Headline
CVE-2017-2839: TALOS-2017-0341 || Cisco Talos Intelligence Group
An exploitable denial of service vulnerability exists within the handling of challenge packets in FreeRDP 2.0.0-beta1+android11. A specially crafted challenge packet can cause the program termination leading to a denial of service condition. An attacker can compromise the server or use man in the middle to trigger this vulnerability.
Summary
An exploitable denial of service vulnerability exists within the handling of challenge packets in FreeRDP 2.0.0-beta1+android11. A specially crafted challenge packet can cause the program termination leading to a denial of service condition. An attacker can compromise the server or use man in the middle to trigger this vulnerability.
Tested Versions
FreeRDP 2.0.0-beta1+android11 - Windows, OSX, Linux
Product URLs
http://www.freerdp.com/
CVSSv3 Score
6.5 - CVSS:3.0/AV:N/AC:L/PR:N/UI:R/S:U/C:N/I:N/A:H
CWE
CWE-252: Unchecked Return Value
Details
FreeRDP is a remote desktop protocol implementation available for all of the major operating systems. Many of the commercial remote desktop protocol applications actually use this library as their core. The vulnerability arises due to failure to check the return value result.
BOOL license_read_platform_challenge_packet(rdpLicense* license, wStream* s)
{
BYTE MacData[16];
UINT32 ConnectFlags = 0;
if (Stream_GetRemainingLength(s) < 4)
return FALSE;
license->EncryptedPlatformChallenge->type = BB_ANY_BLOB;
license_read_binary_blob(s, license->EncryptedPlatformChallenge); [1]
license->EncryptedPlatformChallenge->type = BB_ENCRYPTED_DATA_BLOB;
if (Stream_GetRemainingLength(s) < 16)
return FALSE;
if (!license_decrypt_platform_challenge(license)) [2]
The license structure is populated at, [1],and a return value check is omitted. This newly populated license is then passed into a decryption function directly. Below is the code for license_read_binary_blob, [1].
BOOL license_read_binary_blob(wStream* s, LICENSE_BLOB* blob)
{
UINT16 wBlobType;
Stream_Read_UINT16(s, wBlobType); /* wBlobType (2 bytes) */
Stream_Read_UINT16(s, blob->length); /* wBlobLen (2 bytes) */
if (Stream_GetRemainingLength(s) < blob->length) [3]
return FALSE;
...
blob->type = wBlobType;
blob->data = (BYTE*) malloc(blob->length); [4]
In the read_blob function we can see the length is read in directly from the packet and then checked against the stream length, [3]. This will exit the function if the check fails and return false. Recall that the previous function does not check the return value so the type blob->data,[4], will not be initialized. The license_decrypt_platform_challenge function is shown below:
BOOL license_decrypt_platform_challenge(rdpLicense* license)
{
BOOL rc;
WINPR_RC4_CTX* rc4;
...
rc = winpr_RC4_Update(rc4, license->EncryptedPlatformChallenge->length,
license->EncryptedPlatformChallenge->data, [5]
license->PlatformChallenge->data);
The license object is passed in and the EncryptedPlatformChallenge is used without validation, [5]. Recall the EncryptedPlatformChallenge data field is not set due to incorrect length so when the RC4 function attempts to use it a null pointer access happens and a denial of service condition arises.
Crash Information
Crashed thread log =
: Dispatch queue: com.apple.main-thread
0 com.apple.CoreGraphics 0x00007fff9109bb34 blt_pattern_blend_XXXX32 + 608
1 com.apple.CoreGraphics 0x00007fff91058de4 argb32_mark + 19951
2 libRIP.A.dylib 0x00007fff8f7e4cec RIPLayerBltShape + 1319
3 libRIP.A.dylib 0x00007fff8f7e2713 ripc_Render + 319
4 libRIP.A.dylib 0x00007fff8f7df1a2 ripc_DrawRects + 438
5 com.apple.AppKit 0x00007fff900577cd __backing_store_DrawRects_block_invoke + 39
6 com.apple.AppKit 0x00007fff90056a77 backing_store_delegate + 768
7 com.apple.AppKit 0x00007fff900564fb backing_store_DrawRects + 1047
8 com.apple.CoreGraphics 0x00007fff91050be7 CGContextFillRects + 107
9 com.apple.CoreGraphics 0x00007fff91050b79 CGContextFillRect + 134
10 com.apple.CoreGraphics 0x00007fff91098001 CGContextDrawImages + 3688
11 com.apple.coreui 0x00007fff98fb858e _CUITileImageWithOperation + 365
12 com.apple.coreui 0x00007fff98fb4e78 DrawOnePartElementFromRenditionWithOperation + 993
13 com.apple.coreui 0x00007fff98fbdc5a -[CUIThemeFacet
_drawSpecificRenditionKey:rendition:inFrame:context:alpha:operation:isFocused:isFlipped:] + 594
14 com.apple.coreui 0x00007fff98fbd91a -[CUIThemeFacet
_drawSpecificRenditionKey:inFrame:context:isFocused:isFlipped:] + 163
15 com.apple.coreui 0x00007fff98fbbc32 -[CUIThemeFacet drawInFrame:isFocused:context:] + 137
16 com.apple.coreui 0x00007fff98fd8f68 CUICoreThemeRenderer::DrawWindowFrameStandardNew(CUIDescriptor
const*) + 1558
17 com.apple.coreui 0x00007fff98f5a065 CUIRenderer::Draw(CGRect, CGContext*, __CFDictionary const*,
__CFDictionary const**) + 2341
18 com.apple.coreui 0x00007fff98f5c992 CUIDraw + 175
19 com.apple.AppKit 0x00007fff8ffeed25 __44-[NSAppearance _drawInRect:context:options:]_block_invoke + 64
20 com.apple.AppKit 0x00007fff8fe55e91 -[NSCompositeAppearance _callCoreUIWithBlock:] + 183
21 com.apple.AppKit 0x00007fff8ffeecde -[NSAppearance _drawInRect:context:options:] + 127
22 com.apple.AppKit 0x00007fff900c0699 -[NSThemeFrame _maskCorners:clipRect:] + 259
23 com.apple.AppKit 0x00007fff90612b0d -[NSThemeFrame _drawTransparentTitlebarInRect:] + 173
24 com.apple.AppKit 0x00007fff900bd6b3 -[NSThemeFrame _drawUnifiedToolbar:] + 181
25 com.apple.AppKit 0x00007fff900bd480 -[NSThemeFrame _drawTitleBar:] + 104
26 com.apple.AppKit 0x00007fff900bd411 -[NSThemeFrame _drawFrameInterior:clip:] + 83
27 com.apple.AppKit 0x00007fff900bd3b1 -[NSThemeFrame drawFrame:] + 892
28 com.apple.AppKit 0x00007fff900bcf98 -[NSFrameView drawRect:] + 1098
29 com.apple.AppKit 0x00007fff900bcb33 -[NSThemeFrame drawRect:] + 280
30 com.apple.AppKit 0x00007fff8fffcc86 -[NSView _drawRect:clip:] + 3550
31 com.apple.AppKit 0x00007fff8fffacf5 -[NSView
_recursiveDisplayRectIfNeededIgnoringOpacity:isVisibleRect:rectIsVisibleRectForView:topView:] + 3136
32 com.apple.AppKit 0x00007fff8fff9be0 -[NSThemeFrame
_recursiveDisplayRectIfNeededIgnoringOpacity:isVisibleRect:rectIsVisibleRectForView:topView:] + 334
33 com.apple.AppKit 0x00007fff8fff7feb -[NSView _displayRectIgnoringOpacity:isVisibleRect:rectIsVisibleRectForView:]
+ 2449
34 com.apple.AppKit 0x00007fff8fff33f5 -[NSView displayIfNeeded] + 1950
35 com.apple.AppKit 0x00007fff8fff2c3c -[NSWindow displayIfNeeded] + 232
36 com.apple.AppKit 0x00007fff9067741b ___NSWindowGetDisplayCycleObserver_block_invoke6365 + 476
37 com.apple.AppKit 0x00007fff8fff25d6 __37+[NSDisplayCycle currentDisplayCycle]_block_invoke + 941
38 com.apple.QuartzCore 0x00007fff86da5f71 CA::Transaction::run_commit_handlers(CATransactionPhase) + 85
39 com.apple.QuartzCore 0x00007fff86da542c CA::Context::commit_transaction(CA::Transaction*) + 160
40 com.apple.QuartzCore 0x00007fff86da50ec CA::Transaction::commit() + 508
41 com.apple.QuartzCore 0x00007fff86db0977 CA::Transaction::observer_callback(__CFRunLoopObserver*, unsigned
long, void*) + 71
42 com.apple.CoreFoundation 0x00007fff88e20067
__CFRUNLOOP_IS_CALLING_OUT_TO_AN_OBSERVER_CALLBACK_FUNCTION__ + 23
43 com.apple.CoreFoundation 0x00007fff88e1ffd7 __CFRunLoopDoObservers + 391
44 com.apple.CoreFoundation 0x00007fff88dfeef8 CFRunLoopRunSpecific + 328
45 com.apple.HIToolbox 0x00007fff8f2b7935 RunCurrentEventLoopInMode + 235
46 com.apple.HIToolbox 0x00007fff8f2b7677 ReceiveNextEventCommon + 184
47 com.apple.HIToolbox 0x00007fff8f2b75af _BlockUntilNextEventMatchingListInModeWithFilter + 71
48 com.apple.AppKit 0x00007fff8fe9adf6 _DPSNextEvent + 1067
49 com.apple.AppKit 0x00007fff8fe9a226 -[NSApplication _nextEventMatchingEventMask:untilDate:inMode:dequeue:] +
454
50 com.apple.AppKit 0x00007fff8fe8ed80 -[NSApplication run] + 682
51 com.apple.AppKit 0x00007fff8fe58368 NSApplicationMain + 1176
52 libdyld.dylib 0x00007fff86cf45ad start + 1
log name is: ./crashlogs/1.crashlog.txt
---
exception=EXC_CRASH:signal=11:is_exploitable= no:instruction_disassembly=.byte 0xc4 #bad
opcode:instruction_address=0x00007fff9109bb34:access_type=:access_address=0x0000000000000000:
Exploit Proof-of-Concept
Run included Python server and connect FreeRDP Client to it.
Timeline
2017-05-24 - Vendor Disclosure
2017-07-24 - Public Release
Discovered by Tyler Bohan of Cisco Talos.