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A GitHub-reviewed security advisory, not necessarily under active exploitation the way our CISA KEV advisories are, but a real, disclosed vulnerability in an open-source dependency.
Affected packages
Vulnerable: >= 6.0.1, < 6.4.46
Patched in 6.4.46
Vulnerable: >= 7.0.0, < 7.4.19
Patched in 7.4.19
Vulnerable: >= 8.0.0, < 8.1.7
Patched in 8.1.7
Vulnerable: >= 6.0.1, < 6.4.46
Patched in 6.4.46
Vulnerable: >= 7.0.0, < 7.4.19
Patched in 7.4.19
Vulnerable: >= 8.0.0, < 8.1.7
Patched in 8.1.7
Vulnerable: >= 4.1.0, < 5.4.54
Patched in 5.4.54
Vulnerable: >= 4.1.0, < 5.4.54
Patched in 5.4.54
Details
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.
The pattern now consumes whitespace only when more of the value follows it, so a value can no longer end on whitespace, nothing has to be given back, and every quantifier is possessive. The first release of this fix broke splitting on a whitespace separator: HeaderUtils::split('foo bar', ' ') returned a single value in 6.4.46 to 6.4.47, 7.4.19 to 7.4.20 and 8.1.7 to 8.1.8. Whitespace characters used as separators are now left out of the whitespace branch, which 6.4.48, 7.4.21 and 8.1.9 contain.
The patches for this issue are available here for branch 5.4 (and forward-ported to 6.4, 7.4, 8.1 and 8.2).
Symfony would like to thank r3verii for reporting the issue and Nicolas Grekas for providing the fix.
Symfony would also like to thank Bruno Dias from Checkmarx for his help getting a CVE assigned.
EPSS, exploit probability
Low0.48%
estimated chance of real-world exploitation in the next 30 days, higher than 41.2% of every CVE FIRST.org scores
Refreshed 10/27/2026, via FIRST.org's EPSS model, not CVSS, this measures likelihood of exploitation, not how severe it would be.
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