CAPEC-43 - Exploiting Multiple Input Interpretation Layers

An attacker supplies the target software with input data that contains sequences of special characters designed to bypass input validation logic. This exploit relies on the target making multiples passes over the input data and processing a "layer" of special characters with each pass. In this manner, the attacker can disguise input that would otherwise be rejected as invalid by concealing it with layers of special/escape characters that are stripped off by subsequent processing steps.

The goal is to first discover cases where the input validation layer executes before one or more parsing layers. That is, user input may go through the following logic in an application:

<parser1>

--> <input validator> -->

<parser2>

In such cases, the attacker will need to provide input that will pass through the input validator, but after passing through parser2, will be converted into something that the input validator was supposed to stop.

Severity

Likelihood

Confidentiality

Integrity

Availability

  • Attack Methods 2
  • Injection
  • Modification of Resources
  • Purposes 1
  • Penetration
  • Sec Principles 1
  • Defense in Depth
  • Scopes 3
  • Modify application data
  • Integrity
  • Gain privileges / assume identity
  • Authorization
  • Access_Control
  • Confidentiality
  • Read application data
  • Confidentiality

Medium level:

User input is used to construct a command to be executed on the target system or as part of the file name.

Multiple parser passes are performed on the data supplied by the user.

Initially a fuzzer can be used to see what the application is successfully and escaping and what causes problems. This may be a good starting point.

Manually try to introduce multiple layers of control characters and see how many layers the application can escape.

Step 1 - Determine application/system inputs where bypassing input validation is desired

The attacker first needs to determine all of the application's/system's inputs where input validation is being performed and where he/she wants to bypass it..

Tecnique ID: 1 - Environment(s) env-All

While using an application/system, the attacker discovers an input where validation is stopping him/her from performing some malicious or unauthorized actions.

Indicator ID: 1 - Environment(s) env-All

Type: Positive

When provided with unexpected input, application provides an error message stating that the input was invalid or that access was denied.



Step 1 - Determine which character encodings are accepted by the application/system

The attacker then needs to provide various character encodings to the application/system and determine which ones are accepted. The attacker will need to observe the application's/system's response to the encoded data to determine whether the data was interpreted properly..

Tecnique ID: 1 - Environment(s) env-All

Determine which escape characters are accepted by the application/system. A common escape character is the backslash character, '\'

Tecnique ID: 2 - Environment(s) env-All

Determine whether URL encoding is accepted by the application/system.

Tecnique ID: 3 - Environment(s) env-All

Determine whether UTF-8 encoding is accepted by the application/system.

Tecnique ID: 4 - Environment(s) env-All

Determine whether UTF-16 encoding is accepted by the application/system.

Tecnique ID: 5 - Environment(s) env-All

Determine if any other encodings are accepted by the application/system.

Indicator ID: 1 - Environment(s) env-All

Type: Positive

System provides error message similar to the one it provided when a positive indicator was received for the first step.


Security Control ID: 1

Type: Detective

Detect and alert on appearance of encodings in log messages (e.g. "Unsuccessful login by <joe")


Outcome ID: 1

Type: Success

Application/system accepts at least one high level character encoding where characters can be represented with multiple ASCII characters.

Outcome ID: 2

Type: Failure

Application/system interprets each character separately.


Step 2 - Combine multiple encodings accepted by the application.

The attacker now combines encodings accepted by the application. The attacker may combine different encodings or apply the same encoding multiple times..

Tecnique ID: 1 - Environment(s) env-All

Combine same encoding multiple times and observe its effects. For example, if special characters are encoded with a leading backslash, then the following encoding may be accepted by the application/system: "\\\.". With two parsing layers, this may get converted to "\." after the first parsing layer, and then, to "." after the second. If the input validation layer is between the two parsing layers, then "\\\.\\\." might pass a test for ".." but still get converted to ".." afterwards. This may enable directory traversal attacks.

Tecnique ID: 2 - Environment(s) env-All

Combine multiple encodings and observe the effects. For example, the attacker might encode "." as "\.", and then, encode "\." as "&#92;&#46;", and then, encode that using URL encoding to "%26%2392%3B%26%2346%3B"

Indicator ID: 1 - Environment(s) env-All

Type: Positive

Application/System interprets the multiple encodings properly.


Security Control ID: 1

Type: Preventative

Ensure that the input validation layer is executed after as many parsing layers as possible.

Security Control ID: 2

Type: Preventative

Determine the details of any parsing layers that get executed after the input validation layer (this may be necessary in the case of filesystem access, for example, where the operating system also includes a parsing layer), and ensure that the input validator accounts for the various encodings of illegal characters and character sequences in those layers.


Outcome ID: 1

Type: Success

Attacker bypasses input validation layer(s) and passes data to application that it does not expect.



Step 1 - Leverage ability to bypass input validation

Attacker leverages his ability to bypass input validation to gain unauthorized access to system. There are many attacks possible, and a few examples are mentioned here..

Tecnique ID: 1 - Environment(s) env-All

Gain access to sensitive files.

Tecnique ID: 2 - Environment(s) env-All

Perform command injection.

Tecnique ID: 3 - Environment(s) env-All

Perform SQL injection.

Tecnique ID: 4 - Environment(s) env-All

Perform XSS attacks.

Indicator ID: 1 - Environment(s) env-All

Type: Positive

Success outcome in previous step

Indicator ID: 2 - Environment(s) env-All

Type: Negative

Failure outcome in previous step


Outcome ID: 1

Type: Success

Gaining unauthorized access to system functionality.



An iterative approach to input validation may be required to ensure that no dangerous characters are present. It may be necessary to implement redundant checking across different input validation layers. Ensure that invalid data is rejected as soon as possible and do not continue to work with it.

Make sure to perform input validation on canonicalized data (i.e. data that is data in its most standard form). This will help avoid tricky encodings getting past the filters.

Assume all input is malicious. Create a white list that defines all valid input to the software system based on the requirements specifications. Input that does not match against the white list should not be permitted to enter into the system.