Document Title:
===============
Microsoft Skype v7.2, v7.35 & v7.36 - Stack Buffer Overflow Vulnerability


References (Source):
====================
https://www.vulnerability-lab.com/get_content.php?id=2071

MSRC ID: 38778 
TRK ID: 0461000724

Vulnerability Magazine: https://www.vulnerability-db.com/?q=articles/2017/05/28/stack-buffer-overflow-zero-day-vulnerability-uncovered-microsoft-skype-v72-v735

Video: https://www.vulnerability-lab.com/get_content.php?id=2084


Release Date:
=============
2017-06-26


Vulnerability Laboratory ID (VL-ID):
====================================
2071


Common Vulnerability Scoring System:
====================================
7.2


Vulnerability Class:
====================
Buffer Overflow


Current Estimated Price:
========================
25.000a! - 35.000a!


Product & Service Introduction:
===============================
Skype is an instant messaging app that provides online text message and video chat services. Users may transmit both text and video messages 
and may exchange digital documents such as images, text, and video. Skype allows video conference calls. Skype allows users to communicate 
over the Internet by voice using a microphone, by video using a webcam, and by instant messaging. Skype-to-Skype calls to other users are 
free of charge, while calls to landline telephones and mobile phones (over traditional telephone networks) are charged via a debit-based 
user account system called Skype Credit. Some network administrators have banned Skype on corporate, government, home, and education networks,
citing such reasons as inappropriate usage of resources, excessive bandwidth usage and security concerns. Skype originally featured a hybrid 
peer-to-peer and clientaserver system. Skype has been powered entirely by Microsoft-operated supernodes since May 2012. 

(Copy of the Homepage:  https://en.wikipedia.org/wiki/Skype )


Abstract Advisory Information:
==============================
The vulnerability laboratory core research team discovered a stack buffer overflow vulnerability in the official Microsoft Skype v7.2, v7.3.5.103 & v7.3.6 software.


Vulnerability Disclosure Timeline:
==================================
2017-05-16: Researcher Notification & Coordination (Benjamin Kunz Mejri)
2017-05-17: Vendor Notification (Microsoft Security Response Center - MSRC)
2017-05-24: Vendor Response/Feedback (Microsoft Security Response Center - MSRC)
2017-06-08: Vendor Fix/Patch (Microsoft Service Developer Team)
2017-06-25: Security Acknowledgements (Microsoft Security Response Center - MSRC)
2017-06-26: Public Disclosure (Vulnerability Laboratory)


Discovery Status:
=================
Published


Affected Product(s):
====================
Microsoft Corporation
Product: Skype - Software Client 7.2, 7.35.103 & 7.36.0.101 & 7.36.0.150


Exploitation Technique:
=======================
Local


Severity Level:
===============
High


Technical Details & Description:
================================
A remote and local stack buffer overflow vulnerability has been discovered in the official Microsoft Skype v7.2, v7.3.5 & v7.3.6 software client.
The security vulnerability allows to crash the software application with an unexpected exception error, to overwrite the active process registers 
to execute own malcious codes.

The skype software is using a dll in case of a copy request on the local systen. We place a picture in our clipboard (we take a screenshot in this example), 
this needs to be copied from a remote desktop system. This can be pasted into the local skype message box, by the paste function. Then the picture is taken 
from the clipboard (which is the rdp remote clipboard content) and successfully copies it into the message box.

The security vulnerability is located in the `clipboard format` function of the skype software. Attackers are able to use a remote computer system with a 
shared clipboard, to provoke a stack buffer overflow on transmittion to skype. The issue affects the `MSFTEDIT.DLL` dynamic link library of the windows8 (x86) 
operating system. The limitation of the transmitted size and count for images via print of the remote session clipboard has no secure limitations or restrictions. 
Attackers are able to crash the software with one request to overwrite the eip register of the active software process. Thus allows local or remote attackers to 
execute own codes on the affected and connected computer systems via the skype software.

The attacker opens a local computer system connection and establishes a RDP connection to another system. First the attacker enables the clipboard function for 
the remote session. This is possible by the basic rdp settings in the `Local Devices & Resources settings. Then the attacker moves with a click into the rdp session 
window and pushes the print key. A screenshot is made of the remote session, that is loaded to the local system cache of the first computer system that the attacker uses. 
Then the attacker moves back to the local system into the conversation of skype and copypastes the screenshot of the clipboard (print) to the message body. A clipboard 
error occurs because of the unknown format size and the software crashs with several uncaught access violations or unfiltered exceptions for both parties. The software is 
not terminated and allows to read every dll error by line with offset. 

The error and critical crashs are captured by the software internal dev log called gilasterr.log file. The gilasterr.log file captures internal information of the software 
in case of critical errors for the skype developer teams. During the exploitation process the gilasterr.log file captured the loop crashs and overwrite of the eip register 
as reference. The active offsets allowed us to define a new address to compromise the targeted local or remote computer system.

In a software update of the v7.2, v7.3.5 & v7.3.6 version of skype, a limitation has been implemented for the clipboard function. Due to the implementation, a 
misconfiguration was included by the developers. The cut function allows to paste the image in raw format back to the message box. After the limitation was implemented 
for the pasted image counts and for the byte size of the clipboard, the attacker is still able to exploit the issue. Therefore an attacker uses the developer flaw that 
should fix the new zero-day vulnerability. The attacker copies the content via remote session of the clipboard via cache, then he uses the cut ability of skype with the 
context menu and repaste the input again. After that the images are getting transfered in raw format as text value of the skype code [image] by repasting them to the message 
box query, the vulnerability can be triggered again and the limiation of size and count is bypassed.

The successful attack scenario is not limited to manual exploitation only. Attackers can locally prepare the cache and clipboard of a computersystem to exploit the connected 
remote party computer system using skype.

The security risk of the skype vulnerability is estimated as high with a cvss (common vulnerability scoring system) count of 7.2. Exploitation of the buffer overflow software 
vulnerability requires no user interaction and only a low privilege skype user account. Successful exploitation of the buffer overflow vulnerability results in system and process 
compromise by an overwrite of the registers.

Vulnerable Software Client(s):
[+] Microsoft Skype v7.3.6
[+] Microsoft Skype v7.3.5.103
[+] Microsoft Skype v7.2

Vulnerable Function(s):
[+] clipboard (remote session)

Affected Library:
[+] MSFTEDIT.DLL


Proof of Concept (PoC):
=======================
The buffer overflow vulnerability can be exploited by local and remote attackers without user interaction and with low privileged skype user account.
For security demonstration or to reproduce the vulnerability follow the provided information and steps below to continue.


Manual steps to reproduce the vulnerability ...
1. Install Skype to your windows xp, windows 7 or windows 8 computer system
2. Add a first test contact and use another computer to connect to ip (separate ip and connection)
3. Accept the add request for further communication to both accounts
4. Now, the attacker opens for example a remote desktop session to another system of the first computer for reproduce
Note: Activate in the RDP session the clipboard for copy to share with the local connected computer system
5. Connect via RDP to the new computer system
6. Use the print button to make a screenhot
Note: The data is stored in the cache because of the transmit to the other system
7. Move now back to the regular system without closing the rdp connection
8. Copy the cipboard content inside of the message box
Note: In some cases this already causes the main crash but sometimes the message needs to be delivered to crash on interaction
9. The software crashs and the connected client as well several uncaught and unexpected or unknown errors occur
Note: At that point the attacker is able to overwrite the register of the software process to gain higher privileges
10. Successful reproduce of the buffer overflow vulnerability in the skype software!


Note: The rdp software allows to use the clipboard function to transmit data to the local system. In case of a transmittion via skype by a copy 
of the local system screen via the print key, it is possible to finally exploit the vulnerability. The replace of the screen via print to transmit 
for example large unicode payloads works as well.


--- Debug Logs (WinDBG on Windows XP, Windows 7 & Windows 8) ---
0:000> g
(f2c.1638): Unknown exception - code 000006a6 (first chance)
(f2c.1c18): Unknown exception - code 000006a6 (first chance)
(f2c.1ed4): Unknown exception - code 000006a6 (first chance)
(f2c.1e80): Unknown exception - code 000006a6 (first chance)
(f2c.1c18): Unknown exception - code 000006a6 (first chance)
(f2c.1ed4): Unknown exception - code 000006a6 (first chance)
(f2c.16dc): Access violation - code c0000005 (first chance)
First chance exceptions are reported before any exception handling.
This exception may be expected and handled.
eax=644a1801 ebx=12544290 ecx=644a1801 edx=12544278 esi=12544278 edi=0018f394
eip=00410041 esp=0018dc64 ebp=0018dc7c iopl=0         nv up ei pl nz na po nc
cs=0023  ss=002b  ds=002b  es=002b  fs=0053  gs=002b             efl=00210202
MSFTEDIT!CreateTextServices+0x28a51:
00410041 8b30            mov     esi,dword ptr [eax]  ds:002b:644a1801=????????
0:000> g
(f2c.16dc): Access violation - code c0000005 (first chance)
First chance exceptions are reported before any exception handling.
This exception may be expected and handled.
eax=644a1801 ebx=12544290 ecx=644a1801 edx=12544278 esi=12544278 edi=0018f394
eip=00410041 esp=0018dee4 ebp=0018defc iopl=0         nv up ei pl nz na po nc
cs=0023  ss=002b  ds=002b  es=002b  fs=0053  gs=002b             efl=00210202
MSFTEDIT!CreateTextServices+0x28a51:
00410041 8b30            mov     esi,dword ptr [eax]  ds:002b:644a1801=????????
0:000> g
(f2c.1ed4): Unknown exception - code 000006a6 (first chance)
(f2c.10b8): Unknown exception - code 000006a6 (first chance)
(f2c.1ed4): Unknown exception - code 000006a6 (first chance)
(f2c.16dc): Access violation - code c0000005 (first chance)
First chance exceptions are reported before any exception handling.
This exception may be expected and handled.
eax=644a1801 ebx=12544290 ecx=644a1801 edx=12544278 esi=12544278 edi=0018f394
eip=00410041 esp=0018dc64 ebp=0018dc7c iopl=0         nv up ei pl nz na po nc
cs=0023  ss=002b  ds=002b  es=002b  fs=0053  gs=002b             efl=00210202
MSFTEDIT!CreateTextServices+0x28a51:
00410041 8b30            mov     esi,dword ptr [eax]  ds:002b:644a1801=????????
0:000> u
MSFTEDIT!CreateTextServices+0x28a51:
00410041 8b30            mov     esi,dword ptr [eax]
644a1803 8bce            mov     ecx,esi
644a1805 ff1560056864    call    dword ptr [MSFTEDIT!DllGetActivationFactory+0x75dc0 (64680560)]
644a180b 8bcf            mov     ecx,edi
644a180d ffd6            call    esi
644a180f 8b7f04          mov     edi,dword ptr [edi+4]
644a1812 85ff            test    edi,edi
644a1814 75e9            jne     MSFTEDIT!CreateTextServices+0x28a4f (644a17ff)
0:000> a
...     ...
Skype.exe.1268.dmp
Skype.exe.2664.dmp
Skype.exe.3884.dmp
Skype.exe.6356.dmp


--- Error Event Log Report 1 ---
Version=1
EventType=APPCRASH
EventTime=131394846005041388
ReportType=2
Consent=1
UploadTime=131394846005945068
ReportIdentifier=cf151af6-3add-11e7-bee5-54bef733d5e7
IntegratorReportIdentifier=cf151af5-3add-11e7-bee5-54bef733d5e7
WOW64=1
NsAppName=Skype.exe
Response.BucketId=c2eafb48595741113993cb3466b3a4a1
Response.BucketTable=1
Response.LegacyBucketId=108904300980
Response.type=4
Sig[0].Name=Anwendungsname
Sig[0].Value=Skype.exe
Sig[1].Name=Anwendungsversion
Sig[1].Value=7.35.0.103
Sig[2].Name=Anwendungszeitstempel
Sig[2].Value=590bd88a
Sig[3].Name=Fehlermodulname
Sig[3].Value=MSFTEDIT.DLL
Sig[4].Name=Fehlermodulversion
Sig[4].Value=6.3.9600.17809
Sig[5].Name=Fehlermodulzeitstempel
Sig[5].Value=00410041
Sig[6].Name=Ausnahmecode
Sig[6].Value=c0000005
Sig[7].Name=Ausnahmeoffset
Sig[7].Value=00051801
DynamicSig[1].Name=Betriebsystemversion
DynamicSig[1].Value=6.3.9600.2.0.0.256.48
DynamicSig[2].Name=Gebietsschema-ID
DynamicSig[2].Value=1031
DynamicSig[22].Name=Zusatzinformation 1
DynamicSig[22].Value=1f9b
DynamicSig[23].Name=Zusatzinformation 2
DynamicSig[23].Value=1f9b679267db3137e0d85677258ac18f
DynamicSig[24].Name=Zusatzinformation 3
DynamicSig[24].Value=44fa
DynamicSig[25].Name=Zusatzinformation 4
DynamicSig[25].Value=44fa6b7c31aef8e30a35cae18ad80d90
UI[2]=C:Program Files (x86)SkypePhoneSkype.exe
UI[3]=Skype funktioniert nicht mehr
UI[4]=Windows kann online nach einer LAPsung fA1/4r das Problem suchen.
UI[5]=Online nach einer LAPsung suchen und das Programm schlieAen
UI[6]=SpA$?ter online nach einer LAPsung suchen und das Programm schlieAen
UI[7]=Programm schlieAen
LoadedModule[0]=C:Program Files (x86)SkypePhoneSkype.exe
...     ...
LoadedModule[177]=C:WINDOWSSystem32WMICLNT.dll
LoadedModule[178]=C:WINDOWSsystem32WinSATAPI.dll
LoadedModule[179]=C:WindowsSYSTEM32DDRAW.dll
LoadedModule[180]=C:WindowsSYSTEM32DCIMAN32.dll
LoadedModule[181]=C:WINDOWSSYSTEM32D3DIM700.DLL
LoadedModule[182]=C:WINDOWSSYSTEM32mscms.dll
State[0].Key=Transport.DoneStage1
State[0].Value=1
FriendlyEventName=Nicht mehr funktionsfA$?hig
ConsentKey=APPCRASH
AppName=Skype
AppPath=C:Program Files (x86)SkypePhoneSkype.exe
NsPartner=windows
NsGroup=windows8
ApplicationIdentity=AF2AEB27CB57FBBCA1932A8772681FD2
...     ...
AppCrash_Skype.exe_edbcbb2855eebf494ff386f9eb88be5024b684c0_ce2586b7_09536479
AppCrash_Skype.exe_edbcbb2855eebf494ff386f9eb88be5024b684c0_ce2586b7_162337b5


--- Error Event Log Report 2 ---
Version=1
EventType=APPCRASH
EventTime=131394774156792772
ReportType=2
Consent=1
UploadTime=131394774157463380
ReportIdentifier=1493f024-3acd-11e7-bee5-54bef733d5e7
IntegratorReportIdentifier=1493f023-3acd-11e7-bee5-54bef733d5e7
WOW64=1
NsAppName=Skype.exe
Response.BucketId=c2eafb48595741113993cb3466b3a4a1
Response.BucketTable=1
Response.LegacyBucketId=108904300980
Response.type=4
Sig[0].Name=Anwendungsname
Sig[0].Value=Skype.exe
Sig[1].Name=Anwendungsversion
Sig[1].Value=7.35.0.103
Sig[2].Name=Anwendungszeitstempel
Sig[2].Value=590bd88a
Sig[3].Name=Fehlermodulname
Sig[3].Value=MSFTEDIT.DLL
Sig[4].Name=Fehlermodulversion
Sig[4].Value=6.3.9600.17809
Sig[5].Name=Fehlermodulzeitstempel
Sig[5].Value=55390cb7
Sig[6].Name=Ausnahmecode
Sig[6].Value=c0000005
Sig[7].Name=Ausnahmeoffset
Sig[7].Value=00051801
DynamicSig[1].Name=Betriebsystemversion
DynamicSig[1].Value=6.3.9600.2.0.0.256.48
DynamicSig[2].Name=Gebietsschema-ID
DynamicSig[2].Value=1031
DynamicSig[22].Name=Zusatzinformation 1
DynamicSig[22].Value=58e7
DynamicSig[23].Name=Zusatzinformation 2
DynamicSig[23].Value=58e753590f5b5462c719fa2fec08c5df
DynamicSig[24].Name=Zusatzinformation 3
DynamicSig[24].Value=d3f4
DynamicSig[25].Name=Zusatzinformation 4
DynamicSig[25].Value=d3f420397e2e2e0b425627582efa4b9e
UI[2]=C:Program Files (x86)SkypePhoneSkype.exe
UI[3]=Skype funktioniert nicht mehr
UI[4]=Windows kann online nach einer LAPsung fA1/4r das Problem suchen.
UI[5]=Online nach einer LAPsung suchen und das Programm schlieAen
UI[6]=SpA$?ter online nach einer LAPsung suchen und das Programm schlieAen
UI[7]=Programm schlieAen
...     ...
AppCrash_Skype.exe_edbcbb2855eebf494ff386f9eb88be5024b684c0_ce2586b7_178c36b3
AppCrash_Skype.exe_edbcbb2855eebf494ff386f9eb88be5024b684c0_ce2586b7_17d4edea


--- Error Event Log Report (Final Stack Buffer Overflow) ---
EventType=BEX
EventTime=131396017042959341
ReportType=2
Consent=1
UploadTime=131396017045953454
ReportIdentifier=762751ed-3bee-11e7-8ab7-eaa72e69e267
IntegratorReportIdentifier=6d4bfe78-0289-4d95-9560-e84dc4fda8c5
WOW64=1
NsAppName=Skype.exe
AppSessionGuid=000003b8-0001-0094-b635-203efacfd201
TargetAppId=W:00065514750fd91b9e1d7f9edd239a15171d00000904!000014ddb0fbc81557d09d66c4c18c51c71ff4218649!Skype.exe
TargetAppVer=2016//08//17:21:47:12!1c3188d!Skype.exe
BootId=4294967295
Response.BucketId=1a45da49e92037f743652f4734fcab85
Response.BucketTable=5
Response.LegacyBucketId=50
Response.type=4
Sig[0].Name=Anwendungsname
Sig[0].Value=Skype.exe
Sig[1].Name=Anwendungsversion
Sig[1].Value=7.27.0.101
Sig[2].Name=Anwendungszeitstempel
Sig[2].Value=57b4db60
Sig[3].Name=Fehlermodulname
Sig[3].Value=StackHash_a309
Sig[4].Name=Fehlermodulversion
Sig[4].Value=0.0.0.0
Sig[5].Name=Fehlermodulzeitstempel
Sig[5].Value=00410041
Sig[6].Name=Ausnahmeoffset
Sig[6].Value=PCH_F2_FROM_ntdll+0x0006EB8C
Sig[7].Name=Ausnahmecode
Sig[7].Value=c0000005
Sig[8].Name=Ausnahmedaten
...     ...
EventType=APPCRASH
EventTime=131396017978156224
ReportType=2
Consent=1
UploadTime=131396017979155734
ReportIdentifier=ae0d806d-3bee-11e7-8ab7-eaa72e69e267
IntegratorReportIdentifier=cb6001f2-8e24-428e-9681-798213e07167
WOW64=1
NsAppName=Skype.exe
AppSessionGuid=000003b8-0001-0094-b635-203efacfd201
TargetAppId=W:00065514750fd91b9e1d7f9edd239a15171d00000904!000014ddb0fbc81557d09d66c4c18c51c71ff4218649!Skype.exe
TargetAppVer=2016//08//17:21:47:12!1c3188d!Skype.exe
BootId=4294967295
Response.BucketId=ec621c6ccac633fce5d1e2a3190c48cb
Response.BucketTable=1
Response.LegacyBucketId=108928879505
Response.type=4
Sig[0].Name=Anwendungsname
Sig[0].Value=Skype.exe
Sig[1].Name=Anwendungsversion
Sig[1].Value=7.27.0.101
Sig[2].Name=Anwendungszeitstempel
Sig[2].Value=57b4db60
Sig[3].Name=Fehlermodulname
Sig[3].Value=StackHash_c2f6
Sig[4].Name=Fehlermodulversion
Sig[4].Value=10.0.14393.479
Sig[5].Name=Fehlermodulzeitstempel
Sig[5].Value=58256ca0
Sig[6].Name=Ausnahmecode
Sig[6].Value=c0000374
...     ...
AppCrash_Skype.exe_d0af9530b99d8758e9db661d96b23628d951d0de_1523a347_02d1c738
AppCrash_Skype.exe_eaf5689fdd6d20b6c7ce5c3281fbab78bcbc4c7_1523a347_1416ac1a


Solution - Fix & Patch:
=======================
The vulnerability can be resolved by a count and size restriction of the clipboard content delivered via a remote system. Improve the path content and 
disallow to perform a request when the data is not physical located on the transmittion computer system. Include a secure exception-handling to prevent uncaught and 
unexpected error exceptions to followup with a jump to another address. Another solution could be to deactivate the copy clipboard in case the data is requested 
of another remote system. Thus would deny that an attacker is however able use the clipboard to trick into an overflow.

Immediately update the skype software core and publish a bulletin to prevent exploitation of the zero-day vulnerability. The impact is not limited to skype but 
demonstrated with it. Research on the error to ensure no other software that uses the same way is affected as well.

Note: The skype developer team resolved the stack buffer overflow vulnerability in the official skype software client version 7.37.178.
Download: https://www.skype.com/de/download-skype/skype-for-windows/downloading/


Security Risk:
==============
The security risk of the stack buffer overflow vulnerability in the microsoft skype software core is estimated as high (CVSS 7.2).


Credits & Authors:
==================
Vulnerability Laboratory [Research Team] - Benjamin Kunz Mejri (http://www.vulnerability-lab.com/show.php?user=Benjamin%20K.M.)


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=========================
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