Blind CSS Exfiltration: exfiltrate unknown web pages
Summary: PortSwigger researcher Gareth Heyes published a technique for blind CSS exfiltration that extracts data from pages where JavaScript is blocked by CSP or filters like DOMPurify, using attribute selectors, CSS variables, and background-image requests to an external server. The method adds the :has selector to trigger requests from elements that cannot carry a background, uses :not() to step through multiple elements of the same type, and uses @import chaining with a timer to pull data without knowing the page structure, extracting input names and values, textarea name attributes, form actions, and anchor links.
What you should know: A site that permits style injection but blocks JavaScript can still leak form field names, values, and links to an attacker-controlled server, so treating CSS as harmless removes a working exfiltration channel from your threat model.
Read the original at PortSwigger Research ↗
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