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  1. Rules Repository
  2. RSPEC-5734

Allowing browsers to sniff MIME types is security-sensitive


    • Type: Security Hotspot Detection
    • Status: Active
    • Resolution: Unresolved
    • Labels:
    • Message:
      Make sure allowing browsers to sniff MIME types is safe here.
    • Default Severity:
    • Impact:
    • Likelihood:
    • Default Quality Profiles:
      Sonar way
    • Targeted languages:
      ABAP, APEX, C#, C, C++, Cobol, CSS, Flex, Go, HTML, Java, JavaScript, Kotlin, Objective-C, PHP, PL/I, PL/SQL, Python, RPG, Ruby, Rust, Scala, Solidity, Swift, T-SQL, TypeScript, VB.Net, VB6, XML
    • Remediation Function:
    • Linear Argument Description:
    • Analysis Level:
      Control-flow Analysis
    • Analysis Scope:
      Main Sources
    • OWASP:


      MIME confusion attacks occur when an attacker successfully tricks a web-browser to interpret a resource as a different type than the one expected. To correctly interpret a resource (script, image, stylesheet ...) web browsers look for the Content-Type header defined in the HTTP response received from the server, but often this header is not set or is set with an incorrect value. To avoid content-type mismatch and to provide the best user experience, web browsers try to deduce the right content-type, generally by inspecting the content of the resources (the first bytes). This "guess mechanism" is called MIME type sniffing.

      Attackers can take advantage of this feature when a website (example.com here) allows to upload arbitrary files. In that case, an attacker can upload a malicious image fakeimage.png (containing malicious JavaScript code or a polyglot content file) such as:


      When the victim will visit the website showing the uploaded image, the malicious script embedded into the image will be executed by web browsers performing MIME type sniffing.

      Ask Yourself Whether

      • Content-Type header is not systematically set for all resources.
      • Content of resources can be controlled by users.

      There is a risk if you answered yes to any of those questions.

      Recommended Secure Coding Practices

      • Implement X-Content-Type-Options header with nosniff value (the only existing value for this header) which is supported by all modern browsers and will prevent browsers from performing MIME type sniffing, so that in case of Content-Type header mismatch, the resource is not interpreted. For example within a <script> object context, JavaScript MIME types are expected (like application/javascript) in the Content-Type header.



        JavaScript RSPEC-5735 Language-Specification Active Unassigned



            • Assignee:
              eric.therond Eric Therond
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              • Created: