Quick answers
- Is CVE-2026-121373 exploited in the wild?
- No. Junglewise Threat Intelligence has no report of CVE-2026-121373 being exploited in the wild.
- How severe is CVE-2026-121373?
- Informational severity, published 4 November 2026.
- Which products and versions are affected?
- Symfony HtmlSanitizer 6.1.x before 6.4.50, 7.x before 7.4.23, 8.x before 8.1.11.
- Is there a fix for CVE-2026-121373?
- Yes: fixed in 6.4.50, 7.4.23, 8.1.11. Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.
- How is CVE-2026-121373 attacked?
- It is attacked over the network, in a software library.
- Am I affected by CVE-2026-121373?
- Check whether you run Symfony HtmlSanitizer at an affected version. Junglewise does that check for you around the clock: connect your code, cloud and domains, and it matches new vulnerabilities like this one against what you actually run and tells you when one reaches you.
How to fix
- Fixed in: 6.4.50, 7.4.23, 8.1.11
Technical details
The MetaRefreshAttributeSanitizer uses an overly restrictive regular expression to parse the HTML standard's "shared declarative refresh steps" grammar for meta content attributes. It fails to recognize valid variations that omit the "url=" keyword, use only whitespace as a separator, or include fractional time values; these bypass UrlSanitizer and are passed through unchanged, defeating host and scheme whitelisting. The vulnerability affects versions 6.1.x before 6.4.50, 7.x before 7.4.23, and 8.x before 8.1.11, and is reachable under any configuration that allows meta elements with http-equiv and content attributes.
Affected products
- Symfony HtmlSanitizer 6.1.x before 6.4.50, 7.x before 7.4.23, 8.x before 8.1.11
Timeline
- 2026-11-04: disclosed