Cyber Resilience

CVE-2026-33806

Fastify 5.3.2 – 5.8.5

Published
15 April 2026
Modified
15 July 2026
Patch / advisory
View advisory
CVSS Score v3.1 7.5
Click a component to see what it means
Raw vectorCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:N
EPSS Score 0.0041 34th percentile
Risk Priority 57 floored blend · peak EPSS

Summary

CVE-2026-33806 is a high-severity Improper Validation of Specified Type of Input (CWE-1287) vulnerability in Fastify Fastify. Its CVSS base score is 7.5 (High).

Operationally, exploitation aligns with the MITRE ATT&CK technique Exploitation for Privilege Escalation (T1068); ranked at the 34th percentile by exploit likelihood (below the median); it is not currently listed in the CISA KEV catalog.

The strongest mitigations our analysis identified map to 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-2026-33806 is a vulnerability in Fastify applications that utilize schema.body.content for per-content-type body validation. It allows attackers to entirely bypass schema validation by prepending a space to the Content-Type header, while the body is still parsed correctly. This issue is a regression introduced in Fastify versions 5.3.2 and later, stemming from the fix for CVE-2025-32442, and is classified under CWE-1287 with 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).

Remote attackers require no privileges or user interaction to exploit this vulnerability over the network with low complexity. By crafting an HTTP request with a space-prepended Content-Type header, such as " application/json" instead of "application/json", they can bypass intended body validation schemas. This enables the processing of malformed or malicious payloads that would otherwise be rejected, leading to high integrity impacts such as improper data handling or injection of invalid content into the application.

Advisories from the Fastify GitHub security page (GHSA-mg2h-6x62-wpwc) and OpenJSF CNA recommend upgrading to Fastify v5.8.5 or later to mitigate the issue. No workarounds are available, emphasizing the need for prompt patching in affected applications.

EU & UK References

Vulnerability Data

Impact: Fastify applications using schema.body.content for per-content-type body validation can have validation bypassed entirely by prepending a space to the Content-Type header. The body is still parsed correctly but schema validation is skipped. This is a regression introduced in fastify…

more

>= 5.3.2 by the fix for CVE-2025-32442 Patches: Upgrade to fastify v5.8.5 or later. Workarounds: None. Upgrade to the patched version.

CWE(s)
CWE-1287CWE-1289

Related Threats

MITRE ATT&CK Enterprise Techniques

T1068 Exploitation for Privilege Escalation Privilege Escalation
Adversaries may exploit software vulnerabilities in an attempt to elevate privileges.
attack.mitre.org →
T1190 Exploit Public-Facing Application Initial Access
Adversaries may attempt to exploit a weakness in an Internet-facing host or system to initially access a network.
attack.mitre.org →
T1203 Exploitation for Client Execution Execution
Adversaries may exploit software vulnerabilities in client applications to execute code.
attack.mitre.org →
T1210 Exploitation of Remote Services Lateral Movement
Adversaries may exploit remote services to gain unauthorized access to internal systems once inside of a network.
attack.mitre.org →
T1212 Exploitation for Credential Access Credential Access
Adversaries may exploit software vulnerabilities in an attempt to collect credentials.
attack.mitre.org →
Derived from this CVE’s CWE(s) via the direct CWE→ATT&CK cross-walk.

CVEs Like This One

CVE-2025-32442Same product: Fastify Fastify
CVE-2026-25223Same product: Fastify Fastify
CVE-2026-3419Same product: Fastify Fastify
CVE-2026-3635Same product: Fastify Fastify
CVE-2026-25224Same product: Fastify Fastify
CVE-2026-16117Same vendor: Fastify
CVE-2026-2880Same vendor: Fastify
CVE-2026-15631Same vendor: Fastify
CVE-2026-15074Same vendor: Fastify
CVE-2026-22031Same vendor: Fastify

Affected Assets

fastify
fastify
5.3.2 — 5.8.5

Mitigating Controls

Control response

Prevent
Stop it (NIST 800-53)

—

Detect
Catch it (NIST detect / respond)

—

Harden
Shrink the surface (DISA STIG)

—

Validate
Prove the fix (OWASP ASVS)
  • V5.2.2
  • V9.1.3
  • V10.4.6
  • V1.1.1

Mitigating Controls (NIST 800-53 r5) AI

SI-10 Information Input Validation good match

SI-10 directly requires validation of information inputs, which structurally prevents type-validation failures from being introduced or exploitable.

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.

PR.PS-06 mostly match
prevents

Secure SDLC practices directly require proper input type validation during development.

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.

A.8.29 Security testing in development and acceptance mostly match
finds

Security testing can detect type-validation flaws but does not prevent them during development.

A.8.25 Secure development life cycle partial match
prevents

Secure development lifecycle mandates input validation and type checking to prevent improper type handling.

A.8.26 Application security requirements partial match
prevents

Application security requirements explicitly include validation of input data types and formats.

A.8.27 Secure system architecture and engineering principles partial match
prevents

Secure architecture principles promote defensive input handling and type enforcement at system boundaries.

A.8.28 Secure coding partial match
prevents

Secure coding standards directly require proper type validation of all external inputs.

References