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A Buffer Overflow Example

A Buffer Overflow Example. Buffer Overflow. Buffer Overflow – A simple program. Buffer Overflow – A simple program. Our goal is to execute the hack() function . Could be worst!. Buffer Overflow – Internal aspects of the program. Stack before the scanf call. Stack after the scanf call.

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A Buffer Overflow Example

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  1. A Buffer Overflow Example

  2. Buffer Overflow

  3. Buffer Overflow – A simple program

  4. Buffer Overflow – A simple program • Our goal is to execute the hack() function Could be worst!

  5. Buffer Overflow – Internal aspects of the program Stack before the scanf call Stack after the scanf call print_it() and hack() functions addresses

  6. Buffer Overflow – Running the program The code of ‘A’ is 0x41!

  7. Buffer Overflow – The x86 stack Ends of the stack Call parameters Stack Frame (Of who calls the subroutine) Return Address %EBP register Preserve original register values Stack Frame (currently) Make space for local variables in subroutines %ESP Stack Pointer (32 bits)

  8. Buffer Overflow – The attack Ends of the stack Call parameters Stack Frame (Of who calls the subroutine) Write chars until change the return address Return Address %EBP register Preserve original register values Stack Frame (currently) Make space for local variables in subroutines Buffer s stays here

  9. Buffer Overflow – Running once more We want to put this address here

  10. Buffer Overflow – The attack

  11. Buffer Overflow – The attack It’s used a String larger than s buffer is expecting and that stays over the return value The return address was changed Mallicious code

  12. Buffer Overflow – But where came from the hack() function?? Usually buffers don’t have enought space to store all the code that is going to be executed (i.e. s[] buffer does not have enought space to the binary code of hack()), so we only need to make the program to read data from someplace, and store it in a way that is possible to locate it later. In this case is just necessary to make the returning address point to the right location (e.g., a image!)

  13. Buffer Overflow – But... • Hackers could not have access to the source code to discover the buffer sizes... It’s very easy to incrementally enlarge the input data until a crash occur. This way, the buffer size is discovered.

  14. Buffer Overflow – The correct code Don’t use “insecure functions”, or when it’s absolutely necessary to use them special cares are needed. E.g., gets()/fgets() scanf()/fscanf() strcat()/strcpy() ...

  15. Bibliography

  16. Credits... The original (and Portuguese) version of these presentation belongs to Professor Paulo Marques Paulo Marques Departamento de Eng. Informática Universidade de Coimbra pmarques@dei.uc.pt

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