![]() The kernel driver will process the IRP request and perform some actions based on the IRP request action that was requested by the app.exe. Whenever we’re calling into the kernel driver, we must use the I/O manager that passes the IRP request to it. That in turn calls into the kernel mode through the KiFastCallEntry function. In any user mode application, we’re using the ntdll.dll library which calls into the kernel mode with the use of the sysenter instruction. All the purple elements are already provided by the operating system. The green elements are the ones we have to write: app.exe is a user mode application that calls the kernel mode driver (also a green element). an Object Identifier (OID) with a length of 1, the length of the input was not validated to contain enough bytes for the OID and the x509_note_OID function was given an out-of-bounds pointer and incorrect size.Before we actually take a look at the code and explain it in detail, I would like to first explain the whole concept that we’re going to use, just to better understand it. Thus, when the last bytes of the test input were 06 01, i.e. In the ASN.1 decoder code shown below, we can see that the length check is only performed when the multi-byte format is used and the 0x80 bit is set in the length field. ![]() However, when the length is 127 bytes or less, the length can be directly encoded into the one-byte length field. ![]() The top bit of the length field, 0x80, is set to indicate that the length is encoded using the multi-byte format. When an item in the ASN.1 specification is longer than 127 bytes, the length is encoded into the following N bytes and the length field is used to specify how many bytes contain the input’s length. ![]() Looking through the ASN.1 decoder’s code, it quickly becomes clear that the length check is only performed when using a multi-byte ASN.1 length specification. ![]()
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