CVE-2023-51701Public exploitNetwork attack vector

Security-check bypass via untrimmed Content-Type parsing

Published Jan 8, 2024 · Updated Jun 3, 2025

HTTP request smuggling in Fastify @fastify/reply-from before 9.6.0 allows remote attackers to bypass security checks with a crafted request. The forwarding path split Content-Type at the semicolon without trimming the media type, unlike Fastify's parser, so whitespace before a charset caused inconsistent JSON handling. The plugin must front an upstream server, and the bypass can pass a prohibited JSON body through proxy-side validation.

CVSS severity5.3
Medium
EPSS probability0.48%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

HTTP request smuggling in Fastify @fastify/reply-from before 9.6.0 allows remote attackers to bypass security checks with a crafted request. The forwarding path split Content-Type at the semicolon without trimming the media type, unlike Fastify's parser, so whitespace before a charset caused inconsistent JSON handling. The plugin must front an upstream server, and the bypass can pass a prohibited JSON body through proxy-side validation.

The record

CVE
CVE-2023-51701
Published
Jan 8, 2024
Updated
Jun 3, 2025
Vendor
Fastify
Product
@fastify/reply-from
Classifications
CWE-444, CAPEC-33, T1190
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. Jan 8, 2024CVE publishedPublication date reported by the CVE source.
  2. Jun 3, 2025Record 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=< 9.6.0

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

Fastify · @fastify/reply-fromversion=< 9.6.0

Published CVSS scores

7.5NIST NVDCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:N
5.3GitHubCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:N/A:N

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.

Daily unique IPsNo honeypot observations are available for this CVE in the selected window.

No observations available

Sep 9, 2026Sep 15, 2026
Latest reporting daySep 15, 2026
Latest daily unique IPsUnavailable
Prior 30-day averageUnavailable
SourceShadowserver honeypots (KEV)
Vectornetwork
Privilegesunauthenticated
Known exploitationUnconfirmed
Public exploitPublished

Weakness, pattern, technique

CWE-444Inconsistent Interpretation of HTTP Requests ('HTTP Request/Response Smuggling')
CAPEC-33HTTP Request Smuggling
T1190Exploit Public-Facing Application

Public exploit references

Labels summarize the accepted research assessment. They do not indicate a test against your environment.

Technologies

Your stack

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Your stack

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