COMMON ATTACK PATTERN ENUMERATION AND CLASSIFICATIONLive MITRE record

CAPEC-209

XSS Using MIME Type Mismatch

An adversary creates a file with scripting content but where the specified MIME type of the file is such that scripting is not expected. The adversary tricks the victim into accessing a URL that responds with the script file. Some browsers will detect that the specified MIME type of the file does not match the actual type of its content and will automatically switch to using an interpreter for the real content type. If the browser does not invoke script filters before doing this, the adversary's script may run on the target unsanitized, possibly revealing the victim's cookies or executing arbitrary script in their browser.

Abstraction

Detailed

Typical severity

Medium

Likelihood of attack

Medium

Understand the attack

What is this attack pattern?

An adversary creates a file with scripting content but where the specified MIME type of the file is such that scripting is not expected. The adversary tricks the victim into accessing a URL that responds with the script file. Some browsers will detect that the specified MIME type of the file does not match the actual type of its content and will automatically switch to using an interpreter for the real content type. If the browser does not invoke script filters before doing this, the adversary's script may run on the target unsanitized, possibly revealing the victim's cookies or executing arbitrary script in their browser.

Execution flow

How does the attack proceed, step by step?

1

Explore

Survey the application for stored user-controllable inputs

Using a browser or an automated tool, an adversary follows all public links and actions on a web site. They record all areas that allow a user to upload content through an HTTP POST request. This is typically found in blogs or forums.

Techniques used

  • Use a spidering tool to follow and record all links and analyze the web pages to file upload features
  • Use a proxy tool to record all links visited during a manual traversal of the web application.
  • Use a browser to manually explore the website and analyze how it is constructed. Many browsers' plugins are available to facilitate the analysis or automate the discovery.
2

Experiment

Probe identified potential entry points for MIME type mismatch

The adversary uses the entry points gathered in the "Explore" phase as a target list and uploads files with scripting content, but whose MIME type is specified as a file type that cannot execute scripting content. If the application only checks the MIME type of the file, it may let the file through, causing the script to be executed by any user who accesses the file.

Techniques used

  • Upload a script file with a MIME type of text/plain to a forum and then access the uploaded file to see if the script is executed. If possible, the script displays a unique identifier so the adversary knows for certain it was executed when testing.

Store malicious XSS content

Once the adversary has determined which file upload locations are vulnerable to MIME type mismatch, they will upload a malicious script disguised as a non scripting file. The adversary can have many goals, from stealing session IDs, cookies, credentials, and page content from a victim.

Techniques used

  • Use a tool such as BeEF to store a hook into the web application. This will alert the adversary when the victim has accessed the content and will give the adversary control over the victim's browser, allowing them access to cookies, user screenshot, user clipboard, and more complex XSS attacks.
3

Exploit

Get victim to view stored content

In order for the attack to be successful, the victim needs to view the stored malicious content on the webpage.

Techniques used

  • Send a phishing email to the victim containing a URL that will direct them to the malicious stored content.
  • Simply wait for a victim to view the content. This is viable in situations where content is posted to a popular public forum.

Requirements

What does the attack require?

Prerequisites

  • The victim must follow a crafted link that references a scripting file that is mis-typed as a non-executable file.
  • The victim's browser must detect the true type of a mis-labeled scripting file and invoke the appropriate script interpreter without first performing filtering on the content.

Skills required

Not stated in the MITRE record.

Resources required

  • The adversary must have the ability to source the file of the incorrect MIME type containing a script.

Real world

MITRE example instances

Example instance 1Detail
For example, the MIME type text/plain may be used where the actual content is text/javascript or text/html. Since text does not contain scripting instructions, the stated MIME type would indicate that filtering is unnecessary. However, if the target application subsequently determines the file's real type and invokes the appropriate interpreter, scripted content could be invoked.
Example instance 2Detail
In another example, img tags in HTML content could reference a renderable type file instead of an expected image file. The file extension and MIME type can describe an image file, but the file content can be text/javascript or text/html resulting in script execution. If the browser assumes all references in img tags are images, and therefore do not need to be filtered for scripts, this would bypass content filters.
CAPEC-209: XSS Using MIME Type Mismatch — DayBreach Saldırı Sözlüğü · DayBreach