Interpretation Conflict Affecting @fastify/express package, versions <4.0.5
Threat Intelligence
Snyk has a proof-of-concept or detailed explanation of how to exploit this vulnerability.
The probability is the direct output of the EPSS model, and conveys an overall sense of the threat of exploitation in the wild. The percentile measures the EPSS probability relative to all known EPSS scores. Note: This data is updated daily, relying on the latest available EPSS model version. Check out the EPSS documentation for more details.
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Test your applications- Snyk IDSNYK-JS-FASTIFYEXPRESS-16068303
- published15 Apr 2026
- disclosed15 Apr 2026
- creditFred K. Schott
Introduced: 15 Apr 2026
NewCVE-2026-33808 (opens in a new tab)How to fix?
Upgrade @fastify/express to version 4.0.5 or higher.
Overview
@fastify/express is an Express compatibility layer for Fastify
Affected versions of this package are vulnerable to Interpretation Conflict via improper URL normalization gaps. An attacker can gain unauthorized access to protected routes by manipulating the URL path with duplicate slashes or semicolon delimiters, causing middleware to be bypassed.
PoC
const fastify = require('fastify')
async function start() {
const app = fastify({
logger: false,
ignoreDuplicateSlashes: true, // documented Fastify option
})
await app.register(require('@fastify/express'))
// Standard Express middleware auth pattern
app.use('/admin', function expressAuthGate(req, res, next) {
const auth = req.headers.authorization
if (!auth || auth !== 'Bearer admin-secret-token') {
res.statusCode = 403
res.setHeader('content-type', 'application/json')
res.end(JSON.stringify({ error: 'Forbidden by Express middleware' }))
return
}
next()
})
// Protected route
app.get('/admin/dashboard', async (request) => {
return { message: 'Admin dashboard', secret: 'sensitive-admin-data' }
})
await app.listen({ port: 3000 })
console.log('Listening on http://localhost:3000')
}
start()