How severe is CVE-2021-21321?

CVE-2021-21321 has a CVSS score of 10/10, rated CRITICAL. Immediate patching is strongly recommended.

Which packages are affected by CVE-2021-21321?

CVE-2021-21321 affects the following packages: fastify-reply-from (npm). Ecosystems affected: npm.

How do I fix CVE-2021-21321?

Update fastify-reply-from to 4.0.2 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2021-21321 is resolved across your whole dependency graph.

How do I detect CVE-2021-21321 in my npm dependencies?

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

How do I mitigate CVE-2021-21321 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-2021-21321 actively exploited in the wild?

No public exploit code has been indexed for CVE-2021-21321 yet. This does not mean the vulnerability cannot be exploited — absence of public exploits does not imply safety. Apply the recommended fix and use O3 Security to monitor your exposure.

What is the EPSS score for CVE-2021-21321?

CVE-2021-21321 has an EPSS (Exploit Prediction Scoring System) score of 1.8%, placing it in the 78th 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-2021-21321?

CVE-2021-21321 is classified as Improper Input Validation (CWE-20). This weakness type describe the underlying flaw category, which helps determine the potential impact and the right class of mitigation.

When was CVE-2021-21321 published, and has it been updated?

CVE-2021-21321 was published on March 2, 2021 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

CRITICAL severity

CVE-2021-21321 — fastify-reply-from

CRITICALFix: fastify/fastify-reply-from@dea227d

CVE-2021-21321 is a critical-severity (CVSS 10) Improper Input Validation vulnerability in fastify-reply-from. A fix is available for fastify-reply-from — see the affected versions and patch details below.

Prefix escape

Also known asGHSA-qmw8-3v4g-gwj4

Published

Mar 2, 2021

Updated

Aug 7, 2026

Affected

1 pkg

Patched

1 / 1

Exploits

None indexed

Exploitation data as of Oct 7, 2026 · OSV.dev, NVD, FIRST.org (EPSS)

EPSS Exploitation Probability

via FIRST.org ↗

1.8%probability of exploitation in next 30 days

Lower Risk0.00%

Lower risk than most CVEs78th percentile — riskier than 78% of all scored CVEsHighest risk

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

How urgent is this, really

CVE-2021-21321 by exploitation likelihood (EPSS) against impact (CVSS). Outside the shaded patch-first corner.

Where this sits among everything scored

Of 385,987 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-reply-fromnpm

3Kdownloads / week

Description

Impact

By crafting a specific URL, it is possible to escape the prefix of the proxied backend service. If the base url of the proxied server is /pub/, a user expect that accessing /priv on the target service would not be possible. Unfortunately, it is.

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:H/I:H/A:N

Patches

A patch have been submitted by Corey Farrell [email protected], the reporter. All releases after v4.0.2 include the fix.

Workarounds

There are no workaround available.

For more information

If you have any questions or comments about this advisory:

Open an issue in fastify-reply-from
Email us at [email protected]

Affected Packages

1 total 1 fixed

Ecosystem	Package	Vulnerable range	Fix
📦npm	`fastify-reply-from`	all versions	4.0.2`npm install fastify-reply-from@4.0.2`

Affected Products

1 product · 1 configurations

Application

fastify-reply-fromfastify-reply-from_project

< 4.0.2

range

Detection & mitigation playbook

Open-source dependency

Detect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for fastify-reply-from, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
Fix
Update fastify-reply-from to 4.0.2 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2021-21321 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 By crafting a specific URL, it is possible to escape the prefix of the proxied backend service. If the base url of the proxied server is `/pub/`, a user expect that accessing `/priv` on the target service would not be possible. Unfortunately, it is. [CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:H/I:H/A:N](https://www.first.org/cvss/calculator/3.1#CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:H/I:H/A:N) ### Patches A patch have been submitted by Corey Farrell [email protected], the reporter. All releases after v4.0.2 include the fix. ### Workarounds There are no workaround available. ### For more inf