MsMpEng is the Malware Protection service that is enabled by default on Windows 8, 8.1, 10, Windows Server 2012, and so on. Additionally, Microsoft Security Essentials, System Centre Endpoint Protection and various other Microsoft security products share the same core engine. MsMpEng runs as NT AUTHORITY\SYSTEM without sandboxing, and is remotely accessible without authentication via various Windows services, including Exchange, IIS, and so on. On workstations, attackers can access mpengine by sending emails to users (reading the email or opening attachments is not necessary), visiting links in a web browser, instant messaging and so on. This level of accessibility is possible because MsMpEng uses a filesystem minifilter to intercept and inspect all system filesystem activity, so writing controlled contents to anywhere on disk (e.g. caches, temporary internet files, downloads (even unconfirmed downloads), attachments, etc) is enough to access functionality in mpengine. MIME types and file extensions are not relevant to this vulnerability, as MsMpEng uses it's own content identification system. Vulnerabilities in MsMpEng are among the most severe possible in Windows, due to the privilege, accessibility, and ubiquity of the service. The core component of MsMpEng responsible for scanning and analysis is called mpengine. Mpengine is a vast and complex attack surface, comprising of handlers for dozens of esoteric archive formats, executable packers and cryptors, full system emulators and interpreters for various architectures and languages, and so on. All of this code is accessible to remote attackers. NScript is the component of mpengine that evaluates any filesystem or network activity that looks like JavaScript. To be clear, this is an unsandboxed and highly privileged JavaScript interpreter that is used to evaluate untrusted code, by default on all modern Windows systems. This is as surprising as it sounds. We have written a tool to access NScript via a command shell for testing, allowing us to explore and evaluate it: $ mpscript main(): Please wait, initializing engine... main(): Ready, type javascript (history available, use arrow keys) > 6 * 9 JavaScriptLog(): 54 > document.location.hostname JavaScriptLog(): www.myserver.com > "abcd" + String.fromCharCode(0x3f) JavaScriptLog(): abcd? > /[y]e+(s|S)/.exec("yes")[0] // C++ regex engine running unsandboxed as SYSTEM on attacker controlled REGEX? JavaScriptLog(): yes > for (i in document) log(i) JavaScriptLog(): appendChild JavaScriptLog(): attributes JavaScriptLog(): childNodes JavaScriptLog(): createElement JavaScriptLog(): createTextNode JavaScriptLog(): getElementById JavaScriptLog(): getElementsByTagName JavaScriptLog(): write JavaScriptLog(): writeln JavaScriptLog(): referrer JavaScriptLog(): cookie JavaScriptLog(): location JavaScriptLog(): undefined > window.ScriptEngineBuildVersion JavaScriptLog(): [object Function] > window.ScriptEngineBuildVersion() JavaScriptLog(): 8831 We have discovered that the function JsDelegateObject_Error::toString() reads the "message" property from the this object, but fails to validate the type of the property before passing it to JsRuntimeState::triggerShortStrEvent(). In pseudocode, the code does something like this: prophash = JsObject::genPropHash("message", 0); RuntimeState::getThisPtr(&thisptr) if (JsObject::get(thisptr, prophash, &message)) { JsRuntimeState::triggerShortStrEvent("error_tostring", message); } The method assumes that message is a string, but it can be of any type, so this type confusion allows an attacker to pass arbitrary other objects. JsRuntimeState::triggerShortStrEvent() calls JsString::numBytes() on the passed object, which will invoke a method from the object's vtable. int __fastcall JsString::numBytes(JsString this) { if ( this == 0x12 ) return 0; if ( (this & 0x12) == 0x12 ) return this >> 5; return this->vtbl->GetLength(this); } Nscript supports "short" strings, with length and values contained in the handle and "long" strings with out-of-line memory. If the string is "long" (or appears to be due to type confusion), a vtable call is made to retrieve the length. Integer handles are represented as four-byte values with the final bit set to one by the engine. The integer itself is left shifted by one bit, and the final bit set to create the handle. Handles to most objects, including strings are represented as the value of the pointer to the object with no modification. Therefore, this type confusion allows an integer to be specified and treated as pointer (though the bits need to shifted to get the correct value in the handle, and only odd pointer values are possible). To reproduce this vulnerability, download the attached testcase. The debugging session below was captured after visiting a website that did this: <a href="testcase.txt" download id=link> <script> document.getElementById("link").click(); </script> 3: kd> !process PROCESS 8805fd28 SessionId: 0 Cid: 0afc Peb: 7ffdf000 ParentCid: 01c8 DirBase: bded14e0 ObjectTable: bfb99640 HandleCount: 433. Image: MsMpEng.exe 3: kd> !token -n _EPROCESS 8805fd28, _TOKEN 00000000 TS Session ID: 0 User: S-1-5-18 (Well Known Group: NT AUTHORITY\SYSTEM) 3: kd> .lastevent Last event: Access violation - code c0000005 (first chance) debugger time: Fri May 5 18:22:14.740 2017 (UTC - 7:00) 3: kd> r eax=00000010 ebx=1156c968 ecx=41414141 edx=115730f8 esi=68bd9100 edi=41414141 eip=68b1f5f2 esp=0208e12c ebp=0208e134 iopl=0 nv up ei ng nz ac po cy cs=001b ss=0023 ds=0023 es=0023 fs=003b gs=0000 efl=00010293 mpengine!FreeSigFiles+0xec822: 001b:68b1f5f2 8b07 mov eax,dword ptr [edi] ds:0023:41414141=???????? 3: kd> lmv mmpengine start end module name 68790000 6917a000 mpengine (export symbols) mpengine.dll Loaded symbol image file: mpengine.dll Image path: c:\ProgramData\Microsoft\Microsoft Antimalware\Definition Updates\{1C2B7358-645B-41D0-9E79-5FA3E5C4EB51}\mpengine.dll Image name: mpengine.dll Timestamp: Thu Apr 06 16:05:37 2017 (58E6C9C1) CheckSum: 00A1330D ImageSize: 009EA000 Translations: 0000.04b0 0000.04e4 0409.04b0 0409.04e4 3: kd> u mpengine!FreeSigFiles+0xec822: 001b:68b1f5f2 8b07 mov eax,dword ptr [edi] 001b:68b1f5f4 56 push esi 001b:68b1f5f5 8b7008 mov esi,dword ptr [eax+8] 001b:68b1f5f8 8bce mov ecx,esi 001b:68b1f5fa ff15c0450e69 call dword ptr [mpengine!MpContainerWrite+0x35f3a0 (690e45c0)] 001b:68b1f600 8bcf mov ecx,edi 001b:68b1f602 ffd6 call esi <--- Jump to attacker controlled address 001b:68b1f604 5e pop esi Before executing JavaScript, mpengine uses a number of heuristics to decide if evaluation is necessary. One such heuristic estimates file entropy before deciding whether to evaluate any javascript, but we've found that appending some complex comments is enough to trigger this. The attached proof of concept demonstrates this, but please be aware that downloading it will immediately crash MsMpEng in it's default configuration and possibly destabilize your system. Extra care should be taken sharing this report with other Windows users via Exchange, or web services based on IIS, and so on. As mpengine will unpack arbitrarily deeply nested archives and supports many obscure and esoteric archive formats (such as Amiga ZOO and MagicISO UIF), there is no practical way to identify an exploit at the network level, and administrators should patch as soon as is practically possible. We have verified that on Windows 10, adding a blanket exception for C:\ is enough to prevent automatic scanning of filesystem activity (you can still initiate manual scans, but it seems prudent to do so on trusted files only, making the action pointless).