No interruption of visitors. The complete article can be downloaded from: Currently the stack protection built into Windows can be defeated. In a way, they had to. Windows Server — Defeating the stack protection mechanism If the cookies do not match then it is assumed that the buffer has been overflowed and the process is stopped. Microsoft is committed to security. As part of the security in depth model adopted by Microsoft for their latest Windows version a new stack protection mechanism was incorporated into their compiler that was intended to help mitigate the risk posed by stack based buffer overflow vulnerabilities by attempting to prevent their exploitation.
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This can be accomplished in a number of ways, such as by overwriting a return address on the stack with a bogus address in order to cause an access violation exception to be raised. When an exception is raised, the exception dispatcher will attempt to enumerate the list of exception registration records for the thread and call the exception handler that is associated with each record. By corrupting the next pointer and exception handler function pointer of one of the exception registration records, the exception dispatcher can be made to execute code from an arbitrary address as specified by the corrupt exception handler function pointer.
In many cases, an attacker will choose to overwrite the exception handler function pointer with an address that contains instructions that are equivalent to a pop reg, pop reg, ret. This allows an attacker to reliably execute arbitrary code by transferring control to the EstablisherFrame that the exception dispatcher passes as the second parameter when calling an exception handler.
This works because the EstablisherFrame parameter holds the address of the attacker-controlled exception registration record.
Attackers have also used heap spraying in conjunction with an SEH overwrite to reliably execute arbitrary code. The following diagram illustrates what an SEH overwrite would typically look like from an exploitation perspective: The mitigation technique: SEHOP There are two general approaches that can be considered when attempting to mitigate the SEH overwrite exploitation technique.
The first approach involves making changes to the compiled versions of code such that executable files are made to contain metadata that the platform would need to properly mitigate this technique.
Microsoft pursued this approach and released a functional mitigation with Visual Studio Unfortunately, the need to rebuild executables in combination with the inability to completely handle cases where an exception handler is pointed outside of an image file make the SafeSEH approach less attractive. The second approach involves adding dynamic checks to the exception dispatcher that do not rely on having metadata derived from a binary. This mitigation technique is made possible because of an implicit side effect of an SEH overwrite.
Since the next pointer is corrupted, the integrity of the exception handler chain is broken. This step occurs when a thread first begins executing in user mode. Since exception registration records are always inserted at the head of the exception handler list, the symbolic record is guaranteed to be the final exception registration record. The second step consists of walking the exception handler list at the time that an exception is being dispatched to ensure that the symbolic record can be reached and that it is valid.
This step happens when the exception dispatcher is notified that an exception has occurred in user mode. If the symbolic record cannot be reached, the exception dispatcher can assume that the exception handler list is corrupt and that an SEH overwrite may have occurred. The exception dispatcher is then able to safely terminate the process. If the symbolic record is found, the exception dispatcher is able to proceed as it normally would and call each of the registered exception handlers.
The primary reason this feature was disabled by default on Windows Vista SP1 was due to a lack of adequate application compatibility data. Wrapping up We are continuing to investigate new and enhanced exploit mitigation techniques and feel that SEHOP is a valuable addition that can help protect users. We encourage users to enable this feature if it is not enabled by default in order to better protect themselves against the SEH overwrite exploitation technique. Sep, Microsoft Security Intelligence Report volume 5.
Preventing the Exploitation of Structured Exception Handler (SEH) Overwrites with SEHOP
David Litchfield has been playing with Microsoft products, as far as security is concerned, since and in the past year and a half or two David Litchfield has seen a marked difference with some very positive moves made. In a way, they had to. With the public relations crisis caused by worms such as Code Red Microsoft needed to do something to stem the flow of customers moving away from the Windows OS to other platforms. We will see more; but David Litchfield is confident that the number of security vulnerabilities that will be discovered in Windows Server will be a fraction of those found in Windows Acknowledging that there have been holes found and that, yes, more will come to light in the future this paper is going to look at how, currently, the stack based protection built into Windows Server to protect against buffer overflow vulnerability exploitation can be bypassed. The development of this mechanism is one of the right moves made in the direction of security.
DEFEATING W2K3 STACK PROTECTION PDF
‘Defeating the Stack Based Buffer Overflow Prevention Mechanism of Microsoft Windows 2003 Server’