CAPEC 53

Summary

Attack Execution Flow

1. An attacker first probes to figure out what restrictions on input are placed by filter, such as a specific characters on the end of the URL.

2. The attacker then injects a string of their choosing with a null terminator (using an alternate encoding such as %00), followed by a backslash (%5C), followed by some additional characters that are required to keep the filter happy

3. The malicious string then passes through the filter and passed to the underlying API. Everything after the null terminator is ignored. This may give an attacker the opportunity to access file system resources to which they should not have access and do other things.

   Some popular forms in which this takes place:

   PATH%00%5C

   PATH[0x00][0x5C]

   PATH[alternate encoding of the NULL][additional characters required to pass filter]

Likelihood: High

Description

Skill or Knowledge Level: Medium

An attacker needs to understand alternate encodings, what the filter looks for and the data format acceptable to the target API

Test the program with various inputs and observe the behavior of the filter. Overtime it should be possible to understand what the filter is expecting.

Null characters are observed by the filter. The filter needs to be able to understand various encodings of the Null character, or only canonical data should be passed to it.

Properly handle Null characters. Make sure canonicalization is properly applied. Do not pass Null characters to the underlying APIs.

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.