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fix(react-provider): prevent XSS theme vulnerability during SSR #35717

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Merged
Hotell merged 6 commits into from
Feb 6, 2026
+37 −1
Merged

fix(react-provider): prevent XSS theme vulnerability during SSR #35717

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de64eb9
fix(react-provider): prevent XSS theme vulnerability during SSR
Hotell Feb 2, 2026
c310832
change file
Hotell Feb 2, 2026
8754ae7
fixup! fix(react-provider): prevent XSS theme vulnerability during SSR
Hotell Feb 2, 2026
8b2d886
fixup! fixup! fix(react-provider): prevent XSS theme vulnerability du…
Hotell Feb 2, 2026
5810b73
escape per declaration
Hotell Feb 2, 2026
0b3312f
refactor: run only once and dictionarize the escape map/matches
Hotell Feb 3, 2026
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7 changes: 7 additions & 0 deletions change/@fluentui-react-provider-6afbfd01-d257-41ee-99b8-bfe470e89675.json
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Original file line number Diff line number Diff line change
@@ -0,0 +1,7 @@
{
"type": "patch",
"comment": "fix: prevent XSS theme vulnerability during SSR",
"packageName": "@fluentui/react-provider",
"email": "martinhochel@microsoft.com",
"dependentChangeType": "patch"
}
13 changes: 13 additions & 0 deletions ...nents/react-provider/library/src/components/FluentProvider/createCSSRuleFromTheme.test.ts
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Original file line number Diff line number Diff line change
Expand Up @@ -16,4 +16,17 @@ describe('createCSSRuleFromTheme', () => {
`".selector { --borderRadiusLarge: 10px; --colorBackgroundOverlay: rgba(0, 0, 0, 0.4); }"`,
);
});

it('prevents XSS by replacing angle brackets that could inject HTML', () => {
const theme = {
colorBrandBackground: '</style><script>alert("xss")</script>',
} as PartialTheme;

const result = createCSSRuleFromTheme('.selector', theme);
expect(result).not.toContain('<');
expect(result).not.toContain('>');
expect(result).toMatchInlineSnapshot(
`".selector { --colorBrandBackground: \\\\3C /style\\\\3E \\\\3C script\\\\3E alert(\\"xss\\")\\\\3C /script\\\\3E ; }"`,
);
});
});
18 changes: 17 additions & 1 deletion ...components/react-provider/library/src/components/FluentProvider/createCSSRuleFromTheme.ts
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Original file line number Diff line number Diff line change
@@ -1,5 +1,21 @@
import type { PartialTheme } from '@fluentui/react-theme';

const CSS_ESCAPE_MAP = {
'<': '\\3C ',
'>': '\\3E ',
};
/**
* Escapes characters that could break out of a <style> tag during SSR.
*
* IMPORTANT: Do not strip quotes. Theme values legitimately include quoted font families and other CSS.
* We only need to ensure the generated text cannot terminate the style tag and inject HTML.
*/
function escapeForStyleTag(value: string): string {
// Escape as CSS code points so the resulting CSS still represents the same characters.
// Using CSS escapes prevents the HTML parser from seeing a literal '<' / '>' and closing <style>.
return value.replace(/[<>]/g, match => CSS_ESCAPE_MAP[match as keyof typeof CSS_ESCAPE_MAP]);
}

/**
* Creates a CSS rule from a theme object.
*
Expand All @@ -11,7 +27,7 @@ export function createCSSRuleFromTheme(selector: string, theme: PartialTheme | u
return `${cssVarRule}--${cssVar}: ${theme[cssVar]}; `;
}, '');

return `${selector} { ${cssVarsAsString} }`;
return `${selector} { ${escapeForStyleTag(cssVarsAsString)} }`;
}

return `${selector} {}`;
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