CVE-2021-21322Network attack vector

Unauthenticated prefix escape via crafted URL

Published Mar 2, 2021 · Updated Aug 3, 2024

@fastify/http-proxy before 4.3.1 allows remote attackers to access upstream paths outside the configured proxy prefix via crafted URLs. The proxy's URL rewriting accepts a path that does not remain under the configured prefix, forwarding requests to otherwise hidden backend paths. No authentication is required, so any network client reaching the proxy can obtain responses from those unintended upstream locations, with confidentiality and integrity impact bounded by backend permissions.

CVSS severity9.8
Critical
EPSS probability1.63%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

@fastify/http-proxy before 4.3.1 allows remote attackers to access upstream paths outside the configured proxy prefix via crafted URLs. The proxy's URL rewriting accepts a path that does not remain under the configured prefix, forwarding requests to otherwise hidden backend paths. No authentication is required, so any network client reaching the proxy can obtain responses from those unintended upstream locations, with confidentiality and integrity impact bounded by backend permissions.

The record

CVE
CVE-2021-21322
Published
Mar 2, 2021
Updated
Aug 3, 2024
Vendor
Fastify
Product
@fastify/http-proxy
Classifications
CWE-20, T1190
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. Mar 2, 2021CVE publishedPublication date reported by the CVE source.
  2. Aug 3, 2024Record updatedLatest update available in the CVE record.

Exploitability

Present is not the same as exploitable

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Is a vulnerable build present?

Compare these published version ranges with your installed build and any vendor patches.

  1. Affected versionversion=< 4.3.1

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

Fastify · @fastify/http-proxyversion=< 4.3.1

Published CVSS scores

7.5NVDAV:N/AC:L/Au:N/C:P/I:P/A:P
10.0GitHubCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:H/I:H/A:N
9.8NVDCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H

CVSS describes severity. EPSS estimates exploitation probability.

Attacks

What attackers are doing with it

Daily unique IPs observed by Shadowserver honeypots for known exploited vulnerabilities (KEVs). Missing observations do not establish an absence of attacks.

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No observations available

Sep 10, 2026Sep 16, 2026
Latest reporting daySep 16, 2026
Latest daily unique IPsUnavailable
Prior 30-day averageUnavailable
SourceShadowserver honeypots (KEV)
Vectornetwork
Privilegesunauthenticated
Known exploitationUnconfirmed
Public exploitUnconfirmed

Weakness, pattern, technique

CWE-20Improper Input Validation
T1190Exploit Public-Facing Application

Public exploit references

No public exploit references are available in this record.

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Technologies

Your stack

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Your stack

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