CVE-2021-21321

CRITICALNVD 10.010.0—
EchelonGraph scoreMEDIUM confidence

This critical-severity CVE scores 10.0 under NVD CVSS v3. EPSS exploit probability: 1.8%, top 24% of all CVEs by exploit prediction. GitHub Security Advisory data not yet ingested — confidence will rise once GHSA publishes (typical lag: hours to days for open-source ecosystem CVEs; never for infrastructure-only CVEs).

Triggered by: NVD CVSS baseline
Sources: epss, nvd
Trending — 3 sources updated this week
10.0
EchelonGraph verdictPlan a fixSerious severity, but no confirmed exploitation yet.
  • High severity, but no confirmed exploitation yet
CISA-KEV: Not listedEPSS: 2%CVSS: 10.0Exploit: NoneExposed: 0

No vendor fix yet — apply a workaround or compensating control (WAF / firewall / segmentation) and watch for a patch.

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.

CVSS v3
10.0
EG Score
10.0(medium)
EG Risk
50(Track)
EG Risk 50/100SSVC: Track

EG Risk is EchelonGraph's 0–100 priority score: it fuses intrinsic severity with real-world exploitation and automatability so you can rank equal-severity CVEs and fix the most dangerous first. Higher = act sooner. Distinct from the 0–10 EG Score (severity).

How it’s computed
Severity100% × 45%
Exploitation2% × 40%
Automatability30% × 15%
Action: Routine — remediate on your standard cadence.
EPSS
76.5%
KEV
Not listed

Published

March 2, 2021

Last Modified

November 21, 2024

Advisory Details (2)

Auto-updated Jul 18, 2026
Patch available. Sources: github_commit, github.
github Patch Available

Prefix escape · Advisory · fastify/fastify-reply-from · GitHub

https://github.com/fastify/fastify-reply-from/security/advisories/GHSA-qmw8-3v4g-gwj4
github_commit Patch Available

commit dea227dda606 (fastify/fastify-reply-from)

Patch available: fastify/fastify-reply-from v8.4.2 (contains commit dea227dda606)

https://github.com/fastify/fastify-reply-from/commit/dea227dda606900cc01870d08541b4dcc69d3889

Vendor Advisories for CVE-2021-21321(1)

These vendors published their own advisory mentioning this CVE — often with vendor-specific remediation steps + affected product lists not in NVD.

Affected Packages

(1 across 1 ecosystem)
npm(1)
PackageVulnerable rangeFixed inDependents
fastify-reply-from—4.0.2—

Weakness Classification(1)

MITRE Common Weakness Enumeration — the root-cause categories this CVE belongs to.

Data Freshness Timeline

(refreshed 11× in last 7d / 49× in last 30d)

Each row is a source pipeline that fetched or updated this CVE on that date, with what changed. For example, "NVD update" means NVD published or revised its analysis for this CVE; "MITRE cvelistV5" means we ingested or refreshed it from the CNA feed. Most recent first.

Showing the most recent 100 of 102 total refreshes for this CVE.

  1. 2026-07-23 01:57 UTCEG score recompute
  2. 2026-07-22 22:43 UTCEG score recompute
  3. 2026-07-22 14:05 UTCEPSS rescore
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  40. 2026-06-13 12:43 UTCOSV refresh
  41. 2026-06-12 23:10 UTCEPSS rescore
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  72. 2026-05-26 20:16 UTCEG score recompute
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Frequently asked(5)

What is CVE-2021-21321?
CVE-2021-21321 is a critical vulnerability published on March 2, 2021. 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…
When was CVE-2021-21321 disclosed?
CVE-2021-21321 was first published in the National Vulnerability Database on March 2, 2021, with the most recent update on November 21, 2024. EchelonGraph re-ingests CVE updates from NVD on a 2-hour cycle, so this page reflects the latest published state.
Is CVE-2021-21321 actively exploited?
CVE-2021-21321 is not currently on CISA's Known Exploited Vulnerabilities catalog. FIRST EPSS estimates a 76.5% percentile likelihood of exploitation in the next 30 days — higher percentiles indicate greater predicted risk.
What is the CVSS score of CVE-2021-21321?
CVE-2021-21321 has a CVSS v3 base score of 10.0 (NVD).
How do I remediate CVE-2021-21321?
Patch to the fixed version published by the affected vendor. Where vendor advisories exist for CVE-2021-21321, EchelonGraph cross-links them in the Vendor Advisories panel below — those typically contain the canonical remediation steps, fixed version numbers, and any vendor-specific mitigations.

Dependency Blast Radius

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