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Open Sesame: Picking Locks with Cortana

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Slides of our Blackhat USA 2018 talk:
Many new devices are trying to fit into our life seamlessly. As a result, there’s a quest for a “universal access methods” for all devices. Voice activation seems to be a natural candidate for the task and many implementations for it surfaced in recent years. A few notable examples are Amazon’s Alexa, Google’s Assistant and Microsoft’s Cortana.

The problem starts when these “Universal” access methods, aimed for maximal comfort, meet the very “specific” use-case of the enterprise environment which requires comfort to be balanced with other aspects, such as security. Microsoft Cortana is used on Mobile and IoT devices, but also in the enterprise computers as it comes enabled by default with Windows10 and always ready to respond to users’ commands even when the machine is locked.

Allowing interaction with a locked machine is a dangerous architectural decision, and earlier this year, we exposed the Voice of Esau (VoE) exploit for a Cortana vulnerability. The VoE exploit allowed attackers to take over a locked Windows10 machine by combining voice commands and network fiddling to deliver a malicious payload to the victim machine.

In this presentation, we will reveal the “Open Sesame” vulnerability, a much more powerful vulnerability in Cortana that allows attackers to take over a locked Windows machine and execute arbitrary code. Exploiting the “Open Sesame” vulnerability attackers can view the contents of sensitive files (text and media), browse arbitrary web sites, download and execute arbitrary executables from the Internet, and under some circumstances gain elevated privileges. To make matters even worse, exploiting the vulnerability does not involve ANY external code, nor shady system calls, hence making code focused defenses such as Antivirus, Anti-malware and IPS blind to the attack.

We would conclude by suggesting some defense mechanisms and compensating controls to detect and defend against such attacks.

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Open Sesame: Picking Locks with Cortana

  1. 1. Open Sesame: Picking Locks with Cortana Ron Marcovich, Yuval Ron, Amichai Shulman, Tal Be'ery 1
  2. 2. Amichai Shulman • Independent Security Researcher • Advisor for multiple cyber security start up companies • Former CTO and Co-Founder of Imperva • Blackhat, RSA, Infosec speaker • @amichaishulman Tal Be’ery • Co-Founder @ Kzen Networks • Formerly VP Research @Aorato (Acquired by Microsoft), Imperva, Singtel Innov8 VC • Blackhat, RSA, SAS speaker • @talbeerysec 2 Who are we?
  3. 3. Also Featuring… Yuval Ron Twitter: @RonYuval LinkedIn: ronyuval Ron Marcovich Twitter: @RonMarcovich LinkedIn: ronmarcovich B.Sc. Software Engineering students at the Technion, Israel Institute of Technology. Both will start their M.Sc. In Computer Science this year. 3
  4. 4. Agenda • Understanding Cortana • What is it, how does it work and key elements • Attacking Cortana on all fronts • Cortana agent: Open Sesame (CVE-2018-8140) • Cortana actions: The voice of Esau • Cortana cloud: Malicious skills • Protecting against Cortana attacks • Voice Firewalls: NewSpeak • Summary and Conclusions 4
  5. 5. Understanding Cortana 5
  6. 6. What is Cortana? • "Your intelligent assistant across your life." • Translate human intent into computer actions • Retrieve data • Browse the web • Launch programs 6
  7. 7. What is Cortana? • Multi-platform: Mobile, PC, devices • Multi inputs (“intents”): keyboard, mouse, voice, touch, … 7
  8. 8. Cortana Architecture 8 Cortana Service Speech to Text Text to Intent (Action) Cortana Skill Internet 3rd party web service Cortana Client Speech Text Text Intent + p Intent + p Card data Speech Text Resolve! Card Action Provider (Azure Bot) Intent to Card (Azure Bot)
  9. 9. Cortana Architecture - Example 9 Cortana Service Speech to Text Text to Intent (Action) Action Provider (Azure Bot) Internet 3rd party web service Cortana Client Speech Who is George Washington Who is George Washington Search Query = “George Washington” Card data Speech Who is George Washington Resolve! Action Provider (Azure Bot) Intent to Card (Azure Bot) Search Query = “George Washington”
  10. 10. Cortana Agent • Very fat Client • Can do a lot of stuff! • Merely an execution engine • Exposes a powerful Javascript API • Works on a locked devices • By Default! • SpeechRuntime.exe listens for “Hey Cortana” • SearchUI.exe has the “Cortana Logic” 10
  11. 11. Cortana Cloud Service • Processing and decision making is done in the cloud • Two phases • Audio processing – Speech to Text • wss://websockets.platform.bing.com/ws/cu/v3 • Binary + JSON • Semantic processing – Text to Intent & Intent to Card • https://www.bing.com/speech_render - GET request, HTML response • https://www.bing.com/DialogPolicy - GET / POST request, Javascript response • Machine Learning • Improve speech recognition • Extend intent resolution capabilities 11
  12. 12. Audio Processing Phase Client Server Connection.context(JSON) Audio stream (BIN) IntermediateResult (XML) Audio stream (BIN) IntermediateResult (XML) Audio stream (BIN) Audio.stream.hypothesis PhraseResult (XML) 12
  13. 13. Semantic Processing Phase 13
  14. 14. Cortana Skills • Cortana can be extended with cloud based “skills” • A Skill is an Azure bot registered to the Cortana channel • Receive all user input after an invocation name • Interacts with the Cortana client using Cards that include voice, text and LIMITED COMMANDS 14
  15. 15. Cortana Skills 15
  16. 16. • Fat client executes on locked screen • Many possible actions • Action choice by cloud logic • Can be changed without any apparent sign on the device • Might depend on Machine Learning • Choice of action can be affected by unknown 3rd parties Summary 16
  17. 17. 17
  18. 18. Putting Murphy to Work • Set up a research project with the Technion • Undergraduate students exploring different aspects of the system • Some avenues we explored • Local input to Cortana • Intents that invoke exploitable actions • Intents that retrieve malicious content • Capabilities of 3rd party Cortana skills 18
  19. 19. Attacking Cortana 19 Cortana Service Speech to Text Text to Intent (Action) Cortana Skill Internet 3rd party web service Cortana Client Speech Text Text Intent + p Intent + p Card data Speech Text Resolve! Card Action Provider (Azure Bot) Intent to Card (Azure Bot) Expressing bad intents Local commands through lock screen Malicious skills Bad content provider
  20. 20. Open Sesame 20
  21. 21. CVE-2018-8140 (Open Sesame) 21 Grabbing Information
  22. 22. CVE-2018-8140 (Open Sesame) 22 Taking over
  23. 23. Open Sesame: Attack Model • Impact: • by Abusing The “Open Sesame” vulnerability, “Evil Maid” attackers can gain full control over a locked machine • Evil Maid attack model: • Attackers have physical access for a limited time, but the Computer is locked • Why it’s called Evil Maid? • Think of the laptop you left in your room last night when you went out… • But also borders control, computers in the office during breaks and night, … • But isn’t that exactly what Locked Screen suppose to stop? 23
  24. 24. Lock Screen: You Had One Job • Lock Screen is not magic! • Lock Screen is merely another “Desktop” ( Winlogon desktop ) with very limited access • The security stems from the reduced attack surface • If Microsoft adds more apps on Lock Screen: The attack surface expands → security is reduced 24
  25. 25. Lock Screen Evolution : Then 25
  26. 26. Lock Screen Evolution: Now 26
  27. 27. “Open Sesame” Root Cause • Lock screen restricts keyboard, but allows Cortana invocation through voice • Once Cortana is invoked, the Lock Screen no longer restricts it • Cortana is free to accept input from the keyboard too • The fix: Make Cortana Search UI state aware. Different behavior when the UI is locked • Shift of responsibility: • In the past, the OS made sure the UI is not accessible when computer is locked, therefore developers do not need to think about it. • Now, it’s the developers’ responsibility 27
  28. 28. Disclosure Timeline 16 APR 18: We report CVE-2018- 8140 to MS 23 APR 18: McAfee reports CVE-2018- 8140 to MS 12 JUN 18: MS patch (Very quick + Bug Bounty!) 26 JUN 18: We report CVE-2018- 8369 to MS 28
  29. 29. “Open Sesame” Summary • Impact: Evil Maid Attackers can gain full control on a locked machine • The fix is • Tactical: making Cortana Search aware of UI state • Not Strategical: Cortana still gets keyboard input and launches processes from a locked screen in some other scenarios • There are more where it came from: CVE-2018-8369 • Design lessons: Adding more capabilities to Lock Screen is very tempting, but dangerous 29
  30. 30. Cruel Intentions: The Voice of Esau 30
  31. 31. Attacking Cortana: Cruel Intentions 31 Cortana Service Speech to Text Text to Intent (Action) Cortana Skill Internet 3rd party web service Cortana Client Speech Text Text Intent + p Intent + p Card data Speech Text Resolve! Card Action Provider (Azure Bot) Intent to Card (Azure Bot) Expressing bad intents Local commands through Lock Screen
  32. 32. Voice of Esau Attack • Evil Maid Attack (First presented in Kaspersky SAS 2018) • Attackers: 1. Achieve Man-in-the-Middle position: Plug into the network interface 2. Use Cortana on locked screen to invoke insecure (Non-HTTPS) browsing 3. Intercept request, respond with malicious payload • Exploit browser vulnerabilities • Capture domain credentials 32
  33. 33. The VOE Attack - Evil Maid (Local) I’m in! but the computer is locked! Hi Cortana! Go to bbc.com Browse http://www.bbc.com I’m BBC and here’s my malicious payload! http://www.bbc.com 33
  34. 34. The VOE Attack Demo 34
  35. 35. VOE Attack – Lateral movement • Use initial compromise to install agent on compromised machine • Achieve Man-in-the-Middle position • Some local routing attack: e.g. ARP spoofing • Invoke Cortana insecure browsing • Play sound file – “GOTO BBC DOT COM” • RDP (Remote Desktop Protocol) sound file to target • NLA must be disabled for it to work • Intercept traffic of targeted machines and compromise, as in before. 35
  36. 36. RDP: A Silent Killer 36
  37. 37. Cortana over RDP Demo 37
  38. 38. VOE Disclosure Timeline 16 JUN 17: We report VOE to MS 29 JUN 17: MS Cloud patch (no CVE) 8 MAR 18: Our talk @ Kaspersky SAS 25 JUN 18: We report CVE-2018- 8271 (and more) to MS 38
  39. 39. The Voice of Esau • Impact: Evil Maid or even remote attacker can invoke unsafe browsing on a locked machine. Using additional vulns attacker can gain full control • The fix is • Tactical: making Cortana cloud aware of UI state and safely Bing instead of direct browse in certain scenarios • Not Strategical: Cortana may still allow unsafe browsing in some other scenarios • There are more where it came from: CVE-2018-8271 (and more) • Design lessons: Adding more capabilities to Lock Screen is tempting but dangerous 39
  40. 40. Skill of Death 40
  41. 41. Skill of Death • VOE attack took advantage of existing intent resolution mechanisms • What about adding our own interpretation mechanism? • Skills interact with client through cards • Cards have “limited functionality” 41
  42. 42. Navigate to an attacker controlled server Open malicious MS Office document Skill of Death – Limited Functionality 42
  43. 43. Skill of Death 43
  44. 44. Skill of Death • How can attacker invoke a “malicious” skill? • Invoking a new skill on a machine requires user consent • Cortana Skill can be invoked and granted consent from locked screen! 44
  45. 45. Skill of Death 45
  46. 46. Skill of Death • Timeline • Authorization of skills in locked screen detected March 2018 • Guy Feferman and Afik Friedberg of The Technion, Israel • Takeover methods detected June 2018 • Natanela Brod and Matan Pugach of the Technion, Israel • Fixed on June 25th 2018 • Fixed in the cloud • No formal announcement of fix • Skills can no longer be INVOKED (authorized or not) from locked screen • Adding functionality on locked screen is a slippery slope • Soon you find yourself allowing NON Microsoft code to run over locked screen 46
  47. 47. Protection 47
  48. 48. Preventing Voice Attacks: Speaker Identification • Respond only to me • “try” doesn’t sound very reassuring • “Hey Cortana” can be easily recorded • Can be subverted, see other talk 48
  49. 49. Preventing Voice Attacks: Compensating Controls Take 1 • Take 1: Put a security Microphone on each room? • Disadvantages: • Privacy • Cost • Audio directionality • Audio semantics • Not all attacks are audible • Detection only 49
  50. 50. Preventing Voice Attacks: Compensating Controls Take 2 • NewSpeak: a Network-based Intercepting proxy • TLS/SSL certificate must be installed on monitored devices • In many organization already exists for web gateway monitoring, DLP • Can monitor all Cortana requests and responses • Origin details: IP, computer name, user, UI State, etc. • Request audio and Text to Speech results • Intents and Action cards • Can block or modify all Cortana requests and responses • Much better than previous suggestion: Centrally located, does not rely on audio analogic capture, can mitigate not just detect 50
  51. 51. Network monitoring with NewSpeak 51 I’m the NewSpeak Proxy Hi Cortana! Go to cnn.com Browse http://www.cnn.com Browse http://www.foxnews.com
  52. 52. NEWspeak: DEMO 52
  53. 53. Summing up 53
  54. 54. Summary: Attacking Cortana 54 Cortana Service Speech to Text Text to Intent (Action) Cortana Skill Internet 3rd party web service Cortana Client Speech Text Text Intent + p Intent + p Card data Speech Text Resolve! Card Action Provider (Azure Bot) Intent to Card (Azure Bot) Expressing bad intents Local commands through Lock Screen Malicious skills Bad content provider
  55. 55. Takeaways: Defenders • For the time being: • Disable Cortana voice in corporate environments • Or at least on locked screen • Reconsider when compensating controls are there • “voice firewall”: If voice becomes mainstream, considering specialized solutions is a must for corporate adoption 55 https://www.pcgamer.com/how-to-disable-cortana/
  56. 56. Takeaways: Builders & Breakers • New interfaces are much more than “just an interface” • When introducing innovative concept into existing environments • Secure Coding is not enough • We need Secure System Engineering • We found 3 different CVEs and numerous issues that enables attackers to bypass the lock screen 56
  57. 57. Questions? 57

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