Affected versions

Symfony versions >=6.1, <6.4.40, >=7, <7.4.12, >=8, <8.0.12 of the Symfony HTML Sanitizer component are affected by this security issue.

The issue has been fixed in Symfony 6.4.40, 7.4.12, 8.0.12.

Description

Symfony\Component\HtmlSanitizer\TextSanitizer\UrlSanitizer::parse() (used by UrlSanitizer::sanitize() and therefore by every HtmlSanitizer config that allows links or media) accepts URLs that contain Unicode explicit-direction BiDi formatting characters: U+202A-U+202E (LRE / RLE / PDF / LRO / RLO) and U+2066-U+2069 (LRI / RLI / FSI / PDI). These characters are passed through unchanged into the href / src attributes produced by HtmlSanitizer. When the resulting HTML is rendered in a browser, the override characters reverse or alter the visual ordering of the URL text, so the displayed link can differ arbitrarily from the actual destination: a classic visual-spoofing / phishing primitive against viewers of sanitized content.

Resolution

UrlSanitizer::parse() now rejects URLs containing the explicit-direction BiDi formatting code points (U+202A-U+202E, U+2066-U+2069) before invoking the underlying URL parser. As an unrelated companion fix in the same patch, spaces inside path/query/fragment are now percent-encoded rather than rejected outright, while spaces in the scheme/authority remain rejected by the post-encoding whitespace check.

The patch for this issue is available here for branch 5.4.

Credits

We would like to thank Nicolas Grekas for reporting the issue and providing the fix.

Published in #Security Advisories #Symfony