‘Advanced Exploitation in Exec-Shield (Fedora Core Case Study)’


The below linked paper discusses some technique to attack applications on exec-shield kernel found in the Fedora Core operating system.’


‘The information has been provided by dong-hun you.
The original article can be found at: http://x82.inetcop.org/h0me/papers/FC_exploit/FC_exploit.txt


2 – Brief features of Fedora Core system (gcc + glibc + exec-shield)
Fedora Core is a project run by Redhat and it provides technical background. Fedora Core (F/C since now), unlike old RedHat OS, has stack and heap hacking prevent system called exec-shield. Exec-shield makes its specialty of blocking all the existing attacks concern stack, buffer, function pointer overflow and overwriting data structure or injecting code on that data structure. You can find more by looking ANNOUNCE-exec-shield on Reference 7.16

2.1 – non-executable randomization stack, malloc heap, randomization library
Stack and data area, heap area allocated by malloc() function are now non-executable, and it makes classic shellcode useless. Area that library is mapped ,merely, has execute privilege. But even this will be re-mapped on every single execution. This randomizing algorithm ,unlike that of Redhat Linux 9.0, is very unpredictable. It looks very similar to Openwall Project (7.12) of Solar Designer and PaX kernel system (7.13)

2.2 – Addressing system under 16mb (NULL pointer dereference protection)
Kernel re-maps all PROT_EXEC mapping in ascii-armor area and make its address system less than 16mb. This idea came from the fact that hackers use 4byte library address when they make overflow attempt on old 32bit system. By doing this, memory address now has null in the address it means many memory related hacking technique such as return-into-libc(7.2) are now hard to be used. Sometimes, there is a chance to enter null into overflowed buffer easily. But on this paper, dong-hun is not going to talk about this.

In addition, recent changes on glibc make some library function address end with null or 0x20.

2.3 – PIE technology (changed gcc)
PIE stands for Position Independent Executables is a similar concept with old PIC. This is a way to protect application from being exploited by attacks such as buffer overflow.

2.4 – Changed method of accessing function parameter (changed glibc)
F/C 3 has glibc 2.3.3 on it. This glibc handles the commands that passed into system() and exec family function with %ebp register. With the stack overflow happening, hacker can manipulate %ebp register, so there is still a good chance to indicate certain command to execute.’

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