Refitter C# REST API Client Code Injection via Content-Type

Publication date: 2026-10-30

Last updated on: 2026-10-30

Assigner: GitHub, Inc.

Description

Refitter is a tool for generating a C# REST API client using the Refit library. Prior to 2.1.2, content-type keys in the content maps of `requestBody/responses` are emitted verbatim into a Refit `[Headers("Accept: <ct>", "Content-Type: <ct>")]` attribute string without escaping quotation marks. A crafted content-type key in an attacker-influenced OpenAPI description survives default validation without `--skip-validation`, breaks out of the attribute string and enclosing interface and namespace, and can introduce a top-level file class while leaving valid generated C#. A `[ModuleInitializer]` method in that class executes attacker code when the generated assembly is loaded at application startup, without requiring a call to the client. The behavior was verified with Refitter 2.0.0 and .NET 8. This issue is fixed in version 2.1.2.

CVSS Scores

EPSS Scores

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Meta Information

Published

2026-10-30

Last Modified

2026-10-30

Generated

2026-10-31

AI Q&A

2026-10-31

EPSS Evaluated

N/A

NVD

CVE-2026-63344

Affected Vendors & Products

Vendor	Product	Version / Range
christianhelle	refitter	< 2.1.2

Helpful Resources

Exploitability

CWE

KEV

CWE ID	Description
CWE-94	The product constructs all or part of a code segment using externally-influenced input from an upstream component, but it does not neutralize or incorrectly neutralizes special elements that could modify the syntax or behavior of the intended code segment.
CWE-116	The product prepares a structured message for communication with another component, but encoding or escaping of the data is either missing or done incorrectly. As a result, the intended structure of the message is not preserved.

Attack-Flow Graph

Executive Summary

Refitter is a tool that generates a C# REST API client using the Refit library. Prior to version 2.1.2, it had a vulnerability where content-type keys in requestBody/responses were not properly escaped when inserted into a Refit attribute string. This allowed crafted content-type keys to break out of the attribute string and interface, introducing a top-level class with a [ModuleInitializer] method that executes attacker code when the assembly loads.

Impact Analysis

This vulnerability allows an attacker to execute arbitrary code on your system when the generated assembly is loaded, without requiring any interaction from you. It could lead to full system compromise, data theft, or further network infiltration if the generated client is used in your application.

Mitigation Strategies

Update Refitter to version 2.1.2 or later to address the vulnerability. Avoid using untrusted OpenAPI descriptions with Refitter unless validation is enabled.

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