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I wonder if it does. It seems to me that there is no way for participants to authenticate each other securely. There are, though, signaling servers, but I don't think that they are capable of key distribution.

Is there something I've missed?

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WebRTC uses DTLS-SRTP and according to RFC 5764 DTLS-SRTP supports certificates for both client and server. However in the context of WebRTC the direct connection established between the users is established by dynamically downloaded software running in the browser. In other words the user does not control the software so even if the software did provide him with ways to secure the connection it could be replaced by other software that does not.

This is the same problem as faced by all kind of web based mail encryption systems.

It's nicely treated in this RFC draft:

In general, it is important to understand that unlike a conventional SIP proxy, the calling service (i.e., the Web server) controls not only the channel between the communicating endpoints but also the application running on the user's browser. While in principle it is possible for the browser to cut the calling service out of the loop and directly present trusted information (and perhaps get consent), practice in modern browsers is to avoid this whenever possible.

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As Elias has pointed, we can found a useful information in this draft. Specifically, in section 4.3.2 it is said:

4.3.2. Protecting Against During-Call Attack

Protecting against attacks during a call is a more difficult proposition. Even if the calling service cannot directly access keying material (as recommended in the previous section), it can simply mount a man-in-the-middle attack on the connection, telling Alice that she is calling Bob and Bob that he is calling Alice, while in fact the calling service is acting as a calling bridge and capturing all the traffic. Protecting against this form of attack requires positive authentication of the remote endpoint such as explicit out-of-band key verification (e.g., by a fingerprint) or a third-party identity service as described in [I-D.ietf-rtcweb-security-arch].

Since WebRTC itself doesn't provide a positive authentication it can't protect you from active MITM.

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