This document discusses possible man-in-the-middle attacks when a back-to-back user agent (B2BUA) is involved in a SIP call. It outlines three modes of B2BUAs - media relay, media aware, and media terminator - and the associated risks. It proposes mitigations like mandatory authenticated identity management to prevent attacks when the B2BUA modifies signaling or media. The goal is to maintain end-to-end security between the original endpoints while allowing flexibility for B2BUAs.

1. DTLS-SRTP Handling in SIP B2BUAs
draft-ram-straw-b2bua-dtls-srtp
IETF-91
Hawaii, Nov 12, 2014
Presenter: Tirumaleswar Reddy
Authors: Ram Mohan, Tirumaleswar Reddy,
Gonzalo Salgueiro, Victor Pascual
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2. Agenda
B2BUA modes and possible MITM attacks
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3. B2BUA Modes
1.Media Relay
2.Media Aware
3.Media Terminator
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4. Legitimate Media Relay
Media
Forwards packets without inspection or modification
Only modifies the L3 and L4 headers
Signaling
It MUST forward the received certificate fingerprint without any modifications
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5. Malicious Media Relay
Media
Forwards packets with inspection or modification
Signaling
Modifies the certificate fingerprint and signals its own fingerprint
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6. Possible Mitigation
Mandate authenticated identity management in SIP (draft-ietf-stir- rfc4474bis)
signed-identity-digest carries the signed hash of certificate fingerprint
Mandate Identity headers to be present
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7. 2. The outbound proxy for the Alice’s domain verifies that this is from Alice and adds an assertion(based of 4474bis) that is it from alice@atlanta.com
3. This assertion is signed with the atlanta.com certificate from a well known certificate authority
4. The B2BUA here just changes UDP/IP header and does not modify payload
INVITE
Challenge
INVITE
INVITE
INVITE
Alice
atlanta.com
atlanta.com
bob@biloxy.com
1.Alice calls Bob
Authenticated identity management
Alice
Alice’s
Proxy
SIP Cloud
bob
Alice’s B2BUA
(Back-to- Back User Agent)

8. B2BUA Modes
1.Media Relay
2.Media Aware
3.Media Terminator
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9. Legitimate Media Aware
Media
Modifies the RTP header
Signaling
Terminates the DTLS connection and acts as a DTLS proxy
-Changes the certificate fingerprint and signals its own fingerprint
-Decrypts and re-encrypts the payload
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10. Malicious Media Aware
Media
Inspects or modifies the payload.
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11. 2. The outbound proxy for the Alice’s domain verifies that this is from Alice and adds an assertion(based of 4474bis) that is it from alice@atlanta.com
3. This assertion is signed with the atlanta.com certificate from a well known certificate authority
4. The B2BUA changes the RTP header
INVITE
Challenge
INVITE
INVITE
INVITE
Alice
atlanta.com
atlanta.com
bob@biloxy.com
1.Alice calls Bob
B2BUA in the same administrative domain
Alice
Alice’s
Proxy
SIP Cloud
bob
Alice’s B2BUA
(Back-to- Back User Agent)

12. Possible mitigations
Option 1> SRTP for cloud services (draft-cheng-srtp-cloud-00) proposes a mechanism where confidentiality and message authentication is independent of the RTP header
Option 2> Trust the B2BUA
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13. 2. The outbound proxy for the Alice’s domain verifies that this is from Alice and adds an assertion(based of 4474bis) that is it from alice@atlanta.com
3. This assertion is signed with the atlanta.com certificate from a well known certificate authority
4. The B2BUA changes the RTP header
INVITE
Challenge
INVITE
INVITE
INVITE
Alice
atlanta.com
ISP
bob@biloxy.com
1.Alice calls Bob
B2BUA in different administrative domain
Alice
Alice’s
Proxy
SIP Cloud
bob
B2BUA
(Back-to- Back User Agent)

14. Possible mitigation
SRTP for cloud services (draft- cheng-srtp-cloud-00) proposes a mechanism where confidentiality and message authentication is independent of the RTP header
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15. B2BUA Modes
1.Media Relay
2.Media Aware
3.Media Terminator
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16. Media Terminator
Media terminator modifies the payload
Terminates the DTLS connection, acts as a DTLS proxy
-Changes the certificate fingerprint and signals its own fingerprint
-Decrypts and re-encrypts the payload
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17. Possible attacks
Breaks end-to-end security.
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18. 2. The outbound proxy for the Alice’s domain verifies that this is from Alice and adds an assertion(based of 4474bis) that is it from alice@atlanta.com
3. This assertion is signed with the atlanta.com certificate from a well known certificate authority
4. The B2BUA modifies the payload
INVITE
Challenge
INVITE
INVITE
INVITE
Alice
atlanta.com
ISP
bob@biloxy.com
1.Alice calls Bob
B2BUA in same administrative domain
Alice
Alice’s
Proxy
SIP Cloud
bob
B2BUA
(Back-to- Back User Agent)

19. Possible mitigations
Clients can be configured to maintain the B2BUA server's certificate fingerprints. This way the client is aware that B2BUA is playing the role of a media- proxy.
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20. 2. The outbound proxy for the Alice’s domain verifies that this is from Alice and adds an assertion(based of 4474bis) that is it from alice@atlanta.com
3. This assertion is signed with the atlanta.com certificate from a well known certificate authority
4. The B2BUA modifies the payload
INVITE
Challenge
INVITE
INVITE
INVITE
Alice
atlanta.com
ISP
bob@biloxy.com
1.Alice calls Bob
B2BUA in different administrative domain
Alice
Alice’s
Proxy
SIP Cloud
bob
B2BUA
(Back-to- Back User Agent)

21. Possible mitigations
Discourage media terminator mode.
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22. Next Steps
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DTLS-SRTP Handling in SIP B2BUAs

23. Backup
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24. B2BUA Modes
Media Relay
- Only changes UDP/IP header- e.g.: topology hiding, privacy
Media Aware
- relay which can change RTP/RTCP headers- e.g.: monitors RTCP for QoS, mux/demuxes RTP/RTCP on same 5-tuple
Media Terminator
- Transcoders, Conference Servers
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