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#6261 Improve CSS sanitizer #6262

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Merged
sosnovsky merged 3 commits into from
Jul 20, 2026
Merged

#6261 Improve CSS sanitizer #6262

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bbbbec4
feat: improve css sanitizer
martgil Jul 16, 2026
448b97d
Merge branch 'master' into issue-6261-update-css-sanitizer
martgil Jul 16, 2026
58cb7cc
wip
martgil Jul 20, 2026
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130 changes: 88 additions & 42 deletions extension/js/common/platform/xss.ts
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Original file line number Diff line number Diff line change
Expand Up @@ -49,9 +49,78 @@ export class Xss {
private static ADD_ATTR = ['email', 'page', 'addurltext', 'longid', 'index', 'target', 'fingerprint', 'cryptup-data'];
private static FORBID_ATTR = ['background'];
private static HREF_REGEX_CACHE: RegExp | undefined;
private static FORBID_CSS_STYLE =
/(?:^|;)\s*(?:z-index|position|display|visibility|opacity|transform|clip-path|clip|top|left|right|bottom|pointer-events|font-size|line-height|width|height|text-indent|filter)\s*:[^;]*;?/gi;
private static EMOJI_REGEX = /(?![*#0-9]+)[\p{Emoji}\p{Emoji_Modifier}\p{Emoji_Component}\p{Emoji_Modifier_Base}\p{Emoji_Presentation}]/gu;
/* eslint-disable @typescript-eslint/naming-convention */
private static readonly ALLOWED_EMAIL_CSS_PROPERTIES = new Set<string>([
// Colors
'color',
'background-color',
// Fonts
'font-family',
'font-style',
'font-variant',
'font-weight',
'letter-spacing',
'word-spacing',
// Text
'text-align',
'text-decoration',
'text-decoration-color',
'text-decoration-line',
'text-decoration-style',
'text-transform',
'vertical-align',
'white-space',
'word-break',
'overflow-wrap',
// Spacing
'margin',
'margin-top',
'margin-right',
'margin-bottom',
'margin-left',
'padding',
'padding-top',
'padding-right',
'padding-bottom',
'padding-left',
// Borders
'border',
'border-top',
'border-right',
'border-bottom',
'border-left',
'border-color',
'border-style',
'border-width',
'border-top-color',
'border-top-style',
'border-top-width',
'border-right-color',
'border-right-style',
'border-right-width',
'border-bottom-color',
'border-bottom-style',
'border-bottom-width',
'border-left-color',
'border-left-style',
'border-left-width',
'border-radius',
'border-top-left-radius',
'border-top-right-radius',
'border-bottom-right-radius',
'border-bottom-left-radius',
// Lists and tables
'list-style-type',
'border-collapse',
'border-spacing',
'caption-side',
'empty-cells',
// Direction and writing
'direction',
'unicode-bidi',
]);
/* eslint-enable @typescript-eslint/naming-convention */

public static sanitizeRender = (selector: string | HTMLElement | JQuery, dirtyHtml: string) => {
// browser-only (not on node)
Expand Down Expand Up @@ -115,14 +184,8 @@ export class Xss {

// Handle style attributes
if (node.hasAttribute('style')) {
// mitigation rather than a fix, which will involve updating CSP, see https://github.com/FlowCrypt/flowcrypt-browser/issues/2648
let style = node.getAttribute('style') || '';
style = Xss.sanitizeCssStyle(style);
if (style && Xss.FORBID_CSS_STYLE.test(style)) {
const updatedStyle = style.replace(Xss.FORBID_CSS_STYLE, '');
node.setAttribute('style', updatedStyle);
} else if (style) {
// if style was modified but still present, update it
const style = Xss.sanitizeCssStyle(node.getAttribute('style') || '');
if (style) {
node.setAttribute('style', style);
} else {
node.removeAttribute('style');
Expand Down Expand Up @@ -278,40 +341,23 @@ export class Xss {
};

/**
* Decode CSS escape sequences before applying security checks
*/
private static normalizeCssEscapes = (css: string): string => {
return css.replace(/\\(?:\r\n|[\n\r\f])|\\([0-9a-fA-F]{1,6}\s?|.)/g, (_match: string, escaped: string | undefined) => {
if (typeof escaped === 'undefined') {
return '';
}
if (/^[0-9a-fA-F]/.test(escaped)) {
const codePoint = Number.parseInt(escaped.trim(), 16);
return codePoint > 0 && codePoint <= 0x10ffff ? String.fromCodePoint(codePoint) : '';
}
return escaped;
});
};

/**
* Remove @import rules and any url(...) that would cause an out‑of‑band request.
* Only data: and cid: URLs are allowed.
* Sanitize a CSS style string by keeping only explicitly allowed properties.
* Uses the browser's native CSS parser for robustness (handles comments,
* escape sequences, shorthand, vendor prefixes, etc.).
*/
private static sanitizeCssStyle = (css: string): string => {
return css
.replace(/\/\*[\s\S]*?\*\//g, '')
.split(';')
.map(part => this.normalizeCssEscapes(part.trim()))
.filter(part => {
return !/^(z-index|position|display|visibility|opacity|transform|clip-path|clip|top|left|right|bottom|pointer-events|font-size|line-height|width|height|text-indent|filter)\s*:/i.test(
part
);
})
.filter(part => {
// remove url + import safely
return !/@import|url\(/i.test(part);
})
.join('; ');
if (!css || typeof document === 'undefined') {
return '';
}
const style = document.createElement('span').style;
style.cssText = css;
for (const property of Array.from(style)) {
const value = style.getPropertyValue(property).trim();
if (!this.ALLOWED_EMAIL_CSS_PROPERTIES.has(property.toLowerCase()) || !value) {
style.removeProperty(property);
}
}
return style.cssText;
};

/**
Expand Down
30 changes: 30 additions & 0 deletions test/source/tests/browser-unit-tests/unit-Xss.js
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Original file line number Diff line number Diff line change
Expand Up @@ -49,3 +49,33 @@ BROWSER_UNIT_TEST_NAME(`Xss.htmlSanitizeAndStripAllTags preserves leading tabs`)
}
return 'pass';
})();

BROWSER_UNIT_TEST_NAME(`Xss.htmlSanitizeKeepBasicTags strips image-set() CSS tracking`);
(async () => {
const dirty = `<div style="background-image: image-set(&quot;https://attacker.example/track.png&quot; 1x)">message body</div>`;
const clean = Xss.htmlSanitizeKeepBasicTags(dirty, 'IMG-KEEP');
if (/image-set/i.test(clean)) {
throw Error(`image-set() was not stripped from sanitized HTML: ${clean}`);
}
return 'pass';
})();

BROWSER_UNIT_TEST_NAME(`Xss.htmlSanitizeKeepBasicTags strips -webkit-image-set() CSS tracking`);
(async () => {
const dirty = `<div style="background-image: -webkit-image-set(url(&quot;https://attacker.example/track.png&quot;) 1x)">message body</div>`;
const clean = Xss.htmlSanitizeKeepBasicTags(dirty, 'IMG-KEEP');
if (/-webkit-image-set/i.test(clean)) {
throw Error(`-webkit-image-set() was not stripped from sanitized HTML: ${clean}`);
}
return 'pass';
})();

BROWSER_UNIT_TEST_NAME(`Xss.htmlSanitizeKeepBasicTags strips url() in CSS`);
(async () => {
const dirty = `<div style="background-image: url('https://attacker.example/track.png')">message body</div>`;
const clean = Xss.htmlSanitizeKeepBasicTags(dirty, 'IMG-KEEP');
if (/url\(/i.test(clean)) {
throw Error(`url() was not stripped from sanitized HTML: ${clean}`);
}
return 'pass';
})();
4 changes: 2 additions & 2 deletions test/source/tests/decrypt.ts
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Original file line number Diff line number Diff line change
Expand Up @@ -2301,8 +2301,8 @@ XZ8r4OC6sguP/yozWlkG+7dDxsgKQVBENeG6Lw==
expect(combined.includes('url(')).to.equal(false);
expect(combined.includes('@import')).to.equal(false);
expect(combined.includes('po/**/sition:fixed')).to.equal(false);
// verify safe styles survive
expect(combined.includes('color:green')).to.equal(true);
// verify safe styles survive (browser-normalized output includes a space after the colon)
expect(combined.includes('color: green')).to.equal(true);
await inboxPage.close();
})
);
Expand Down
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