CVE-2021-43803Network attack vector

Remote server crash via unhandled URL parsing error

Published Dec 9, 2021 · Updated Aug 4, 2024

Improper input validation in Vercel Next.js 11.1.0 through 11.1.2 and 12.0.0 through 12.0.4 allows remote attackers to crash servers. The request handler lets ERR_INVALID_URL or DecodeError exceptions from malformed URL parsing escape instead of returning HTTP 400. Exposure requires Node.js above 15.0.0 with next start or a custom server and no upstream filtering of invalid requests.

CVSS severity7.5
High
EPSS probability44.82%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Improper input validation in Vercel Next.js 11.1.0 through 11.1.2 and 12.0.0 through 12.0.4 allows remote attackers to crash servers. The request handler lets ERR_INVALID_URL or DecodeError exceptions from malformed URL parsing escape instead of returning HTTP 400. Exposure requires Node.js above 15.0.0 with next start or a custom server and no upstream filtering of invalid requests.

The record

CVE
CVE-2021-43803
Published
Dec 9, 2021
Updated
Aug 4, 2024
Vendor
Vercel
Product
Next.js
Classifications
CWE-20, T1499.004
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. Dec 9, 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

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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=>= 11.1.0, < 11.1.3
  2. Affected versionversion=>= 12.0.0, < 12.0.5

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

Vercel · Next.jsversion=>= 11.1.0, < 11.1.3; version=>= 12.0.0, < 12.0.5

Published CVSS scores

7.5GitHubCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/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-20Improper Input Validation
T1499.004Application or System Exploitation

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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