CVE-2026-2880
@fastify/middie has Improper Path Normalization when Using Path-Scoped Middleware
Impact
## Summary A path normalization inconsistency in `@fastify/middie` can result in authentication/authorization bypass when using path-scoped middleware (for example, `app.use('/secret', auth)`). When Fastify router normalization options are enabled (such as `ignoreDuplicateSlashes`, `useSemicolonDelimiter`, and related trailing-slash behavior), crafted request paths may bypass middleware checks while still being routed to protected handlers. ## Impact An unauthenticated remote attacker can access endpoints intended to be protected by middleware-based auth/authorization controls by sending specially crafted URL paths (for example, `//secret` or `/secret;foo=bar`), depending on router option configuration. This may lead to unauthorized access to protected functionality and data exposure. ## Affected versions - Confirmed affected: `@fastify/middie@9.1.0` - All versions prior to the patch are affected. ## Patched versions - Fixed in: *9.2.0* ## Details The issue is caused by canonicalization drift between: 1. `@fastify/middie` path matching for `app.use('/prefix', ...)`, and 2. Fastify/find-my-way route lookup normalization. Because middleware and router did not always evaluate the same normalized path, auth middleware could be skipped while route resolution still succeeded. ## Workarounds Until patched version is deployed: - Avoid relying solely on path-scoped middie guards for auth/authorization. - Enforce auth at route-level handlers/hooks after router normalization. - Disable risky normalization combinations only if operationally feasible. ## Resources - Fluid Attacks Disclosure Policy: https://fluidattacks.com/advisories/policy - Fluid Attacks advisory URL: https://fluidattacks.com/advisories/jimenez ## Credits - **Cristian Vargas** (Fluid Attacks Research Team) — discovery and report. - **Oscar Uribe** (Fluid Attacks) — coordination and disclosure.
Affected packages
| Ecosystem | Package | Affected | Fixed in |
|---|---|---|---|
| npm | @fastify/middie | < 9.2.0 | 9.2.0 |
Affected products
| Vendor | Product | Affected versions |
|---|---|---|
| Fastify | Fastify\/Middie | before 9.2.0 |
Weakness classification
References
- NVD record
- CVE.org record
- GitHub advisory
- https://github.com/fastify/middie/security/advisories/GHSA-8p85-9qpw-fwgw
- https://github.com/advisories/GHSA-8p85-9qpw-fwgw
- https://nvd.nist.gov/vuln/detail/CVE-2026-2880
- https://github.com/fastify/middie/commit/140e0dd0359d890fec7e6ea1dcc5134d6bd554d4
- https://fluidattacks.com/advisories/jimenez
- https://fluidattacks.com/advisories/policy
- https://github.com/fastify/middie/releases/tag/v9.2.0