CVE-2026-25223
Fastify ≤ 5.7.2
Raw vector
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:NSummary
CVE-2026-25223 is a high-severity Interpretation Conflict (CWE-436) vulnerability in Fastify Fastify. Its CVSS base score is 7.5 (High).
Operationally, exploitation aligns with the MITRE ATT&CK technique Exploit Public-Facing Application (T1190); ranked in the top 48% of CVEs by exploit likelihood; it is not currently listed in the CISA KEV catalog.
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-2026-25223 is a validation bypass vulnerability in Fastify, a fast and low-overhead web framework for Node.js, affecting versions prior to 5.7.2. The flaw allows attackers to completely circumvent request body validation schemas that are specified based on the Content-Type header. This is achieved by appending a tab character (\t) followed by arbitrary content to the Content-Type header, enabling the server to process the request body as the original content type without enforcing the associated validation schema. The issue is classified under CWE-436 and carries a CVSS v3.1 base score of 7.5 (AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:N).
Any unauthenticated remote attacker who can send HTTP requests to an affected Fastify server can exploit this vulnerability with low complexity and no user interaction required. By crafting a malicious Content-Type header, the attacker bypasses body validation, allowing the submission of payloads that violate schema rules but are still parsed and processed according to the intended content type. This could lead to integrity impacts, such as injecting malformed data, executing unintended logic, or enabling further attacks depending on the application's validation usage.
The vulnerability has been patched in Fastify version 5.7.2. Mitigation involves upgrading to this version or later. Details on the fix are documented in the Fastify GitHub security advisory (GHSA-jx2c-rxcm-jvmq), the patching commit (32d7b6add39ddf082d92579a58bea7018c5ac821), and related code changes in content-type-parser.js and validation.js.
OWASP Top 10 for Web (2025)
EU & UK References
- 🇪🇺 ENISA EUVD: EUVD-2026-5157
Vulnerability Data
Fastify is a fast and low overhead web framework, for Node.js. Prior to version 5.7.2, a validation bypass vulnerability exists in Fastify where request body validation schemas specified by Content-Type can be completely circumvented. By appending a tab character (\t)…
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followed by arbitrary content to the Content-Type header, attackers can bypass body validation while the server still processes the body as the original content type. This issue has been patched in version 5.7.2.
- CWE(s)
Related Threats
MITRE ATT&CK Enterprise Techniques
CVEs Like This One
Affected Assets
Mitigating Controls
Control response
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V1.3.6V1.3.8V2.2.1V3.7.2
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.
CWE-179 is a coding flaw enabling input-validation bypasses; it neither impairs post-incident forensic/root-cause processes (RS.AN-03) nor is itself mitigated or exposed by those processes.
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.