How EMET works

12 Sep 2010

I'm interested in how easily EMET is able to turn DEP, ASLR and other security features on in executables, so I decided to take a look. I chose an executable, turned on ProcMon from Sysinternals, and used EMET to turn on the security on that executable. I checked the executable afterward to ensure it's actually the same as before (so they're not bit fiddling). I then looked through ProcMon's output, and found that it adds a registry key to HKLM\SOFTWARE\Microsoft\Windows NT\CurrentVersion\AppCompatFlags\Custom\ with the name of the file, and with REG_QWORD values that are always the same. For example, I have: HKLM\SOFTWARE\Microsoft\Windows NT\CurrentVersion\AppCompatFlags\Custom\firefox.exe\{e1c810aa-f7cc-4aaf-ada1-181863075f9b}.sdb = "8c 05 84 fb f0 4e cb 01" HKLM\SOFTWARE\Microsoft\Windows NT\CurrentVersion\AppCompatFlags\Custom\firefox.exe\{f8c4cc07-6dc4-418f-b72b-304fcdb64052}.sdb = "8c 05 84 fb f0 4e cb 01"

Those GUID's then correlate back to keys at HKLM\SOFTWARE\Microsoft\Windows NT\CurrentVersion\AppCompatFlags\InstalledSDB\ which identify files at the following locations: C:\WINDOWS\AppPatch\Custom\Custom64\{e1c810aa-f7cc-4aaf-ada1-181863075f9b}.sdb C:\WINDOWS\AppPatch\Custom\{f8c4cc07-6dc4-418f-b72b-304fcdb64052}.sdb

I then googled for info on .sdb files, and came to the page for a tool called sdb2xml. When I ran that on the .sdb file, I got an XML file showing each of the processes I had turned EMET on for, but I'll just show the portion relevant to firefox, along with the rest of the info:

<?xml version="1.0" encoding="IBM437" standalone="yes"?>
<SDB xmlns:xs="" path="file.sdb">
      <INDEX_TAG type="xs:short">28676</INDEX_TAG>
      <INDEX_KEY type="xs:short">24577</INDEX_KEY>
      <INDEX_BITS type="xs:base64Binary" />
      <INDEX_TAG type="xs:short">28691</INDEX_TAG>
      <INDEX_KEY type="xs:short">24577</INDEX_KEY>
      <INDEX_BITS type="xs:base64Binary" />
      <INDEX_TAG type="xs:short">28679</INDEX_TAG>
      <INDEX_KEY type="xs:short">24577</INDEX_KEY>
      <INDEXFLAGS type="xs:int">1</INDEXFLAGS>
      <INDEX_BITS type="xs:base64Binary" />
    <OS_PLATFORM type="xs:int">1</OS_PLATFORM>
    <NAME type="xs:string">EMET Database</NAME>
    <DATABASE_ID_x0028_GUID_x0029_ type="xs:string" baseType="xs:base64Binary">{f8c4cc07-6dc4-418f-b72b-304fcdb64052}</DATABASE_ID_x0028_GUID_x0029_>
        <NAME type="xs:string">EnableDEP</NAME>
        <FLAGS_PROCESSPARAM type="xs:long">131072</FLAGS_PROCESSPARAM>
        <NAME type="xs:string">EMET Shim</NAME>
        <DLLFILE type="xs:string">EMET.dll</DLLFILE>
          <INCLUDE />
          <MODULE type="xs:string">*</MODULE>
      <NAME type="xs:string">firefox.exe</NAME>
      <APP_NAME type="xs:string">EMET Apps</APP_NAME>
      <EXE_ID_x0028_GUID_x0029_ type="xs:string" baseType="xs:base64Binary">{7e589b34-a304-44ef-b715-bb037cb5221f}</EXE_ID_x0028_GUID_x0029_>
        <NAME type="xs:string">*</NAME>
        <NAME type="xs:string">EMET Shim</NAME>
        <SHIM_TAGID type="xs:int">236</SHIM_TAGID>
        <NAME type="xs:string">EnableDEP</NAME>
        <FLAG_TAGID type="xs:int">214</FLAG_TAGID>
    <STRTAB_ITEM type="xs:string">EMET Database</STRTAB_ITEM>
    <STRTAB_ITEM type="xs:string">EnableDEP</STRTAB_ITEM>
    <STRTAB_ITEM type="xs:string">EMET Shim</STRTAB_ITEM>
    <STRTAB_ITEM type="xs:string">EMET.dll</STRTAB_ITEM>
    <STRTAB_ITEM type="xs:string">*</STRTAB_ITEM>
    <STRTAB_ITEM type="xs:string">EMET Apps</STRTAB_ITEM>
    <STRTAB_ITEM type="xs:string">firefox.exe</STRTAB_ITEM>

This suggests that when an executable with the name "firefox.exe" gets run, the DLL "EMET.dll" gets loaded. This file is at C:\WINDOWS\AppPatch\EMET.dll, and running it through IDA we see a call to an internal function labeled aSetprocessdepp which calls GetModuleHandle()/GetProcAddress() on kernel32.dll/SetProcessDEPPolicy. I didn't know such a function existed, but we can check it out in the MSDN at SetProcessDEPPolicy Function and see that it just turns DEP on or off (duh!). Just for fun, continuing to read the MSDN article, we can see that a process can also have it's DEP setting changed by use of the function UpdateProcThreadAttribute or when creating the process in the call to CreateProcess you can specify that same attribute list in the STARTUPINFOEX structure.

Although EMET keys off of the name of the file, you can also use "file size, checksum, version, and date" according to the MSDN article on the Application Compatibility Database.

Also, I found it odd that the .pdb file listed inside the EMET.dll is c:\src\redteam\src\tools\defense\emet\src\razzle\shimdll\objfre\i386\EMET.pdb when the EMET tool is produced by the BlueHat team. What color are you!?!

I couldn't figure out how EMET is enabling the other security features (like ASLR). I guess it's trickier then just making an API call.