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When generating CSS from external data (JSON, user input, theme files), validate and sanitize both CSS property names and values before injecting them into style elements to prevent style-breaking input and XSS/style injection.

Motivation: Building CSS via string concatenation (e.g. --term-${key}: ${value};) can allow malicious or malformed keys/values to break the stylesheet or inject content. Use browser APIs and input validation to make this safe.

How to apply:

• Prefer CSSOM APIs instead of raw string injection: use element.style.setProperty(propertyName, value).
• Escape property names with CSS.escape when used in a context that requires serialization.
• Validate values with a whitelist or conservative regex: disallow characters that can break CSS such as ;, } or HTML tag delimiters, and limit length. For colors/units consider explicit parsers or allowlists (hex colors, rgb(), numbers+units).
• If full fidelity is required, parse/serialize values using strict rules rather than passing arbitrary strings into a style element.

Example (adapted from the discussion):

// unsafe: --term-${key}: ${value}; // safer approach: function isValidKey(key) { // allow lowercase letters, numbers, and hyphens (adjust to your naming rules) return /^[a-z0-9-]+$/.test(key); }

function isValidValue(value) { if (typeof value !== ‘string’ || value.length > 200) return false; // disallow characters that can close or break CSS rules return !/[;{}<>]/.test(value); }

async function applyTheme(themeJson, styleEl) { for (const [key, value] of Object.entries(themeJson)) { if (!isValidKey(key) || !isValidValue(String(value))) continue; // reject invalid entries

// use CSS custom property API which avoids injecting raw CSS text
// ensure the property name is valid per your rules
styleEl.style.setProperty(`--term-${CSS.escape(key)}`, String(value));   } }

Notes:

• CSS.escape is broadly available and safe for serializing identifiers; still validate keys to match your naming expectations.
• For values that are colors or units, prefer strict parsing (e.g., validate hex, rgb(), or numeric+unit) rather than a permissive regex.
• When creating

References: [0]

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