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CVE-2021-21321— Prefix escape

Quick assessment

Affected
fastify fastify-reply-from
Exploitation
Public or AI PoC available; prioritize validation
Recommended action
Check the vendor advisory and references for a fixed version. If immediate upgrade is impossible, restrict exposure and increase monitoring.

Matteo Collina fastify-reply-from是 （Matteo Collina）开源的一个应用软件。用于将当前的http请求转发到另一台服务器。 fastify-reply-from before version 4.0.2 存在安全漏洞，该漏洞源于通过创建特定的URL，可以转义代理后端服务的前缀。

CVSS 10.0 · Critical EPSS 1.82% · P78

Possible ATT&CK Techniques 1 AI

T1190 · Exploit Public-Facing Application
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I. Basic Information for CVE-2021-21321

Vulnerability Information
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Vulnerability Title
Prefix escape
Source: CVE Program / CVE List V5
Vulnerability 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.
Source: CVE Program / CVE List V5
CVSS Information
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:H/I:H/A:N
Source: CVE Program / CVE List V5
Vulnerability Type
输入验证不恰当
Source: CVE Program / CVE List V5
Vulnerability Title
fastify-reply-from 存在输入验证错误漏洞
Source: CNNVD (China National Vulnerability Database)
Vulnerability Description
Matteo Collina fastify-reply-from是 （Matteo Collina）开源的一个应用软件。用于将当前的http请求转发到另一台服务器。 fastify-reply-from before version 4.0.2 存在安全漏洞，该漏洞源于通过创建特定的URL，可以转义代理后端服务的前缀。
Source: CNNVD (China National Vulnerability Database)
CVSS Information
N/A
Source: CNNVD (China National Vulnerability Database)
Vulnerability Type
N/A
Source: CNNVD (China National Vulnerability Database)

Affected Products

Vendor Product Affected Versions CPE Subscribe
fastify fastify-reply-from < 4.0.2 -

II. Public POCs for CVE-2021-21321

# POC Description Source Link Shenlong Link
AI-Generated POC Premium
Qwen3.6-35B-A3B · 7696 chars
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III. Intelligence Information for CVE-2021-21321

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Patches & Fixes for CVE-2021-21321 (1)

Vendor Advisories for CVE-2021-21321 (1)

Other References for CVE-2021-21321 (1)

IV. Related Vulnerabilities

Same product: fastify-reply-from
Same vendor: fastify
Same weakness: CWE-20

V. Comments for CVE-2021-21321

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