CAPEC-53 Postfix, Null Terminate, and Backslash

If a string is passed through a filter of some kind, then a terminal NULL may not be valid. Using alternate representation of NULL allows an adversary to embed the NULL mid-string while postfixing the proper data so that the filter is avoided. One example is a filter that looks for a trailing slash character. If a string insertion is possible, but the slash must exist, an alternate encoding of NULL in mid-string may be used.

1. [Explore] [Survey the application for user-controllable inputs] Using a browser, an automated tool or by inspecting the application, an adversary records all entry points to the application. 2. [Experiment] [Probe entry points to locate vulnerabilities] The adversary uses the entry points gathered in the "Explore" phase as a target list and injects postfix null byte(s) followed by a backslash to observe how the application handles them as input. The adversary is looking for areas where user input is placed in the middle of a string, and the null byte causes the application to stop processing the string at the end of the user input. 3. [Exploit] [Remove data after null byte(s)] After determined entry points that are vulnerable, the adversary places a null byte(s) followed by a backslash such that they bypass an input filter and remove data after the null byte(s) in a way that is beneficial to them.

• Null terminators are not properly handled by the filter.

• [Medium] An adversary needs to understand alternate encodings, what the filter looks for and the data format acceptable to the target API

• Integrity: Modify Data • Confidentiality: Read Data • Confidentiality, Access Control, Authorization: Gain Privileges

• 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 an allowlist that defines all valid input to the software system based on the requirements specifications. Input that does not match against the allowlist should not be permitted to enter into the system.