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CVE-2021-21321

31
FAUCET Score

CVE-2021-21321 is a critical vulnerability affecting the fastify-reply-from npm package, a fastify plugin for HTTP request forwarding. It allows an attacker to bypass intended prefix restrictions and access unauthorized backend services by crafting a specific URL. With a CVSS score of 10.0, this vulnerability is easily exploitable over the network with low complexity, potentially leading to complete compromise of confidentiality, integrity, and availability. While no active exploitation, public exploit code, or significant community discussion has been observed, the high severity warrants immediate patching to version 4.0.2 or later.

Impacted Technologies

VendorProductVersion(s)CPE
< 4.0.2CPE matchmatch criteria
cpe:2.3:a:fastify-reply-from_project:fastify-reply-from:*:*:*:*:*:node.js:*:*

CVSS Data

CVSS version used by this source: 3.1

10.0CRITICAL

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

Attack Vector
NETWORK
Attack Complexity
LOW
Privileges Required
NONE
User Interaction
NONE
Scope
CHANGED
Confidentiality Impact
HIGH
Integrity Impact
HIGH
Availability Impact
NONE
Exploitability Score
3.9
Impact Score
5.8
CvssVersion
3.1

Exploit Intelligence

EPSS Score
1.82%
Probability of exploitation in next 30 days
EPSS Percentile
76.5%
Percentile rank of EPSS score among Peer Group
As of 2026-07-28
Model: v2026.06.15
This CVE's current EPSS score of 0.0182 is in the 65th percentile among its peer group of 36,897 CVEs.

Social Chatter

The average CVE in this peer group has 0.0 Twitter, 0.1 Reddit, 0.3 Bluesky, 0.3 Mastodon, and 2.4 GitHub mentions.

Media Mentions

No media coverage found for this CVE.

The average CVE in this peer group has 0.3 InfoSec Media, 0.0 Vendor Blog, and 0.0 Security Researcher mentions.

Remediation

Patch Available

Vendor Patches (3)

github_advisorypatch availablevia nvd_reference
View patch
npmpatch availablevia ghsa
Product: fastify-reply-fromFixed in: 4.0.2
redhatno patchvia redhat_api
Product: Red Hat Advanced Cluster Management for Kubernetes 2Fixed in: rhacm2/search-ui-rhel8

Vendor Advisories (2)

npmGHSA-qmw8-3v4g-gwj4critical

Prefix escape

Mar 3, 2021
redhatCVE-2021-21321Critical

fastify-reply-from: crafted URL allows prefix scape of the proxied backend service

Feb 23, 2021

References

github.com / fastify/fastify-reply-from/commit/dea227dda606900cc01870d08541b4dcc69d3889
PatchThird Party Advisory
github.com / fastify/fastify-reply-from/security/advisories/GHSA-qmw8-3v4g-gwj4
Third Party Advisory
npmjs.com / package/fastify-reply-from
ProductThird Party Advisory