CVE-2021-21321

Prefix escape

CRITICAL
10
CVSS Score
Published: March 2, 2021 at 03:35 AM UTC
Updated: June 17, 2026 at 03:35 AM UTC
2 products affected across 2 vendorsFastify Reply FromReply From

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Severity
CRITICAL10
Exploit prediction
EPSS 2%

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.

Affected

Our own answer: ranges merged, affected separated from platform-only, with confidence and review flags.

v < 4.0.2
Affected range stops below 4.0.2, so it looks safe — our inference, not a vendor statement
v < 4.0.2
Affected range stops below 4.0.2, so it looks safe — our inference, not a vendor statement

Remediation

No guide yet

No remediation guidance has been written for this CVE yet. What the record does show:

CVSS

10CRITICALGitHub_M
Attack Vector
NETWORK
Complexity
LOW
Privileges
NONE
User Interaction
NONE
Scope
CHANGED
Confidentiality
HIGH
Integrity
HIGH
Availability
NONE
Vector?

Quick Info

StatusMODIFIED
CWE
CWE-20

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