CVE-2026-27120: Leaf-kit html escaping does not work on characters that are part of extended grapheme cluster

February 19, 2026 (updated February 23, 2026)

htmlEscaped in leaf-kit will only escape html special characters if the extended grapheme clusters match, which allows bypassing escaping by using an extended grapheme cluster containing both the special html character and some additional characters. In the case of html attributes, this can lead to XSS if there is a leaf variable in the attribute that is user controlled.

References

github.com/advisories/GHSA-4hfh-fch3-5q7p
github.com/vapor/leaf-kit
github.com/vapor/leaf-kit/commit/8919e39476c3a4ba05c28b71546bb9195f87ef34
github.com/vapor/leaf-kit/security/advisories/GHSA-4hfh-fch3-5q7p
nvd.nist.gov/vuln/detail/CVE-2026-27120

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