How severe is CVE-2021-21321?

CVE-2021-21321 has a CVSS 3.x base score of 10.0, rated critical severity. It is exploitable over network with low attack complexity, requires no privileges and no user interaction. Impact on confidentiality is high, integrity high, and availability none.

Is CVE-2021-21321 being actively exploited?

It is not currently listed in CISA's KEV catalog. Its EPSS exploit-prediction score is 2% (78th percentile), an estimate of the probability of exploitation in the next 30 days.

What products are affected by CVE-2021-21321?

CVE-2021-21321 affects Fastify-reply-from Project Fastify-reply-from. See the affected-products list for the exact vulnerable versions.

How do I fix CVE-2021-21321?

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

When was CVE-2021-21321 published?

CVE-2021-21321 was published on 2021-03-02 and last updated on 2026-06-17.

CVE-2021-21321

CVE-2021-21321 is a critical-severity vulnerability in Fastify-reply-from Project Fastify-reply-from with a CVSS 3.x base score of 10.0. It is not currently listed as actively exploited by CISA, and its EPSS exploit-prediction score is low. The underlying weakness is classified as CWE-20.

Key facts

Severity: Critical (CVSS 3.x base score 10.0)
CVSS v2: 7.5
EPSS exploit prediction: 2% (78th percentile)
Actively exploited: Not listed in CISA KEV
Weakness: CWE-20
Affected product: Fastify-reply-from Project Fastify-reply-from
Published: 2021-03-02
Last modified: 2026-06-17

Description

fastify-reply-from is an npm package which is a fastify plugin to forward the current http request to another server. In fastify-reply-from before version 4.0.2, 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. In affected versions, it is possible. This is fixed in version 4.0.2.

Frequently asked questions

What is CVE-2021-21321?: fastify-reply-from is an npm package which is a fastify plugin to forward the current http request to another server. In fastify-reply-from before version 4.0.2, 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. In affected versions, it is possible. This is fixed in version 4.0.2.
How severe is CVE-2021-21321?: CVE-2021-21321 has a CVSS 3.x base score of 10.0, rated critical severity. It is exploitable over network with low attack complexity, requires no privileges and no user interaction. Impact on confidentiality is high, integrity high, and availability none.
Is CVE-2021-21321 being actively exploited?: It is not currently listed in CISA's KEV catalog. Its EPSS exploit-prediction score is 2% (78th percentile), an estimate of the probability of exploitation in the next 30 days.
What products are affected by CVE-2021-21321?: CVE-2021-21321 affects Fastify-reply-from Project Fastify-reply-from. See the affected-products list for the exact vulnerable versions.
How do I fix CVE-2021-21321?: Review the linked vendor and NVD advisories for patched versions and mitigations, then upgrade or apply the recommended workaround. Given its critical severity, prioritise patching exposed systems.
When was CVE-2021-21321 published?: CVE-2021-21321 was published on 2021-03-02 and last updated on 2026-06-17.

References

Affected products (1)

cpe:2.3:a:fastify-reply-from_project:fastify-reply-from:*:*:*:*:*:node.js:*:*

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