9.1

CVE-2026-33804: fastify fastify\/middie Interpretation Conflict

CVSS: 9.1v3.1CriticalAnalyzed
AliasesGHSA-v9ww-2j6r-98q6
Published 16-04-2026Last modified 16-04-2026
Open JSON

Vulnerability summary

CVE-2026-33804 is a critical (CVSS 9.1) Interpretation Conflict vulnerability in fastify fastify\/middie. EPSS estimates a 0.49% chance of exploitation in the next 30 days, higher than 40.9% of scored vulnerabilities.

Overall risk
70/100
0100
High · default modelHow is this calculated?
CVSS score
9.1/10
010
Critical · v3.1 (NVD)
EPSS score
0.49%
0%100%
Low · +0.21 pp in 30 days
KEVNot listedIn no KEV catalog
RansomwareNone recordedNo known use recorded
Public exploits0No public exploits recorded

Timeline

  1. 16-04-2026
    PublishedVulnerability first disclosed.
  2. 16-04-2026
    Last modifiedVulnerability information updated.

Description

@fastify/middie versions 9.3.1 and earlier are vulnerable to middleware bypass when the deprecated Fastify ignoreDuplicateSlashes option is enabled. The middleware path matching logic does not account for duplicate slash normalization performed by Fastify's router, allowing requests with duplicate slashes to bypass middleware authentication and authorization checks. This only affects applications using the deprecated ignoreDuplicateSlashes option. Upgrade to @fastify/middie 9.3.2 to fix this issue. There are no workarounds other than disabling the ignoreDuplicateSlashes option.

CVSS metrics

3 scores · 3 sources · 1 version

EPSS trend

Probability

0.49%

Chance of exploitation in the next 30 days

Percentile

41%

Versus all other CVEs

30-day change

+0.21 pp

From 0.28% on 20-09-2026

20-09-202620-10-2026
EPSS score
0.49%

Known exploited

Compare KEV catalogs

Not in any of the 3 tracked catalogs.

Public exploits

0 found

No public exploits indexed.

Weaknesses

CWE · root cause

Affected systems

@fastify1 product
  • middie< 9.3.2
@fastify/middie1 product
Fastify1 product

3 vendors · 3 products

References

4 links