What is CVE-2021-21321?

CVE-2021-21321 is a critical-severity improper input validation vulnerability in fastify-reply-from (npm), affecting versions < 4.0.2. It is fixed in 4.0.2. The application does not adequately validate input before processing it, allowing unexpected values to reach sensitive code paths.

How severe is CVE-2021-21321?

CVE-2021-21321 has a CVSS score of 10.0 (Critical). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.

Which versions of fastify-reply-from are affected by CVE-2021-21321?

fastify-reply-from (npm) versions < 4.0.2 is affected.

Is there a fix for CVE-2021-21321?

Yes. CVE-2021-21321 is fixed in 4.0.2. Upgrade to this version or later.

Is CVE-2021-21321 exploitable, and should I be worried?

Whether CVE-2021-21321 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk.

What actually determines whether CVE-2021-21321 is exploitable, and how bad it is?

Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.

How do I fix CVE-2021-21321?

Upgrade fastify-reply-from to 4.0.2 or later.

CVE-2021-21321 in fastify-reply-from

Q: How severe is CVE-2021-21321?

CVE-2021-21321 has a CVSS score of 10.0 (Critical). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.

Q: Which versions of fastify-reply-from are affected by CVE-2021-21321?

fastify-reply-from (npm) versions < 4.0.2 is affected.

Q: Is there a fix for CVE-2021-21321?

Yes. CVE-2021-21321 is fixed in 4.0.2. Upgrade to this version or later.

Q: Is CVE-2021-21321 exploitable, and should I be worried?

Whether CVE-2021-21321 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk.

Q: What actually determines whether CVE-2021-21321 is exploitable, and how bad it is?

Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.

Q: How do I fix CVE-2021-21321?

Upgrade fastify-reply-from to 4.0.2 or later.

Summary

Prefix escape

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]

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

The application does not adequately validate input before processing it, allowing unexpected values to reach sensitive code paths. Typical impact: varies by context: data corruption, logic bypass, or denial of service.

CVE-2021-21321 has a CVSS score of 10.0 (Critical). The vector is network-reachable, no privileges required, and no user interaction. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. A fixed version is available (4.0.2); upgrading removes the vulnerable code path.

Affected versions

fastify-reply-from (< 4.0.2)

Security releases

fastify-reply-from → 4.0.2 (npm)

Kodem intelligence

Severity tells you how bad this could be in the worst case. It does not tell you whether you are exposed. Exploitability and impact are functions of runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A vulnerable package can sit in your dependency tree and never run.

Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter. Kodem's runtime-powered SCA identifies whether this CVE is reachable in your applications.

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See it in your environment New to Kodem? Get a demo →

Remediation advice

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

Frequently Asked Questions

What is CVE-2021-21321? CVE-2021-21321 is a critical-severity improper input validation vulnerability in fastify-reply-from (npm), affecting versions < 4.0.2. It is fixed in 4.0.2. The application does not adequately validate input before processing it, allowing unexpected values to reach sensitive code paths.
How severe is CVE-2021-21321? CVE-2021-21321 has a CVSS score of 10.0 (Critical). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
Which versions of fastify-reply-from are affected by CVE-2021-21321? fastify-reply-from (npm) versions < 4.0.2 is affected.
Is there a fix for CVE-2021-21321? Yes. CVE-2021-21321 is fixed in 4.0.2. Upgrade to this version or later.
Is CVE-2021-21321 exploitable, and should I be worried? Whether CVE-2021-21321 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
What actually determines whether CVE-2021-21321 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
How do I fix CVE-2021-21321? Upgrade fastify-reply-from to 4.0.2 or later.