How severe is CVE-2021-23426?

CVE-2021-23426 has a CVSS score of 7.5/10, rated HIGH. Immediate patching is strongly recommended.

Which packages are affected by CVE-2021-23426?

CVE-2021-23426 affects the following packages: Proto (npm). Ecosystems affected: npm.

How do I fix CVE-2021-23426?

No patched version of Proto has shipped for CVE-2021-23426 yet. Where your build allows, override or pin the dependency away from the vulnerable range, and apply any maintainer-recommended mitigation.

How do I detect CVE-2021-23426 in my npm dependencies?

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

How do I mitigate CVE-2021-23426 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-2021-23426 actively exploited in the wild?

Yes. There are 1 known exploit references for CVE-2021-23426, 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-2021-23426?

CVE-2021-23426 has an EPSS (Exploit Prediction Scoring System) score of 0.9%, placing it in the 57th 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.

When was CVE-2021-23426 published, and has it been updated?

CVE-2021-23426 was published on September 2, 2021 and was last updated on November 8, 2023. 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-2021-23426 — Proto

HIGH

CVE-2021-23426 is a high-severity (CVSS 7.5) vulnerability in Proto. 1 public exploit reference exists, so weaponization risk is real. No vendor fix is recorded yet; mitigation options are listed below.

Prototype Pollution in Proto

Published

Sep 2, 2021

Updated

Nov 8, 2023

Affected

1 pkg

Patched

See advisory

Exploits

1 known

Exploitation data as of Oct 10, 2026 · OSV.dev, NVD, FIRST.org (EPSS)

EPSS Exploitation Probability

via FIRST.org ↗

0.9%probability of exploitation in next 30 days

Lower Risk0.00%

Lower risk than most CVEs57th percentile — riskier than 57% of all scored CVEsHighest risk

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

How urgent is this, really

CVE-2021-23426 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, and reverse-dependency count shows how many other packages break if it stays unpatched.

0other npm packages depend on this — each one inherits the vulnerability until it's patched upstream

Protonpm

11downloads / week

Description

This affects all versions of package Proto. It is possible to inject pollute the object property of an application using Proto by leveraging the merge function.

Affected Packages

1 total

Ecosystem	Package	Vulnerable range	Fix
📦npm	`Proto`	all versions	No fix

Affected Products

1 product · 1 configurations

Application

protoproto_project

all

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 all versions of package Proto. It is possible to inject pol…

snyk.ioNVDSep 2021

Detection & mitigation playbook

Open-source dependency

Detect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for Proto, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
Remediation status
No patched version of Proto has shipped for CVE-2021-23426 yet. Where your build allows, override or pin the dependency away from the vulnerable range, and apply any maintainer-recommended mitigation.
Mitigate without a patch
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

This affects all versions of package Proto. It is possible to inject pollute the object property of an application using Proto by leveraging the merge function.

O3 Security · Impact-Aware SCA

Is CVE-2021-23426 in your dependencies?

Find it across npm, including transitive dependencies.

Scan my dependencies How O3 SCA works