What’s Vulnerable › Vulnerabilities

CVE-2026-56858: Fix Javascript regexp context tracking in html/template

• Severity: medium (CVSS 6.1 v3.1)
• Exploited in the wild: not listed in CISA KEV
• EPSS: 0.3% probability of exploitation in the next 30 days
• Public exploit or detection template: none known
• Published: 2026-08-13 · updated 2026-09-03
• Weakness: CWE-79

Description

Previously, pathological inputs could close an unescaped '/' early, allowing for attack-controlled data to inject arbitrary content, potentially leading to XSS.

Affected products

• Go Stdlib: before 1.25.13; from 1.26.0-0 before 1.26.6; from 1.27.0-0 before 1.27.0-rc.3 — fixed in 1.25.13, 1.26.6, 1.27.0-rc.3
• Go standard library html/template: before 1.25.13; from 1.26.0-0 before 1.26.6; from 1.27.0-0 before 1.27.0-rc.3 — fixed in 1.25.13, 1.26.6, 1.27.0-rc.3

How to fix

Upgrade to a fixed version: Go Stdlib 1.25.13, 1.26.6, 1.27.0-rc.3; Go standard library html/template 1.25.13, 1.26.6, 1.27.0-rc.3. Check the vendor advisory for mitigations.

References

• https://go.dev/cl/807100
• https://go.dev/issue/80435
• https://groups.google.com/g/golang-announce/c/94pEornpRlI
• https://pkg.go.dev/vuln/GO-2026-6091

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