Compression Oracle Attacks on VPN Networks
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Security researchers have done a good amount of practical attacks in the past using chosen plain-text attacks on compressed traffic to steal sensitive data. In spite of how popular CRIME and BREACH were, little was talked about how this class of attacks was relevant to VPN networks. Compression oracle attacks are not limited to just TLS protected data. In this talk, we try these attacks on browser requests and responses which usually tunnel their HTTP traffic through VPNs. We also show a case study with a well-known VPN server and their plethora of clients. We then go into practical defenses and how mitigations in HTTP/2's HPACK and other mitigation techniques are the way forward rather than claiming 'Thou shall not compress traffic at all.' One of the things that we would like to showcase is how impedance mismatches in these different layers of technologies affect security and how they don't play well together.
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Compression Oracle Attacks on VPN Networks
- 1. Compression Oracle Attacks on VPN Networks Nafeez Defcon 26
- 2. About Nafeez - @sketpic_fx Interested in AppSec and writing software Maker @ assetwatch.io, Attacker Surface Discovery as a Service
- 3. Overview Compression Side Channel and Encryption History of attacks VPNs and how they use compression Demo - Voracle Tool How to find if your "VPN" is vulnerable Way forward
- 4. Data Compression LZ77 Replace redundant patterns 102 Characters Everything looked dark and bleak, everything looked gloomy, and everything was under a blanket of mist 89 Characters Everything looked dark and bleak, (-34,18)gloomy, and (-54,11)was under a blanket of mist
- 5. Data Compression Huffman Coding Replace frequent bytes with shorter codes https://en.wikipedia.org/wiki/Huffman_coding
- 6. Data Compression DEFLATE - LZ77 + Huffman Coding ZLIB, GZIP are well known DEFLATE libraries
- 7. Compression Side Channel First known research in 2002
- 8. The Side Channel Length of encrypted payloads
- 9. Compression Oracle Chosen Plain Text Attack Brute force the secret byte by byte Force a compression between the chosen byte and the existing bytes in the secret
- 10. Compression Oracle Cookie: secret=637193 -some-data- Cookie: secret=1 Cookie: secret=637193 -some-data- (-34,15)1 Encrypted Payload Length = 43
- 11. Compression Oracle Cookie: secret=637193 -some-data- Cookie: secret=2 Cookie: secret=637193 -some-data- (-34,15)2 Encrypted Payload Length = 43
- 12. Compression Oracle Cookie: secret=637193 -some-data- Cookie: secret=3 Cookie: secret=637193 -some-data- (-34,15)3 Encrypted Payload Length = 43
- 13. Compression Oracle Cookie: secret=637193 -some-data- Cookie: secret=4 Cookie: secret=637193 -some-data- (-34,15)4 Encrypted Payload Length = 43
- 14. Compression Oracle Cookie: secret=637193 -some-data- Cookie: secret=5 Cookie: secret=637193 -some-data- (-34,15)5 Encrypted Payload Length = 43
- 15. Compression Oracle Cookie: secret=637193 -some-data- Cookie: secret=6 Cookie: secret=637193 -some-data- (-34,16) Encrypted Payload Length = 42
- 16. How can we convert this into an attack using browsers?
- 17. EkoParty 2012 Back in 2012
- 18. Ingredients Attacker on the data path can sniff packet length Browser attaches cookies as part of any cross-domain request Attacker controls HTTP request body
- 19. You get! Chosen Plain Text attack using browsers
- 20. TIME Attack 2013 Tal Be'ery, Amichai Shulman Breachattack.com Timing side channel purely via browsers Extending CRIME to HTTP Responses
- 21. BREACH Attack 2013 Angelo Prado, Neal Harris, Yoel Gluck BreachAttack.com
- 22. So far CRIME style attacks have been mostly targeted on HTTPS Researchers have possibly explored all possible side channels to efficiently leak sensitive data
- 23. Lets talk VPNs TLS VPNS IPSEC L2TP/ PPTP
- 24. TLS VPNs are pretty common these days
- 26. What do most of these SaaS VPNs have in common?
- 28. Compression
- 29. Almost all VPNs support compression by default
- 31. OpenVPN Compression Algorithms LZO LZ4 -LZ77 Family-
- 32. High level overview Authentication & Key Negotiation (Control Channel) Data Channel Encryption
- 33. High level overview Authentication & Key Negotiation (Control Channel) Data Channel Compression Data Channel Encryption
- 35. We have a compress then encrypt on all of data channel
- 36. CRIME + BREACH on VPN Networks
- 37. Existing TLS channel are safe
- 38. Things are safe, if the underlying app layer already uses HTTPS / TLS.
- 39. Things might go bad, if the VPN is helping you to encrypt already encrypted data
- 40. Lets see how this attack works on an HTTP website using an encrypted VPN
- 41. Given a HTTP Website through VPN, Can we leak Sensitive Cookie Data from a Cross-Domain Website?
- 42. Ingredients • VPN Server and Client has compression turned on by default • VPN User using a vulnerable browser • Visits attacker controlled website
- 43. Vulnerable Browser? Yes, the browser plays a huge role in how it sends plain HTTP requests.
- 44. Browser needs to send HTTP requests in single TCP Data Packet
- 45. Google Chrome splits HTTP packets into Header and Body So we can't get the compression window in the same request
- 46. Mozilla Firefox sends them all in a single TCP data packet Now we get the compression window in the same request
- 48. Attack Setup VPN User Vulnerable Browser
- 49. Attack Setup VPN User Vulnerable Browser HTTP WebApp
- 50. Attack Setup VPN User Vulnerable Browser HTTP WebApp Trusted VPN with Compression
- 51. Attack Setup VPN User Vulnerable Browser HTTP WebApp Trusted VPN with Compression
- 52. Attack Setup VPN User Vulnerable Browser HTTP WebApp Trusted VPN with Compression Attacker
- 53. Attack Setup VPN User Vulnerable Browser HTTP WebApp Trusted VPN with Compression Attackerattacker.com
- 54. Attack Setup VPN User Vulnerable Browser HTTP WebApp Trusted VPN with Compression Attackerattacker.com Passive MITM
- 55. Attack Setup VPN User Vulnerable Browser HTTP WebApp Trusted VPN with Compression Attackerattacker.com Passive MITM Injected Ads, Malicous Blogs, etc.
- 56. Attack Setup VPN User Vulnerable Browser HTTP WebApp Trusted VPN with Compression Attackerattacker.com Passive MITM Injected Ads, Malicous Blogs, etc. Can Observe VPN Data packet Lengths
- 57. Attack Setup VPN User Vulnerable Browser HTTP WebApp Trusted VPN with Compression Attackerattacker.com Passive MITM Injected Ads, Malicous Blogs, etc. Can Observe VPN Data packet Lengths Can Send Cross Domain requests to the HTTP WebApp
- 58. Attacker can now conduct CRIME Style attacks on HTTP requests and responses
- 59. Demo
- 61. How to tell if your VPN is vulnerable?
- 62. Ingredients Wireshark Terminal with Curl Connected to your VPN under test Observe VPN Payload Length
- 63. curl -s -o /dev/null -X POST http://website.com -d "--some-data-- Secret=37346282; --blah-- Secret=1 Secret=1" Length = x Curl and Observe Length
- 64. curl -s -o /dev/null -X POST http://website.com -d "--some-data-- Secret=37346282; --blah-- Secret=2 Secret=2" Length = x Curl and Observe Length
- 65. curl -s -o /dev/null -X POST http://website.com -d "--some-data-- Secret=37346282; --blah-- Secret=3 Secret=3" Length = x-1 Curl and Observe Length
- 66. curl -s -o /dev/null -X POST http://website.com -d "--some-data-- Secret=37346282; --blah-- Secret=1 Secret=1" Length = x Curl and Observe Length
- 67. Fix?
- 68. Fixing Compression is an interesting problem
- 69. Selectively disable Compression - HPACK in HTTP2
- 70. Remember when SPDY was vulnerable to CRIME? HPACK selectively disabled header compression for sensitive fields
- 72. For VPNs, Disable compression completely for any plain text transactions
- 73. Turning compression off by default is opinionated.
- 74. OpenVPN chose to warn the implementors more explicitly to turn off data Compression.
- 77. Its time, everything moves to HTTPS
- 78. Takeaway If you are using VPNs to access plain text websites over the internet, its time to move them to HTTPs. Most corporates using VPN still allow plain text HTTP websites, because they think VPN protects them.