Thanks to the ubiquity of Adobe Flash Player, Adobe Flash Player vulnerabilities have become a major target for attackers who want to deliver attacks from web pages. In 2014, some new exploits appeared, targeting two vulnerabilities (CVE-2013-5330 and CVE-2014-0497) in a new feature of Adobe applications – domain memory opcode (or Alchemy opcode). In their VB2014 paper, Chun Feng and Elia Florio analyse the technical details of exploits using these vulnerabilities. They unveil some interesting tricks used by these exploits to make the attacks more reliable and stealthy, and discuss the malware components distributed by the exploits.
Copyright © 2015 Virus Bulletin
According to Adobe, Adobe Flash Player reaches ‘over 1.3 billion people across browsers and OS versions with no install’ . Hence, Adobe Flash Player vulnerabilities have become a major target for attackers who want to deliver attacks from web pages, along with security researchers in public contests such as pwn2own, where the value of a Flash zero-day vulnerability starts at around $70,000 , .
Since 2012, we have seen a small increase in the number of Flash vulnerabilities exploited in real attacks and for malicious intent . The fact that these vulnerabilities are quite different – ranging from canonical heap and integer overflow to type/object confusion and use-after-free (UAF) vulnerabilities – proves that attackers are actively looking into Flash code to find weaknesses that can be abused to execute malicious code.
In particular in 2014, we have seen some new exploits which target two vulnerabilities (CVE-2013-5330 and CVE-2014-0497) in a new feature of Adobe applications – domain memory opcode (also known as Alchemy opcode).
This paper analyses the technical details of exploits using CVE 2013-5330 and CVE-2014-0497. It unveils some interesting tricks used by these exploits to make the attacks more reliable and stealthy, such as improved leaked gadgets using a just-in-time (JIT) spray technique. The malware components distributed by these exploits, namely Win32/Lurk and Win32/Siromost, will also be discussed.
Fortunately, Adobe has introduced changes in Flash designed to break JIT spray techniques, and Microsoft’s Enhanced Mitigation Experience Toolkit (EMET) can provide some help in mitigating these and other memory corruption exploits. Adobe’s changes and EMET both attempt to break some of the exploitation techniques used by attackers, and provide some degree of protection even before the vendor has released a patch. This paper will indicate how EMET can be used successfully to mitigate similar memory corruption exploits, and what mitigations can be more or less effective against these attacks.
Adobe Flash Player is a popular piece of free software which can run ShockWave Flash (SWF) files – an Adobe Flash file format used for multimedia, vector graphics and ActionScript. Adobe Flash Player can be used to execute rich Internet applications, including the delivery of console-quality games to the web browser and the streaming of high-quality video and audio content. According to Adobe, Adobe Flash Player reaches ‘over 1.3 billion people across browsers and [operating system] versions with no install.’ 
Adobe Flash Player can run either from a web browser (as a web browser plug-in) or as a standalone application. The SWF file format supported by Adobe Flash Player can be generated from Adobe products, such as Adobe Flex SDK and Adobe Flash Builder. Developers can create SWF files using ActionScript, a script language based on ECMAScript.
Due to the high prevalence of Adobe Flash Player, attackers have continuously been exploiting the vulnerabilities in the player or leveraging malicious SWF files to deliver attacks. Malicious SWF files are crafted by the attackers and hosted on malicious websites, and the attackers inject malicious scripts into benign websites to redirect to their malicious websites. Users will be infected if they visit these compromised, benign websites with a vulnerable version of Adobe Flash Player.
The remainder of this paper is organized as follows:
Section 2 discusses domain memory opcode (Alchemy opcode) – a new feature introduced in Adobe Flash Player version 11.
Sections 3 and 4 analyse two domain memory opcode related exploits which exploit the CVE-2013-5330 and CVE-2014-0497 vulnerabilities.
Section 5 explains how a malicious SWF file can also be used as a helper file to exploit vulnerabilities in Internet Explorer (as seen in attacks that exploit the CVE 2014 1776, CVE 2014-0322 and CVE-2013-3163 vulnerabilities).
Section 6 presents the mitigation of these attacks from both Adobe and Microsoft.
Section 7 provides a conclusion and outlines future work.
The history of domain memory opcode dates back to 2008, when Adobe released ‘Project Alchemy’ on the Adobe Labs website . Alchemy allows users to compile C and C++ code into ActionScript libraries (AVM2). In 2012, Adobe released domain memory as a premium feature of Adobe Flash Player (thus requiring a separate licence from Adobe). This feature provides fast memory access to ‘domain memory’. In late 2012, Project Alchemy became the Flash Runtime C++ Compiler (FlashCC) . In 2013, Adobe announced that ‘the Flash C++ Compiler (FlashCC) has been contributed to open source as CrossBridge and will be delivered through GitHub’ .
Domain memory opcodes supported by the Adobe Flash Player are listed in Table 1.
|li8||Load 8-bit integer|
|li16||Load 16-bit integer|
|li32||Load 32-bit integer|
|lf32||Load 32-bit float|
|lf64||Load 64-bit float|
|si8||Store 8-bit integer|
|si16||Store 16-bit integer|
|si32||Store 32-bit integer|
|sf32||Store 32-bit float|
|sf64||Store 64-bit float|
|sxi1||Signed extend 1-bit integer to 32 bits|
|sxi8||Signed extend 8-bit integer to 32 bits|
|sxi16||Signed extend 16-bit integer to 32 bits|
Table 1. Domain memory opcodes. These opcodes provide fast read/write access of ByteArray.
Figure 1 shows an example access of ByteArray with domain memory opcodes.
To use the domain memory opcodes, developers need to use ActionScript Compiler (ASC) 2.0, since ASC 1.0 does not directly support these opcodes. Domain memory opcodes are defined as package-level functions inside package avm2.intrinsics.memory (see Figure 2).
Adobe released security updates for CVE-2013-5330 on 12 November 2013 . Versions of Adobe Flash Player for Windows up to and including 11.9.900.117 are affected by this vulnerability.
A bug in Adobe Flash Player causes a failure in memory range validation for domain memory op-codes li*/si*. Though Adobe Flash Player does validate the memory range for li*/si* instructions, there is a logic error in the function that performs the range check. An attacker can craft a SWF file with the ActionScript code shown inFigure 3 to trigger this vulnerability.
In this ActionScript expression, two op_li32 instructions are ‘anded’ together: the first op_li32 is legal, whereas the second is illegal since index -248 is out of bound. However, due to a logic error in the function, the second li32 does not cause a runtime error in a vulnerable version of Adobe Flash Player. As a result, this vulnerability allows the attacker to use crafted li*/si* instructions to read/write an arbitrary memory location, immediately giving the attacker two very powerful exploitation ‘primitives’ that allow them to read/write process memory without the need for sophisticated tricks to neutralize unwanted side effects of typical memory corruption bugs (such as overflows and UAF).
The first exploit sample for CVE-2013-5330 (SHA1: 1514F6F59CE00BD98493C1AC3EED7BF86CB5A4BE) was observed in the wild by security researchers  on 31 January 2014 (more than two months after Adobe had released the patch for it). The sample was distributed by a common exploit kit (unnamed by the researchers) as a SWF file protected with the obfuscator SecureSWF. The sample has been designed as a ‘one-stop’ attack: it contains the vulnerability’s trigger, the shellcode generator, and an encrypted PE file .
The exploit sample sprays the heap with an Object type and makes sure the domain memory ByteArray data starts immediately after the sprayed Object types (Figure 4). It then uses two successive li32 instructions to trigger the vulnerability so it can access the out-of-bound memory. In this case, it uses negative offsets to access the memory backwards and successfully overwrites the VTABLE (virtual table) for the Object. In the new VTABLE, offsets 0x48–0x64 contain the new virtual function pointer (Figure 5), so the virtual function pointer for MethodEnv::getpropertylate_i() is redirected to the controlled address. Later, when the ActionScript tries to access the object, the virtual function MethodEnv::getpropertylate_i() is called, and control is transferred to the controlled address.
Unlike other Adobe Flash exploit samples, which usually transfer control to the return oriented pro-gramming (ROP) gadgets built from an Adobe Flash Player DLL file in order to bypass data execution prevention (DEP), this sample transfers code to the JIT spray gadgets instead.
When the bytes generated by the JIT compiler are executed from offset 6, the mov instructions are no longer mov instructions. Instead, they become JIT gadgets (Table 2). These JIT gadgets make up a call to VirtualProtect(), which makes the shellcode memory executable. Note that the exploit won’t work on Flash versions higher than 11.8, which break the generation of these JIT gadgets.
90 nop 90 nop eb06 jmp 0ca3d8b1
60 pushad 9 nop eb06 jmp 0ca3d8bb
8bd mov ebx,esp eb06 jmp 0ca3d8c5
|;save stack pointer in EBX|
8b21 mov esp,dword ptr [ecx] eb06 jmp 0ca3d8cf
;ESP -> heap object controlled by attacker
44 inc esp 44 inc esp eb06 jmp 0ca3d8d9
44 inc esp 44 inc esp eb06 jmp 0ca3d8e3
|;add +4 to ESP|
6a7f push 7Fh eb06 jmp 0ca3d8ed
58 pop eax 90 nop eb06 jmp 0ca3d8f7
d1c0 rol eax,1 eb06 jmp 0ca3d901
d1c0 rol eax,1 eb06 jmp 0ca3d90b
|;at the end of this EAX=0x1FC|
03e0 add esp,eax eb06 jmp 0ca3d915
|;need to add 0x200 to ESP to find the correct offset that will point the stack to the attacker’s data|
58 pop eax 90 nop eb06 jmp 0ca3d91f
|;EAX is popped from the stack, attacker has placed an API address here|
55 push ebp 90 nop eb06 jmp 0ca3d929
|;VirtualProtect copied prologue #1|
8bec mov ebp,esp eb06 jmp 0ca3d933
|;VirtualProtect copied prologue #2|
ffe0 jmp eax eb06 jmp 0ca3d93d
Table 2. JIT spray gadgets.
The dropped PE file (SHA1: 05446C67FF8C0BAFFA969FC5CC4DD62EDCAD46F5) is detected as TrojanSpy:Win32/Lurk . It registers itself as a PNG image filter (CLSID: A3CCEDF7-2DE2-11D0-86F4-00A0C913F750) so it will be loaded when the web browser needs to decode a PNG image file. Win32/Lurk forwards the exports to the original PNG filter (pngfilt.dll) to make sure the PNG image can be decoded correctly. Win32/Lurk downloads a PE file from a remote server and injects the PE file into the web browser process.
Adobe released security updates for CVE-2014-0497 on 4 February 2014. Versions of Adobe Flash Player for Windows up to and including 18.104.22.168 are vulnerable . This vulnerability was introduced with the fix in November for CVE 2013-5330, and thus lasted for a very short time and for a limited number of versions .
Similar to CVE-2013-5330, certain versions of Adobe Flash Player fail to validate the memory range for li*/si* instructions. An attacker can use the ActionScript code snippet shown in Figure 9 to bypass the memory range validation.
The first CVE-2014-0497 exploit sample was observed in the wild in February 2014, and Adobe promptly responded with an out-of-band patch . Similar to the CVE-2013-5330 exploit sample, this sample also contains a vulnerability trigger, shellcode generator, and an encrypted PE file . The exploit sample successfully bypasses the memory range validation and is then able to access out-of-bound memory.
Initially, the bug is used to leak a pointer into the Flash module to bypass address space layout randomization (ASLR); curiously enough the attacker did not take full advantage of the read/write primitive to dynamically discover gadgets in memory; instead, the exploit contains a large set of hard-coded gadget addresses for almost 20 different Flash versions. These ROP gadgets make up a call to VirtualProtect() to make the shellcode memory region executable and bypass DEP protection. When the ROP chain is prepared, the exploit uses the write primitive to overwrite a virtual function pointer in VTABLE and successfully transfers control to the ROP chain function. The function starts with stack pivot ROP gadgets found in a Flash Player DLL and ends with a transfer of control to the shellcode via a jmp esp instruction.
The shellcode drops a PE file (decrypted by ActionScript) as %temp%\a.exe and executes it. The dropped PE file is detected as TrojanDownloader:Win32/Siromost.A, which simply downloads another PE file from a remote server and then executes it .
A malicious SWF file can not only be used to exploit the vulnerability in Adobe Flash Player itself, but also used as a helper to exploit the vulnerability in a web browser.
Since the Internet Explorer (IE) process and the Adobe Flash plug-in share the same address space, it is possible for the attacker to use an IE-specific vulnerability to corrupt data and objects used by Flash. This exploitation technique has been observed in real attacks in at least three different IE exploit cases: CVE-2013-3163, CVE-2014-0322 and, recently, CVE 2014-1776. In these three cases, apart from some minor modifications and improvements, the attacker was able to transform a UAF memory corruption into a basic memory corruption overwrite primitive which, when applied to certain Flash objects, can finally become a powerful read/write anywhere exploitation primitive, as seen in Table 3.
|IE vulnerability||Corruption primitive used against Flash objects||SHA1 of corresponding Flash sample|
|CVE-2013-3163||or dword ptr [esi+8],20000h||81fe2ae7a685014cafc12c3abbcc5ffc9ab27b7e|
|CVE-2014-0322||inc dword ptr [eax+10h]||910de05e0113c167ba3878f73c64d55e5a2aff9a|
|CVE-2014-1776||mov [esi+42h], cx||8dd01c0e60e3cedac0b3914e324c39d8ceb74741|
Table 3. Corruption primitives.
The helper SWF file works as an ‘exploitation framework’ to take advantage of IE’s memory corruption bugs (and those of other browsers) and re-use them against Flash objects. This strategy presents some good advantages for attackers, as described in Table 4.
|Write once, re-use multiple times||The ActionScript code is maintained and improved by attackers as a project and can be applied to any browser memory corruption with some minor changes|
|Exploit divided into multiple pieces||Recovery of all exploit artifacts for security vendors becomes more difficult and the IE exploit part won’t trigger without the Flash counterpart|
|Obfuscation||Flash files can easily be obfuscated and protected and pose some non-trivial challenges for anti virus detection|
|Portability||The framework works well on multiple browsers and doesn’t require major changes to be adapted to all IE versions|
Table 4. Advantages of SWF file helper.
The attacker has to achieve the goal of abusing the freed object in IE in order to corrupt the size of a Flash Vector which has to be located exactly at memory address 0x1a1b2000 and without causing a crash. The specific memory layout crafted by the attacker will cause IE to start using a fake CMarkup object. This will lead into a subfunction of the MSHTML.DLL module, which executes the instruction ‘inc dword ptr [eax+10h]’ twice, with the EAX register pointed exactly to the data allocated as the Flash Vector (see Figure 11).
Since the first DWORD of the Vector is the size of the Vector, the size of Vector v1 has now been increased. Now the size of v1 has been increased. Hence, v1[0x3fe] can be accessed via ActionScript. As v1[0x3fe] is actually the first DWORD of the adjacent, Vector v2, this means that by changing the value of v1[0x3fe], the size of v2 (the next Vector adjacent to v1 in memory) is also changed. The exploit code sets v1[0x3fe] to an oversized value, 0x3ffffff0, so the size of v2 has now been increased to 0x3ffffff0, and the attacker can use v2 to access an arbitrary memory location for read and write actions .
Version 11.8 of Adobe Flash Player has introduced constant blinding in the JIT compiler as a mitiga-tion against JIT spray attacks. For the ActionScript code snippet shown in Figure 12, the constant 0x80000000 has been ‘blinded’ with a random expression, 0xCABAE6C3 ^ 0x4ABAE6C3, which makes the JIT compiler-generated code unpredictable and mitigates the JIT spray attack accordingly.
Microsoft also provides a free tool, the Enhanced Mitigation Experience Toolkit (EMET), which can be helpful in mitigating the attacks that originated from memory corruption exploits for any software, including browsers and plug-ins, such as Adobe Flash Player:
‘[It] is a utility that helps prevent vulnerabilities in software from being successfully exploited. EMET achieves this goal by using security mitigation technologies. These technologies function as special protections and obstacles that an exploit author must defeat to exploit software vulnerabilities. These security mitigation technologies do not guarantee that vulnerabilities cannot be exploited. However, they work to make exploitation as difficult as possible to perform.’ 
EMET can be downloaded (free of charge) from http://www.microsoft.com/emet. The latest stable and supported versions are EMET 4.1 Update 1 and EMET 5.0. The goal of this tool is to enable and provide additional security mitigations that are designed to break common exploitation techniques used by attackers. It is known that exploit mitigations do not completely eliminate the vulnerabilities. With tools like EMET, however, it is possible to raise the cost of developing a successful and reliable working exploit by introducing hardening and checks that will lead the exploit code to terminate or crash unexpectedly. In our lab, a Windows 7 machine equipped with EMET 4.1 (released in early November 2013) was tested against samples from the two Flash exploits analysed in this paper. The results are shown in Table 5.
|EMET 4.1/OS Mitigation||CVE-2013-5330||CVE-2014-0497|
Table 5. EMET mitigation.
In the case of CVE-2013-5330, the attacker attempts to bypass the DEP protection mechanism from the operating system by using JIT-spray techniques. The attacker was also able to bypass ASLR and disclose module memory addresses with the capabilities of the exploit primitive provided by the specific Flash vulnerability (out-of-bound read/write using li32/si32 opcodes), which is a very optimal situation for an exploit writer (normally the attacker would have to work hard to craft this type of primitive). In order to bypass EAF, the shellcode is kept minimal and assembled directly from ActionScript filling the API placeholders and without using any dynamic resolution that requires Export Table parsing. Furthermore, to bypass certain user-mode hooks from security products, the JIT code attempts to call VirtualProtect+5 instead of jumping at the beginning of the VirtualProtect function. To do so, the attacker re-implements the missing prologue in the JIT code.
The first EMET mitigation to be effective against this exploit is AntiDetour, a mechanism which increments randomly the number of bytes detoured for each API and fills the original bytes after APIfunction+5 with INT3 opcode, which will raise an exception. When the JIT code attempts to execute VirtualProtect+5, skipping the hooked prologue (see Table 2), it will end up crashing on this opcode. It’s also interesting to note that even if this exploit doesn’t use a ROP chain (it uses JIT generated gadgets instead), some ROP mitigations from EMET are still triggering and preventing exploitation. After analysis, we realized that the reason is because the JIT code performs some kind of stack-pivoting in order to fetch some of the information needed to carry over further exploitation in the shellcode. Because this stack-pivoting effect is never restored and ESP is kept out of stack boundaries, when the shellcode attempts to call some API, the ROP mitigations will detect this anomaly and also terminate the exploit with StackPivot mitigation.
In the case of CVE-2014-0497 (see Figure 13), our tests showed that the exploit was blocked by multiple EMET mitigations because the attacker used pure ROP techniques in order to bypass DEP. In fact, the exploit attempts to call VirtualProtect using a small ROP chain that is detected by SimExecFlow mitigation (a simulation-based heuristic to detect gadget execution). In addition to this, Caller and StackPivot mitigations are also able to stop and detect anomalies during the execution of the exploit (the stack is pivoted and the code transfer to VirtualProtect doesn’t come from a legitimate function). Finally, EAF mitigation triggers at the final stage when the shellcode is executed and tries to parse the KERNEL32 export table.
While it is possible that a sophisticated attacker with knowledge of EMET and with enough skills and time may be able to target all the mitigations and attempt to bypass them or to use techniques not yet protected by EMET, it has been observed that the tool still represents a strong defence-in-depth strategy and it is helpful in blocking a large class of common attacks also seen in targeted attacks . The major advantage is that EMET provides hardening and mitigations without the need for signature updates, and also works in a generic way for older applications.
This paper discussed the vulnerability and exploits of Adobe Flash Player. In addition, it also revealed how vulnerabilities in other software, such as web browsers, could be exploited more generically by using SWF files. Both Adobe and Microsoft have actively been involved in the mitigation of web attacks. Adobe has introduced constant blinding which mitigates the JIT spray attack, and Microsoft’s EMET and improvements in recent IE versions can also help to mitigate zero-day attacks that leverage Flash-based exploits.
The use of SWF in attacks will continue, and will evolve in the long term due to the popularity of the plug-in and its large market share. We will continue tracking, analysing and eliminating these threats
During our research into these Adobe Flash Player vulnerabilities, we have noticed that sometimes it is fairly difficult to debug SWF files due to the limitations of current debuggers and reversing tools and the availability of cheap-but-powerful obfuscation tools. A byte-code-level SWF debugger could make the analyst’s life much easier.
We urge Adobe to provide a byte-code-level SWF debugger for the anti-malware industry.
We would like to thank the MMPC (Microsoft Malware Protection Center) technical writing team for their great efforts in reviewing this paper.
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Shellcode used by CVE-2013-5330 exploit:
add esp,0E000h push ebx push 2048h push 0 mov eax,offset kernel32!GlobalAlloc (7c80fdbd) call eax mov esi,eax push esi push 2048h mov eax,offset kernel32!GetTempPathA (7c835de2) call eax test eax,eax je 04bf22eb push esi push 0 push 0 push esi mov eax,offset kernel32!GetTempFileNameA (7c861807) call eax test eax,eax je 04bf22eb push 0 push 80h push 2 push 0 push 0 push 40000000h push esi mov eax,offset kernel32!CreateFileA (7c801a28) call eax mov edi,eax cmp edi,0FFFFFFFFh je 04bf22eb push 0 push esp mov eax,18200h push eax push edi mov edi,3EF39F0h pop ebx push edi push ebx mov eax,offset kernel32!WriteFile (7c810e17) call eax test eax,eax je 04bf22eb push ebx mov eax,offset kernel32!CloseHandle (7c809bd7) call eax push esi mov eax,offset kernel32!LoadLibraryA (7c801d7b) call eax pop esp popad ret 8
 N/A = mitigation not applicable for the type of exploitation used
X = mitigation effective to stop the exploit sample
B = the exploit sample uses techniques designed to bypass this mitigation