@fastify/middie: Interpretation Conflict
CVE-2026-85184 is a critical-severity interpretation conflict vulnerability in @fastify/middie, with a CVSS 3.1 base score of 9.1 out of 10. It can be exploited remotely over the network by an unauthenticated attacker, without any action from a user. The record lists version 9.3.4 as the first without the flaw, so updating @fastify/middie to that release or later removes the exposure. There is no confirmed exploitation so far. EPSS estimates a 0.52% chance of exploitation within 30 days, which is low compared with other CVEs.
Critical severity: a successful attack would have severe impact.
What to do
- Update @fastify/middie to 9.3.4 or a later version.
- Read the advisory on cna.openjsf.org for workarounds and exact fixed builds.
- Until you can update, restrict network access to the affected service so it is not reachable from untrusted networks.
Affected products and versions
| Vendor | Product | Affected versions | Fixed in |
|---|---|---|---|
| @fastify/middie | @fastify/middie | 9.1.0 up to, but not including, 9.3.4 | 9.3.4 |
Version data comes from the organisation that assigned the CVE. Confirm against the vendor advisory before deciding you are unaffected.
How the severity score breaks down
- Attack vector
- NetworkCan be exploited remotely over a network, including the internet.
- Attack complexity
- LowNo special conditions are needed; the attack works reliably.
- Privileges required
- NoneNo account or login is needed.
- User interaction
- NoneNo action by a victim is required.
- Scope
- UnchangedImpact stays within the vulnerable component.
- Confidentiality
- HighAll data handled by the component can be read.
- Integrity
- HighData or code can be modified at will.
- Availability
- NoneAvailability is not affected.
Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:N. Scored by CNA.
Official description
@fastify/middie versions >= 9.1.0 and before 9.3.4 decide whether to run path-scoped middleware by matching against the raw request target, while the Fastify router resolves an absolute-form request target to its path before dispatching. Because the two layers evaluate different strings, a request using an absolute-form target reaches the route handler while the path-scoped middleware, such as authentication or authorization, is skipped. An unauthenticated network attacker can use this to bypass path-based access controls in a Fastify application that relies on middie for those controls. Users should upgrade to @fastify/middie 9.3.4 or later.
Source: the CVE record as published by its numbering authority, via NVD and CVE.org.
The underlying weakness: Interpretation Conflict
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.
Timeline
- CVE record published.
- CISA published its SSVC assessment (exploitation: none, automatable: yes, technical impact: total).
- Record last changed materially (score, affected products, status or references).
References
Advisories
- cna.openjsf.org/security-advisories.htmlThird Party Advisory
- github.com/fastify/middie/security/advisories/GHSA-hx87-8wv7-pjv8Mitigation
Other vulnerabilities in @fastify/middie
- 9.1 Critical severity
- 9.1 Critical severity
- 7.5 High severity
- 9.1 Critical severity
- 9.1 Critical severity
- 9.1 Critical severity
Recent Interpretation Conflict vulnerabilities
- 7.5 High severity
- 7.5 High severity
- 9.4 Critical severity
- 8.8 High severity
- 8.8 High severity
- 8.8 High severity
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