CVE-2021-34479Public exploit

Script execution via predictable Markdown preview nonce

Published Jul 14, 2021 · Updated Aug 10, 2026

Cross-site scripting in Visual Studio Code before 1.58 allows context-dependent attackers to execute script via a crafted Markdown preview. The built-in Markdown preview's provideTextDocumentContent method derives its CSP nonce from predictable timestamps, allowing injected script tags to guess an accepted nonce. Exploitation requires a user to preview an attacker-controlled Markdown file; the demonstrated script remains inside the sandboxed renderer.

CVSS severity7.8
High
EPSS probability3.40%
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 Visual Studio Code before 1.58 allows context-dependent attackers to execute script via a crafted Markdown preview. The built-in Markdown preview's provideTextDocumentContent method derives its CSP nonce from predictable timestamps, allowing injected script tags to guess an accepted nonce. Exploitation requires a user to preview an attacker-controlled Markdown file; the demonstrated script remains inside the sandboxed renderer.

The record

CVE
CVE-2021-34479
Published
Jul 14, 2021
Updated
Aug 10, 2026
Vendor
Microsoft
Product
Visual Studio Code
Classifications
T1204.002
Attack vector
local
Privileges
unauthenticated

Timeline

How it unfolded

  1. Jul 14, 2021CVE publishedPublication date reported by the CVE source.
  2. Aug 10, 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=1.0.0 <1.58

What conditions does exploitation require?

Attack vectorlocal
Required privilegesunauthenticated

What is affected?

Microsoft · Visual Studio Codeversion=1.0.0 <1.58

Published CVSS scores

4.3NIST NVDAV:N/AC:M/Au:N/C:N/I:P/A:N
5.5NIST NVDCVSS:3.1/AV:L/AC:L/PR:N/UI:R/S:U/C:N/I:H/A:N
7.8Microsoft CorporationCVSS:3.1/AV:L/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:H

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 10, 2026Sep 16, 2026
Latest reporting daySep 16, 2026
Latest daily unique IPsUnavailable
Prior 30-day averageUnavailable
SourceShadowserver honeypots (KEV)
Vectorlocal
Privilegesunauthenticated
Known exploitationUnconfirmed
Public exploitPublished

Weakness, pattern, technique

T1204.002Malicious File

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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