VulnerabilityPublished 27 Oct 2026

CVE-2026-112362: @fastify/express · Interpretation Conflict

Affects @fastify/expressSource CVE Database V5

Not known to be exploited · CVSS 9.1 Critical : upgrade to 4.0.9 as a priority.

Data as of 27 Oct 2026, 11:57 UTC

CVSS 3.1 base score

9.1Critical

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

Patch: Available – fixed in 4.0.9

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

Attack Vector: Network
Attack Complexity: Low
Privileges Required: None
User Interaction: None
Scope: Unchanged
Confidentiality: High
Integrity: High
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: High: There is total loss of confidentiality, resulting in all resources within the impacted component being divulged to the attacker. Alternatively, access to only some restricted information is obtained, but the disclosed information presents a direct, serious impact.
Integrity: High: There is a total loss of integrity, or a complete loss of protection. For example, the attacker is able to modify any/all files protected by the impacted component. Alternatively, only some files can be modified, but malicious modification would present a direct, serious consequence to 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 27 Oct 2026
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 26 Oct 2026.

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 or the CVSS exploit maturity 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/express to 4.0.9.

Affected software

Product	Affected	Fixed in
@fastify/express@fastify/express	Affected >=4.0.5 <4.0.9	Fixed in 4.0.9
npm@fastify/express`pkg:npm/@fastify/express`Other threats	Affected >=4.0.5 <4.0.9	Fixed in 4.0.9

Check your own hosts for these packages with threat-finder (opens in a new tab), our free open-source scanner.

AI-suggested

Upgrade @fastify/express to version 4.0.9 or later, where the issue is fixed. Alternatively, avoid enabling the non-default ignoreDuplicateSlashes option. As a workaround, enforce authentication and other critical middleware in Fastify hooks such as preHandler that run after the router resolves the request, ensuring middleware is applied regardless of path normalization differences.

Description

@fastify/express versions 4.0.5 through 4.0.8 contain a vulnerability where, if Fastify is configured with the non-default ignoreDuplicateSlashes option, path-scoped Express middleware may be bypassed. This occurs because Fastify's router normalizes duplicate slashes differently than Express when handling absolute-form request targets, allowing requests to reach scoped routes without passing through authentication or authorization middleware. The issue is fixed in version 4.0.9.

Impact

An attacker can bypass path-scoped middleware protections such as authentication, authorization, or rate limiting on scoped routes (e.g., admin paths) by exploiting the path normalization conflict when ignoreDuplicateSlashes is enabled. This leads to unauthorized access to protected handlers without credentials. The vulnerability has a high severity with a CVSS score of 9.1, indicating critical impact on confidentiality and integrity without requiring privileges or user interaction.

Summary

@fastify/express is a plugin that allows Express middleware to run inside Fastify. In versions 4.0.5 through 4.0.8, when Fastify is run with the ignoreDuplicateSlashes option enabled (which is not the default), a conflict arises in how duplicate slashes are normalized between Fastify and Express. Specifically, for absolute-form request targets, Fastify's router collapses duplicate slashes including the separator after the scheme, resolving the request to a scoped path and executing the handler. However, the Express middleware layer does not see the same normalized path, causing it to skip middleware such as authentication, authorization, or rate limiting. This discrepancy allows an unauthenticated attacker to access protected routes if the ignoreDuplicateSlashes option is enabled. The vulnerability is addressed in @fastify/express version 4.0.9, which corrects the path normalization behavior. As a workaround, users can enforce authentication in Fastify hooks like preHandler or avoid enabling ignoreDuplicateSlashes.

CWE-436Interpretation Conflict (opens in a new tab)
Class weaknessOWASP Top 10 A06:2025: Insecure Design (opens in a new tab)
Product A handles inputs or steps differently than Product B, which causes A to perform incorrect actions based on its perception of B's state.
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 Request Splitting (opens in a new tab)
HTTP Response Smuggling (opens in a new tab)
HTTP Response Splitting (opens in a new tab)
From MITRE CWE, MITRE CAPEC and OWASP (Top 10, OpenCRE).

References

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Exploitation evidence

What to do

Affected software

Description

AI analysisMachine-generated, updated 27 Oct 2026, 11:45 UTC

WeaknessesCWE-436

Technical details

References

Community reviews

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