How severe is CVE-2021-43803?

CVE-2021-43803 has a CVSS 3.x base score of 7.5, rated high severity. It is exploitable over network with low attack complexity, requires no privileges and no user interaction. Impact on confidentiality is none, integrity none, and availability high.

Is CVE-2021-43803 being actively exploited?

It is not currently listed in CISA's KEV catalog. Its EPSS exploit-prediction score is 45% (99th percentile), an estimate of the probability of exploitation in the next 30 days.

What products are affected by CVE-2021-43803?

CVE-2021-43803 affects Vercel Next.js. See the affected-products list for the exact vulnerable versions.

How do I fix CVE-2021-43803?

Review the linked vendor and NVD advisories for patched versions and mitigations, then upgrade or apply the recommended workaround. Given its high severity, prioritise patching exposed systems.

When was CVE-2021-43803 published?

CVE-2021-43803 was published on 2021-12-10 and last updated on 2026-06-17.

CVE-2021-43803

CVE-2021-43803 is a high-severity vulnerability in Vercel Next.js with a CVSS 3.x base score of 7.5. Its EPSS exploit-prediction score of 45% places it in the 99th percentile, indicating an elevated likelihood of exploitation. The underlying weakness is classified as CWE-20.

Key facts

Severity: High (CVSS 3.x base score 7.5)
CVSS v2: 4.3
EPSS exploit prediction: 45% (99th percentile)
Actively exploited: Not listed in CISA KEV
Weakness: CWE-20
Affected product: Vercel Next.js
Published: 2021-12-10
Last modified: 2026-06-17

Description

Next.js is a React framework. In versions of Next.js prior to 12.0.5 or 11.1.3, invalid or malformed URLs could lead to a server crash. In order to be affected by this issue, the deployment must use Next.js versions above 11.1.0 and below 12.0.5, Node.js above 15.0.0, and next start or a custom server. Deployments on Vercel are not affected, along with similar environments where invalid requests are filtered before reaching Next.js. Versions 12.0.5 and 11.1.3 contain patches for this issue.

Frequently asked questions

What is CVE-2021-43803?: Next.js is a React framework. In versions of Next.js prior to 12.0.5 or 11.1.3, invalid or malformed URLs could lead to a server crash. In order to be affected by this issue, the deployment must use Next.js versions above 11.1.0 and below 12.0.5, Node.js above 15.0.0, and next start or a custom server. Deployments on Vercel are not affected, along with similar environments where invalid requests are filtered before reaching Next.js. Versions 12.0.5 and 11.1.3 contain patches for this issue.
How severe is CVE-2021-43803?: CVE-2021-43803 has a CVSS 3.x base score of 7.5, rated high severity. It is exploitable over network with low attack complexity, requires no privileges and no user interaction. Impact on confidentiality is none, integrity none, and availability high.
Is CVE-2021-43803 being actively exploited?: It is not currently listed in CISA's KEV catalog. Its EPSS exploit-prediction score is 45% (99th percentile), an estimate of the probability of exploitation in the next 30 days.
What products are affected by CVE-2021-43803?: CVE-2021-43803 affects Vercel Next.js. See the affected-products list for the exact vulnerable versions.
How do I fix CVE-2021-43803?: Review the linked vendor and NVD advisories for patched versions and mitigations, then upgrade or apply the recommended workaround. Given its high severity, prioritise patching exposed systems.
When was CVE-2021-43803 published?: CVE-2021-43803 was published on 2021-12-10 and last updated on 2026-06-17.

References

Affected products (1)

cpe:2.3:a:vercel:next.js:*:*:*:*:*:node.js:*:*

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