CVE-2021-39178Network attack vector

Cross-site scripting via unsandboxed optimized SVG

Published Aug 30, 2021 · Updated Aug 4, 2024

Cross-site scripting in Vercel Next.js 10.0.0 through 11.1.0 allows remote attackers to execute script via an optimized SVG. The Image Optimization API serves an attacker-controlled SVG inline without a restrictive Content-Security-Policy header, so a direct browser visit executes embedded script. Exploitation requires the default image loader, an images.domains entry whose host accepts user-provided SVG, and user interaction; Vercel-hosted deployments are not affected.

CVSS severity7.5
High
EPSS probability1.14%
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 Vercel Next.js 10.0.0 through 11.1.0 allows remote attackers to execute script via an optimized SVG. The Image Optimization API serves an attacker-controlled SVG inline without a restrictive Content-Security-Policy header, so a direct browser visit executes embedded script. Exploitation requires the default image loader, an images.domains entry whose host accepts user-provided SVG, and user interaction; Vercel-hosted deployments are not affected.

The record

CVE
CVE-2021-39178
Published
Aug 30, 2021
Updated
Aug 4, 2024
Vendor
Vercel
Product
Next.js
Classifications
CWE-79, T1189
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. Aug 30, 2021CVE publishedPublication date reported by the CVE source.
  2. Aug 4, 2024Record updatedLatest update available in the CVE record.

Exploitability

Present is not the same as exploitable

Compare your product and version with the public record. A matching version still requires validation against your environment.

Is a vulnerable build present?

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

  1. Affected versionversion=>= 10.0.0, < 11.1.1

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

Vercel · Next.jsversion=>= 10.0.0, < 11.1.1

Published CVSS scores

7.5GitHub Advisory DatabaseCVSS:3.1/AV:N/AC:H/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)
Vectornetwork
Privilegesunauthenticated
Known exploitationUnconfirmed
Public exploitUnconfirmed

Weakness, pattern, technique

CWE-79Improper Neutralization of Input During Web Page Generation ('Cross-site Scripting')
T1189Drive-by Compromise

Public exploit references

No public exploit references are available in this record.

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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