CVE-2023-51701VulnerabilityPublished 8 Jan 2024Updated 3 Jun 2025

CVE-2023-51701: fastify-reply-from · HTTP Request/Response Smuggling

Affects fastify-reply-fromSource CVE Database V5

Not known to be exploited · CVSS 5.3 Medium : upgrade to 9.6.0 when convenient.

Data as of 26 Oct 2026, 06:26 UTC

CVSS 3.1 base score

5.3Medium

How severe the flaw is on the 0–10 CVSS scale.

EPSS: 0.4% chance of exploitation in 30 days – higher than 41% of CVEsExploit Prediction Scoring System: the likelihood attackers will use this flaw soon.
Patch: Available – fixed in 9.6.0

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

Attack Vector: Network
Attack Complexity: Low
Privileges Required: None
User Interaction: None
Scope: Unchanged
Confidentiality: Low
Integrity: None
Availability: None

Bold values are the worst case: easiest to exploit or largest impact.

Attack Vector: Network: The vulnerable component is bound to the network stack and the set of possible attackers extends beyond the other options listed, up to and including the entire Internet. Such a vulnerability is often termed 'remotely exploitable' and can be thought of as an attack being exploitable at the protocol level one or more network hops away (e.g., across one or more routers).
Attack Complexity: Low: Specialized access conditions or extenuating circumstances do not exist. An attacker can expect repeatable success against the vulnerable component.
Privileges Required: None: The attacker is unauthorized prior to attack, and therefore does not require any access to settings or files to carry out an attack.
User Interaction: None: The vulnerable system can be exploited without any interaction from any user.
Scope: Unchanged: An exploited vulnerability can only affect resources managed by the same security authority. In this case, the vulnerable component and the impacted component are either the same, or both are managed by the same security authority.
Confidentiality: Low: There is some loss of confidentiality. Access to some restricted information is obtained, but the attacker does not have control over what information is obtained, or the amount or kind of loss is limited. The information disclosure does not cause a direct, serious loss to the impacted component.
Integrity: None: There is no loss of integrity within the impacted component.
Availability: None: There is no impact to availability within the impacted component.

Definitions: FIRST CVSS v3.1 specification.

Exploitation evidence

Disclosed: evidence found.
CVE published 8 Jan 2024
Detectable: no evidence on record.
No public nuclei template indexed
Weaponised: no evidence on record.
No public exploit code indexed in Metasploit or Exploit-DB
Exploited in the wildExploited: no evidence on record.
Not in CISA KEV and no source reports exploitation
Observed by OffSeq MirageSeen by Mirage: no evidence on record.
No attack traffic recorded by OffSeq Mirage sensors

A missing step is not evidence of safety. Sources: cve.org (opens in a new tab), the Metasploit, Exploit-DB and nuclei-templates index as of 2026-10-26.

Disclosed: The CVE record is public on cve.org. Its publication is t = 0.
Detectable: A public nuclei template can find exposed instances, for defenders and attackers alike.
Weaponised: Public exploit code exists in Metasploit or Exploit-DB, or CISA SSVC reports a public proof of concept.
Exploited in the wild: CISA KEV lists it, CISA SSVC rates exploitation active, or a source reports attacks.
Observed by OffSeq Mirage: OffSeq Mirage honeypot sensors recorded attack traffic for this CVE, or found exposed hosts without attack traffic.

Moments are spaced evenly, not to scale; a dashed line marks a gap of six months or more. Deltas count from t = 0. Sources that give only a date (CISA KEV, Exploit-DB, or a time of exactly 00:00Z) read “same day” rather than hours. Metasploit modules carry the flaw’s disclosure date, not their release date, and nuclei templates carry no date, so both appear above but not on the timeline.

What to do

Upgrade fastify-reply-from to 9.6.0.

Affected software

Product	Affected	Fixed in
fastify fastify-reply-from	Affected <9.6.0	Fixed in 9.6.0

AI-suggested

European organizations should prioritize upgrading '@fastify/reply-from' to version 9.6.0 or later, where the vulnerability is patched. Beyond upgrading, organizations should audit their HTTP header parsing logic and ensure strict validation of 'Content-Type' headers to prevent malformed or ambiguous values. Implementing comprehensive logging and monitoring of HTTP traffic for unusual header formats or anomalies can help detect attempted exploitation. Network-level protections such as Web Application Firewalls (WAFs) should be configured to detect and block HTTP request smuggling patterns, including malformed headers or unexpected whitespace in header values. Security teams should conduct penetration testing focused on HTTP request smuggling to identify any residual vulnerabilities in their proxy configurations. Finally, developers should review and harden any custom middleware or plugins that handle HTTP headers to ensure consistent and secure parsing.

Description

fastify-reply-from is a Fastify plugin to forward the current HTTP request to another server. A reverse proxy server built with `@fastify/reply-from` could misinterpret the incoming body by passing an header `ContentType: application/json ; charset=utf-8`. This can lead to bypass of security checks. This vulnerability has been patched in '@fastify/reply-from` version 9.6.0.

Impact

For European organizations, the impact of this vulnerability can be significant, especially for those relying on Fastify-based reverse proxy setups using the 'fastify-reply-from' plugin. HTTP request smuggling can enable attackers to bypass security controls, potentially exposing internal services or sensitive data that should be protected. This could lead to unauthorized access to internal APIs, session hijacking, or injection of malicious payloads into backend systems. Given the medium severity and limited confidentiality impact, the immediate risk is moderate; however, in complex environments, this vulnerability could be chained with other exploits to escalate privileges or exfiltrate data. Organizations in sectors such as finance, healthcare, and critical infrastructure, which often deploy reverse proxies for load balancing and security, may face increased risk. Additionally, the vulnerability's exploitation does not require authentication or user interaction, increasing the attack surface. The absence of known exploits in the wild suggests that proactive patching can effectively mitigate risk before widespread exploitation occurs.

CWE-444Inconsistent Interpretation of HTTP Requests ('HTTP Request/Response Smuggling') (opens in a new tab)
Base weaknessOWASP Top 10 A06:2025: Insecure Design (opens in a new tab)
The product acts as an intermediary HTTP agent (such as a proxy or firewall) in the data flow between two entities such as a client and server, but it does not interpret malformed HTTP requests or responses in ways that are consistent with how the messages will be processed by those entities that are at the ultimate destination.
Implementation
Use a web server that employs a strict HTTP parsing procedure, such as Apache.
Implementation
Use only SSL communication.
Implementation
Terminate the client session after each request.
OWASP Cheat Sheets
REST Assessment (opens in a new tab)
REST Security (opens in a new tab)
Cross-Site Request Forgery Prevention (opens in a new tab)
Secure Design Principles (opens in a new tab)
OWASP ASVS 4.0.3
V13.2.5 (opens in a new tab)
Attack patterns
HTTP Response Smuggling (opens in a new tab)
HTTP Request Smuggling (opens in a new tab)
From MITRE CWE, MITRE CAPEC and OWASP (Top 10, OpenCRE).

References

Community reviews

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Exploitation evidence

What to do

Affected software

Description

AI analysisMachine-generated, updated 4 Jul 2025, 02:57 UTC

WeaknessesCWE-444

Technical details

References

Community reviews

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