CVE-2026-56858Public exploitNetwork attack vector

Cross-site scripting via stale regexp context

Published Aug 13, 2026 · Updated Aug 13, 2026

Cross-site scripting in Go standard library html/template allows remote attackers to execute JavaScript through crafted template data. The tJS transition mishandles a top-level brace by skipping the normal JavaScript context update, so a following slash is treated as division and jsValEscaper leaves a regexp-closing slash unescaped. Exploitation requires attacker-controlled data in a regexp literal after a top-level brace within a script block and a user who loads the rendered page; successful attacks run same-origin JavaScript in that browser.

CVSS severity6.1
Medium
EPSS probability0.31%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Cross-site scripting in Go standard library html/template allows remote attackers to execute JavaScript through crafted template data. The tJS transition mishandles a top-level brace by skipping the normal JavaScript context update, so a following slash is treated as division and jsValEscaper leaves a regexp-closing slash unescaped. Exploitation requires attacker-controlled data in a regexp literal after a top-level brace within a script block and a user who loads the rendered page; successful attacks run same-origin JavaScript in that browser.

The record

CVE
CVE-2026-56858
Published
Aug 13, 2026
Updated
Aug 13, 2026
Vendor
Go standard library
Product
html/template
Classifications
CWE-79, T1059.007
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. Aug 13, 2026CVE publishedPublication date reported by the CVE source.
  2. Aug 13, 2026Record updatedLatest update available in the CVE record.

Exploitability

Present is not the same as exploitable

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Is a vulnerable build present?

Compare these published version ranges with your installed build and any vendor patches.

  1. Affected versionversion=0 <1.25.13
  2. Affected versionversion=1.26.0-0 <1.26.6
  3. Affected versionversion=1.27.0-0 <1.27.0-rc.3

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

Go standard library · html/templateversion=0 <1.25.13; version=1.26.0-0 <1.26.6; version=1.27.0-0 <1.27.0-rc.3

Published CVSS scores

6.1CISA-ADPCVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:N

CVSS describes severity. EPSS estimates exploitation probability.

Attacks

What attackers are doing with it

Daily unique IPs observed by Shadowserver honeypots for known exploited vulnerabilities (KEVs). Missing observations do not establish an absence of attacks.

Daily unique IPsNo honeypot observations are available for this CVE in the selected window.

No observations available

Sep 9, 2026Sep 15, 2026
Latest reporting daySep 15, 2026
Latest daily unique IPsUnavailable
Prior 30-day averageUnavailable
SourceShadowserver honeypots (KEV)
Vectornetwork
Privilegesunauthenticated
Known exploitationUnconfirmed
Public exploitPublished

Weakness, pattern, technique

CWE-79Improper Neutralization of Input During Web Page Generation ('Cross-site Scripting')
T1059.007JavaScript

Public exploit references

Labels summarize the accepted research assessment. They do not indicate a test against your environment.

Technologies

Your stack

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Your stack

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