===============
Microsoft Skype v7.2, v7.35 & v7.36 - Stack Buffer Overflow Vulnerability
References (Source):
====================
https://www.vulnerability-lab.
MSRC ID: 38778
TRK ID: 0461000724
Vulnerability Magazine: https://www.vulnerability-db.
Video: https://www.vulnerability-lab.
Release Date:
=============
2017-06-26
Vulnerability Laboratory ID (VL-ID):
==============================
2071
Common Vulnerability Scoring System:
==============================
7.2
Vulnerability Class:
====================
Buffer Overflow
Current Estimated Price:
========================
25.000€ - 35.000€
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 client–server system. Skype has been powered entirely by Microsoft-operated supernodes since May 2012.
(Copy of the Homepage: https://en.wikipedia.org/wiki/
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+
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+
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+
00410041 8b30 mov esi,dword ptr [eax] ds:002b:644a1801=????????
0:000> u
MSFTEDIT!CreateTextServices+
00410041 8b30 mov esi,dword ptr [eax]
644a1803 8bce mov ecx,esi
644a1805 ff1560056864 call dword ptr [MSFTEDIT!
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+
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-
IntegratorReportIdentifier=
WOW64=1
NsAppName=Skype.exe
Response.BucketId=
Response.BucketTable=1
Response.LegacyBucketId=
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=
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=
Sig[5].Value=00410041
Sig[6].Name=Ausnahmecode
Sig[6].Value=c0000005
Sig[7].Name=Ausnahmeoffset
Sig[7].Value=00051801
DynamicSig[1].Name=
DynamicSig[1].Value=6.3.9600.
DynamicSig[2].Name=
DynamicSig[2].Value=1031
DynamicSig[22].Name=
DynamicSig[22].Value=1f9b
DynamicSig[23].Name=
DynamicSig[23].Value=
DynamicSig[24].Name=
DynamicSig[24].Value=44fa
DynamicSig[25].Name=
DynamicSig[25].Value=
UI[2]=C:Program Files (x86)SkypePhoneSkype.exe
UI[3]=Skype funktioniert nicht mehr
UI[4]=Windows kann online nach einer Lösung für das Problem suchen.
UI[5]=Online nach einer Lösung suchen und das Programm schließen
UI[6]=Später online nach einer Lösung suchen und das Programm schließen
UI[7]=Programm schließen
LoadedModule[0]=C:Program Files (x86)SkypePhoneSkype.exe
... ...
LoadedModule[177]=C:
LoadedModule[178]=C:
LoadedModule[179]=C:
LoadedModule[180]=C:
LoadedModule[181]=C:
LoadedModule[182]=C:
State[0].Key=Transport.
State[0].Value=1
FriendlyEventName=Nicht mehr funktionsfähig
ConsentKey=APPCRASH
AppName=Skype
AppPath=C:Program Files (x86)SkypePhoneSkype.exe
NsPartner=windows
NsGroup=windows8
ApplicationIdentity=
... ...
AppCrash_Skype.exe_
AppCrash_Skype.exe_
--- Error Event Log Report 2 ---
Version=1
EventType=APPCRASH
EventTime=131394774156792772
ReportType=2
Consent=1
UploadTime=131394774157463380
ReportIdentifier=1493f024-
IntegratorReportIdentifier=
WOW64=1
NsAppName=Skype.exe
Response.BucketId=
Response.BucketTable=1
Response.LegacyBucketId=
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=
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=
Sig[5].Value=55390cb7
Sig[6].Name=Ausnahmecode
Sig[6].Value=c0000005
Sig[7].Name=Ausnahmeoffset
Sig[7].Value=00051801
DynamicSig[1].Name=
DynamicSig[1].Value=6.3.9600.
DynamicSig[2].Name=
DynamicSig[2].Value=1031
DynamicSig[22].Name=
DynamicSig[22].Value=58e7
DynamicSig[23].Name=
DynamicSig[23].Value=
DynamicSig[24].Name=
DynamicSig[24].Value=d3f4
DynamicSig[25].Name=
DynamicSig[25].Value=
UI[2]=C:Program Files (x86)SkypePhoneSkype.exe
UI[3]=Skype funktioniert nicht mehr
UI[4]=Windows kann online nach einer Lösung für das Problem suchen.
UI[5]=Online nach einer Lösung suchen und das Programm schließen
UI[6]=Später online nach einer Lösung suchen und das Programm schließen
UI[7]=Programm schließen
... ...
AppCrash_Skype.exe_
AppCrash_Skype.exe_
--- Error Event Log Report (Final Stack Buffer Overflow) ---
EventType=BEX
EventTime=131396017042959341
ReportType=2
Consent=1
UploadTime=131396017045953454
ReportIdentifier=762751ed-
IntegratorReportIdentifier=
WOW64=1
NsAppName=Skype.exe
AppSessionGuid=000003b8-0001-
TargetAppId=W:
TargetAppVer=2016//08//17:21:
BootId=4294967295
Response.BucketId=
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=
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=
Sig[5].Value=00410041
Sig[6].Name=Ausnahmeoffset
Sig[6].Value=PCH_F2_FROM_
Sig[7].Name=Ausnahmecode
Sig[7].Value=c0000005
Sig[8].Name=Ausnahmedaten
... ...
EventType=APPCRASH
EventTime=131396017978156224
ReportType=2
Consent=1
UploadTime=131396017979155734
ReportIdentifier=ae0d806d-
IntegratorReportIdentifier=
WOW64=1
NsAppName=Skype.exe
AppSessionGuid=000003b8-0001-
TargetAppId=W:
TargetAppVer=2016//08//17:21:
BootId=4294967295
Response.BucketId=
Response.BucketTable=1
Response.LegacyBucketId=
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=
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=
Sig[5].Value=58256ca0
Sig[6].Name=Ausnahmecode
Sig[6].Value=c0000374
... ...
AppCrash_Skype.exe_
AppCrash_Skype.exe_
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/
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.
Disclaimer & Information:
=========================
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