggested Fix Proper Escaping (Recommended): Escape special characters in attribute values during SVG serialization.