How severe is CVE-2021-23597?

CVE-2021-23597 has a CVSS score of 7.5/10, rated HIGH. Immediate patching is strongly recommended.

Which packages are affected by CVE-2021-23597?

CVE-2021-23597 affects the following packages: fastify-multipart (npm). Ecosystems affected: npm.

How do I fix CVE-2021-23597?

Update fastify-multipart to 5.3.1 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2021-23597 is resolved across your whole dependency graph.

How do I detect CVE-2021-23597 in my npm dependencies?

Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for fastify-multipart, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.

How do I mitigate CVE-2021-23597 if there is no patch (or I can't update yet)?

If you can't upgrade right away: gate or disable the affected feature, validate untrusted input at the boundary, and avoid passing attacker-controlled data into the vulnerable path. O3's runtime protection blocks exploitation in production as an interim safeguard until the upgrade lands.

Is CVE-2021-23597 actively exploited in the wild?

Yes. There are 1 known exploit references for CVE-2021-23597, including 1 in-the-wild exploitation observed on GitHub and other sources. Treat this as actively exploitable and prioritize patching immediately. All exploit code should only be run in an isolated sandbox environment for research or authorized testing — never against production systems without explicit written authorization.

What is the EPSS score for CVE-2021-23597?

CVE-2021-23597 has an EPSS (Exploit Prediction Scoring System) score of 2.0%, placing it in the 80th percentile of all CVEs. EPSS is maintained by FIRST.org and estimates the probability that a vulnerability will be exploited in the wild within the next 30 days. This score indicates relatively lower exploitation probability, though the CVSS severity should still guide your patching priority.

What type of vulnerability is CVE-2021-23597?

CVE-2021-23597 is classified as CWE-1321 (CWE-1321). This weakness type describe the underlying flaw category, which helps determine the potential impact and the right class of mitigation.

When was CVE-2021-23597 published, and has it been updated?

CVE-2021-23597 was published on February 11, 2022 and was last updated on August 7, 2026. Advisory data evolves as severity scores, affected ranges, and exploit intelligence are revised — always check the latest version of the advisory before acting.

📦

📦 npm

Not in CISA KEV

HIGH severity

CVE-2021-23597 — fastify-multipart

HIGHFix: fastify/fastify-multipart@a70dc70

CVE-2021-23597 is a high-severity (CVSS 7.5) CWE-1321 vulnerability in fastify-multipart. 1 public exploit reference exists, so weaponization risk is real. A fix is available for fastify-multipart — see the affected versions and patch details below.

Uncaught Exception in fastify-multipart

Also known asGHSA-qh73-qc3p-rjv2SNYK-JS-FASTIFYMULTIPART-2395480

Published

Feb 11, 2022

Updated

Aug 7, 2026

Affected

1 pkg

Patched

1 / 1

Exploits

1 known

Exploitation data as of Sep 28, 2026 · OSV.dev, NVD, FIRST.org (EPSS)

EPSS Exploitation Probability

via FIRST.org ↗

2.0%probability of exploitation in next 30 days

Lower Risk0.00%

Lower risk than most CVEs80th percentile — riskier than 80% of all scored CVEsHighest risk

Probability of exploitation in the next 30 days, from FIRST.org EPSS.

How urgent is this, really

CVE-2021-23597 by exploitation likelihood (EPSS) against impact (CVSS). Outside the shaded patch-first corner.

Where this sits among everything scored

Of 385,738 CVEs with a current EPSS score, this one falls in the < 10% band (highlighted). Counts from FIRST.org, log-scaled.

Real-World Exposure

1 pkg affected

How broadly this vulnerability is actually deployed: weekly install volume shows current usage, a proxy for how much of the ecosystem is exposed.

fastify-multipartnpm

40Kdownloads / week

Description

Impact

This is a bypass of CVE-2020-8136 (https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2020-8136). By providing a name=constructor property it is still possible to crash the application. The original fix only checks for the key __proto__ (https://github.com/fastify/fastify-multipart/pull/116).

All users are recommended to upgrade

Patches

v5.3.1 includes a patch

Workarounds

No workarounds are possible.

References

Read up https://www.fastify.io/docs/latest/Guides/Prototype-Poisoning/

For more information

If you have any questions or comments about this advisory:

Open an issue in https://github.com/fastify/fastify-multipart
Email us at [email protected]

Affected Packages

1 total 1 fixed

Ecosystem	Package	Vulnerable range	Fix
📦npm	`fastify-multipart`	all versions	5.3.1`npm install fastify-multipart@5.3.1`

Affected Products

1 product · 1 configurations

Application

fastify-multipartfastify

< 5.3.1

range

Exploits & PoCs

Research use only. For defensive security, authorized penetration testing, and academic research only. Never execute exploit code against systems without explicit written authorization.

This affects the package fastify-multipart before 5.3.1. By providing a …

snyk.ioNVDFeb 2022

Detection & mitigation playbook

Open-source dependency

Detect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for fastify-multipart, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
Fix
Update fastify-multipart to 5.3.1 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2021-23597 is resolved across your whole dependency graph.
Workarounds
If you can't upgrade right away: gate or disable the affected feature, validate untrusted input at the boundary, and avoid passing attacker-controlled data into the vulnerable path. O3's runtime protection blocks exploitation in production as an interim safeguard until the upgrade lands.

Frequently Asked Questions

### Impact This is a bypass of CVE-2020-8136 (https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2020-8136). By providing a `name=constructor` property it is still possible to crash the application. The original fix only checks for the key `__proto__` (https://github.com/fastify/fastify-multipart/pull/116). All users are recommended to upgrade ### Patches v5.3.1 includes a patch ### Workarounds No workarounds are possible. ### References Read up https://www.fastify.io/docs/latest/Guides/Prototype-Poisoning/ ### For more information If you have any questions or comments about this adv