Return-to-libc attack

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A return-to-libc attack is a computer security attack usually starting with a buffer overflow in which the return address on the call stack is replaced by the address of a function that is already loaded in the binary or via shared library. This allows attackers to defeat the Non-eXecutable stack protection - i.e. a page cannot be marked as write and executable at the same time. In fact, in this way the attacker simply calls preexisting functions without the need to inject malicious code into a program.

The shared library called "libc" provides the C runtime on UNIX style systems. Although the attacker could make the code return anywhere, libc is the most likely target, as it is always linked to the program, and it provides useful calls for an attacker (such as the system() call to execute an arbitrary program, which needs only one argument). This is why the exploit is called "return-to-libc" even when the return address may point to a completely different location.

Protection from return-to-libc attacks[edit]

A non-executable stack can prevent some buffer overflow exploitation, however it cannot prevent a return-to-libc attack because in the return-to-libc attack only existing executable code is used. On the other hand these attacks can only call preexisting functions. Stack-smashing protection can prevent or obstruct exploitation as it may detect the corruption of the stack and possibly flush out the compromised segment.

"ASCII armoring" is a technique that can be used to obstruct this kind of attack. With ASCII armoring, all the system libraries (e.g. libc) addresses contain a NULL byte (0x00). This is commonly done by placing them in the first 0x01010100 bytes of memory (around 16 MB, dubbed the "ASCII armour region"), as every address up to this value contains at least one NULL byte. This makes it impossible to emplace code containing those addresses using string manipulation functions such as strcpy(). However, this technique does not work if the attacker does have a way to overflow NULL bytes into the stack. If the program is too large to fit in the first 16 MB, protection may be incomplete.[1] This technique can also be overcome by a more advanced type of attack known as return-to-plt where, instead of returning to libc, the attacker uses the Procedure Linkage Table (PLT) functions loaded in the binary (e.g. system@plt, execve@plt, sprintf@plt, strcpy@plt, etc.).[2]

Address space layout randomization (ASLR) makes this type of attack extremely unlikely to succeed on 64-bit machines as the memory locations of functions are random. For 32-bit systems ASLR provides little benefit since there are only 16 bits available for randomization, and they can be defeated by brute force in a matter of minutes.[3]

Related attacks[edit]

return-to-plt is a similar attack where, instead of returning to libc, the attacker uses the PLT functions loaded in the binary (e.g. system@plt, execve@plt, sprintf@plt, strcpy@plt, etc...).

Return-oriented programming is an elaboration of the techniques used in this attack, and can be used to execute more general operations by chaining individual smaller attacks that execute a small number of instructions at a time.

See also[edit]


  1. ^ David A. Wheeler (27 Jan 2004). "Secure programmer: Countering buffer overflows". IBM DeveloperWorks. 
  2. ^ Sickness (13 May 2011). "Linux exploit development part 4 - ASCII armor bypass + return-to-plt". 
  3. ^ Shacham, H.; Page, M.; Pfaff, B.; Goh, E. J.; Modadugu, N.; Boneh, D. (October 2004). "On the effectiveness of address-space randomization". Proceedings of the 11th ACM conference on Computer and communications security - CCS '04. pp. 298–307. doi:10.1145/1030083.1030124. ISBN 1-58113-961-6.  edit

External links[edit]