CAPEC-47 - Buffer Overflow via Parameter Expansion

In this attack, the target software is given input that the attacker knows will be modified and expanded in size during processing. This attack relies on the target software failing to anticipate that the expanded data may exceed some internal limit, thereby creating a buffer overflow.






  • Attack Methods 1
  • Injection
  • Purposes 2
  • Penetration
  • Exploitation
  • Scopes 5
  • Gain privileges / assume identity
  • Authorization
  • Access_Control
  • Confidentiality
  • DoS: crash / exit / restart
  • Availability
  • Modify memory
  • Integrity
  • Execute unauthorized code or commands
  • Availability
  • Integrity
  • Confidentiality
  • Read memory
  • Confidentiality

High level: Finding this particular buffer overflow may not be trivial. Also, stack and especially heap based buffer overflows require a lot of knowledge if the intended goal is arbitrary code execution. Not only that the attacker needs to write the shell code to accomplish his or her goals, but the attacker also needs to find a way to get the program execution to jump to the planted shell code. There also needs to be sufficient room for the payload. So not every buffer overflow will be exploitable, even by a skilled attacker.

The program expands one of the parameters passed to a function with input controlled by the user, but a later function making use of the expanded parameter erroneously considers the original, not the expanded size of the parameter.

The expanded parameter is used in the context where buffer overflow may become possible due to the incorrect understanding of the parameter size (i.e. thinking that it is smaller than it really is).

Access to the program source or binary. If the program is only available in binary then a disassembler and other reverse engineering tools will be helpful.

Step 1 -

Consider parts of the program where user supplied data may be expanded by the program. Use a disassembler and other reverse engineering tools to guide the search..

Step 1 -

Find a place where a buffer overflow occurs due to the fact that the new expanded size of the string is not correctly accounted for by the program. This may happen perhaps when the string is copied to another buffer that is big enough to hold the original, but not the expanded string. This may create an opportunity for planting the payload and redirecting program execution to the shell code..

Step 1 -

Write the buffer overflow exploit. To be exploitable, the "spill over" amount (e.g. the difference between the expanded string length and the original string length before it was expanded) needs to be sufficient to allow the overflow of the stack return pointer (in the case of a stack overflow), without causing a stack corruption that would crash the program before it gets to execute the shell code. Heap overflow will be more difficult and will require the attacker to get more lucky, by perhaps getting a chance to overwrite some of the accounting information stored as part of using malloc()..

Ensure that when parameter expansion happens in the code that the assumptions used to determine the resulting size of the parameter are accurate and that the new size of the parameter is visible to the whole system