Machine-written summary
Summary written by Claude Haiku from the source advisory. Not reviewed by a person.
Symfony's http-foundation and symfony packages, versions 4.4.0 to before 4.4.7 and 5.0.0 to before 5.0.7, are affected. When a Response lacks a Content-Type header, Symfony falls back to the format in the request's Accept header, which can mismatch the response content and corrupt the cache. The source describes the fix as Symfony no longer using the Accept header to guess Content-Type, and says to upgrade to 4.4.7 or 5.0.7.
Not analysed yet
We haven’t published our analysis of this Advisory yet. Once a person has reviewed it, this page will say what’s vulnerable, what has to be true for your app to be affected, and how to check.
Severity and exploitation
| Score | Rating | CVSS | Vector | Source |
|---|---|---|---|---|
| 2.6 | Low | 3.1 | CVSS:3.1/AV:N/AC:H/PR:L/UI:R/S:U/C:N/I:N/A:L | CNA (GitHub_M) |
- CISA KEV Not listed.
- EPSS 0.013, the 70th percentile, on 5 Nov 2026: FIRST’s estimate of the chance of exploitation in the next 30 days.
Timeline
- 31 Oct 2026, 14:05 UTC First seen by agentprime
- 31 Oct 2026, 14:05 UTC Confirmed by OSV
Sources
- GitHub Advisory Database: GHSA-mcx4-f5f5-4859
- OSV: GHSA-mcx4-f5f5-4859
- github.com/symfony/symfony/security/advisories/GHSA-mcx4-f5f5-4859
- nvd.nist.gov/vuln/detail/CVE-2020-5255
- github.com/symfony/symfony/commit/dca343442e6a954f96a2609e7b4e9c21ed6d74e6
- github.com/FriendsOfPHP/security-advisories/blob/master/symfony/http-foundation/CVE-2020-5255.yaml
- github.com/FriendsOfPHP/security-advisories/blob/master/symfony/symfony/CVE-2020-5255.yaml
- lists.fedoraproject.org/archives/list/[email protected]/message/C36JLPHUPKDFAX6D5WYFC4ALO2K7RDUQ
- symfony.com/blog/cve-2020-5255-prevent-cache-poisoning-via-a-response-content-type-header
- symfony.com/cve-2020-5255