Symfony: ReDoS in HeaderUtils::split() Reachable Through the Cache-Control Request Header

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Symfony is a PHP framework for web and console applications and a set of reusable PHP components. Prior to versions 5.4.54, 6.4.46, 7.4.19, and 8.1.7, HeaderUtils::split() parsed header values with a pattern whose token alternative also matched whitespace, followed by a (?<!\s) lookbehind. A run of whitespace placed before a separator could be divided among the iterations of the enclosing repetition in exponentially many ways, and the engine tried all of them before settling. HeaderBag calls it for the Cache-Control header whenever a Request is built, so any application handling HTTP requests is reachable, with no authentication. A single request carrying such a header costs close to a second of CPU (813 ms for a run of 17 spaces in an 8 KB header on PHP 8.5, doubling with each extra space). From 18 spaces on, preg_match_all() gives up at pcre.backtrack_limit and split() returns an empty array, so every directive of the header is silently dropped. This issue affects symfony/http-foundation v4.1.0-BETA1 through v5.4.53, v6.0.0-BETA1 through v6.4.45, v7.0.0-BETA1 through v7.4.18, and v8.0.0-BETA1 through v8.1.6.

Source published

15 Oct 2026 14:46:01 UTC

Source modified

15 Oct 2026 18:04:45 UTC

Observed here

23 Oct 2026 15:48:27 UTC

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CVSS 4.0 · 8.7 CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N