CVE-2026-6270
Fastify\/Middie ≤ 9.3.2
Raw vector
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:NSummary
CVE-2026-6270 is a critical-severity Interpretation Conflict (CWE-436) vulnerability in Fastify Fastify\/Middie. Its CVSS base score is 9.1 (Critical).
Operationally, exploitation aligns with the MITRE ATT&CK technique Exploit Public-Facing Application (T1190); ranked at the 40th percentile by exploit likelihood (below the median); it is not currently listed in the CISA KEV catalog; a public proof-of-concept is referenced.
The strongest mitigations our analysis identified map to SA-11 (Developer Testing and Evaluation) and SI-10 (Information Input Validation) — see the control section below for these in your framework.
Deeper analysis AI-assisted summary
Synthesised by an AI model from the NVD description and linked references — a reading aid, not an authoritative source.
CVE-2026-6270 affects @fastify/middie versions 9.3.1 and earlier, a middleware plugin for the Fastify web framework. The vulnerability stems from the failure to register inherited middleware directly on child plugin engine instances. In Fastify applications that register authentication middleware in a parent scope and then add child plugins via @fastify/middie, the child scopes do not inherit the parent middleware. This misconfiguration enables unauthenticated requests to access routes in child plugin scopes, effectively bypassing authentication and authorization checks. The issue is classified under CWE-436 and carries a CVSS v3.1 base score of 9.1 (AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:N).
Remote attackers require no privileges or user interaction to exploit this vulnerability over the network with low complexity. By sending crafted requests to routes defined in child plugin scopes, attackers can bypass parent-level authentication middleware, gaining unauthorized access to sensitive endpoints. Successful exploitation results in high confidentiality and integrity impacts, such as reading or modifying protected data without valid credentials, while availability remains unaffected.
Advisories recommend upgrading to @fastify/middie version 9.3.2, which resolves the inheritance issue. No workarounds are available. Detailed guidance is provided in the OpenJSF security advisories at https://cna.openjsf.org/security-advisories.html, as well as GitHub security advisories for fastify-express (GHSA-hrwm-hgmj-7p9c) and fastify/middie (GHSA-72c6-fx6q-fr5w).
OWASP Top 10 for Web (2025)
EU & UK References
- 🇪🇺 ENISA EUVD: EUVD-2026-23241
Vulnerability Data
@fastify/middie versions 9.3.1 and earlier do not register inherited middleware directly on child plugin engine instances. When a Fastify application registers authentication middleware in a parent scope and then registers child plugins with @fastify/middie, the child scope does not inherit…
more
the parent middleware. This allows unauthenticated requests to reach routes defined in child plugin scopes, bypassing authentication and authorization checks. Upgrade to @fastify/middie 9.3.2 to fix this issue. There are no workarounds.
- CWE(s)
Related Threats
MITRE ATT&CK Enterprise Techniques
CVEs Like This One
Affected Assets
Mitigating Controls
Mitigating Controls (NIST 800-53 r5) AI
Developer testing and evaluation can discover cases where two products interpret the same inputs or state transitions differently.
Strict, consistently applied input validation reduces the chance that one product will accept data the other product rejects or interprets differently.
Applying security engineering principles during design can require unambiguous protocol and data-format specifications that eliminate divergent interpretations between products.
Mitigating Controls (NIST CSF 2.0) AI
Derived directly from the weakness types (CWEs) cited in the NVD entry via our AI-authored CWE→CSF cross-walk (authority under review) — links open the control.
Secure SDLC practices directly reduce the chance of introducing parser or state-machine inconsistencies.
Correlating logs from multiple products can surface discrepancies caused by interpretation conflicts.
Runtime monitoring of software behavior can detect adverse outcomes stemming from differing interpretations.
Supplier risk assessments can identify products whose differing interpretations create systemic exposure.
Mitigating Controls (ISO/IEC 27001:2022 Annex A) AI
Derived directly from the weakness types (CWEs) cited in the NVD entry via our AI-authored CWE→ISO cross-walk (authority under review) — links open the control.
Security testing can detect and correct cases where one component misinterprets another’s state or messages.
Secure development lifecycle can require consistent interface contracts and canonicalization rules that reduce interpretation conflicts between components.
Explicit application security requirements can mandate unambiguous protocol and data-format specifications that prevent divergent interpretations.
Secure architecture principles include well-defined component boundaries and shared data models that limit conflicting state perceptions.
Secure coding standards can enforce canonical input handling and strict protocol compliance to avoid misinterpretation between products.