[asterisk-bugs] [JIRA] (ASTERISK-28657) SIPS TLS connection fails when session ticket extension is used

[Marcel Dischinger (JIRA)]

Marcel Dischinger created ASTERISK-28657:
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             Summary: SIPS TLS connection fails when session ticket extension is used
                 Key: ASTERISK-28657
                 URL: https://issues.asterisk.org/jira/browse/ASTERISK-28657
             Project: Asterisk
          Issue Type: Bug
      Security Level: None
          Components: Channels/chan_sip/General
    Affects Versions: 13.21.0
         Environment: Tested with LTS certified, but according to the code this problem should apply to all newer versions as well.
Using Debian Stretch and the dependencies used there (e.g., OpenSSL 1.1.0k).
Using Asterisk with SIPS.
            Reporter: Marcel Dischinger


The TLS implementation of Asterisk using OpenSSL has a flaw that results in OpenSSL bailing with an internal error when a client tries to use session-reuse with the session ticket extension. This happens when Asterisk is used as a server for SIPS connections, i.e., SIPS-clients set up a TLS connection to Asterisk.

h2. How does this work?
# Client contacts Asterisk to set up a TLS connection
# Certificates are exchanged, keys are derived
# TLS session established
# Asterisk sends a TLS NewSessionTicket extension message (see RFC 5077) to the client
# TLS session is closed
# Client starts a new TLS session, sends the session ticket from 4. with the ClientHello
# Asterisk/OpenSSL should use this ticket to establish a new TLS session reusing the exchanged keys from the previous session.

h2. Bug analysis
>From the asterisk log:
{quote}ERROR tcptls.c: Problem setting up ssl connection: error:00000001:lib(0):func(0):reason(1), Internal SSL error
WARNING tcptls.c: FILE * open {quote}

Reason for this is described in the warning for ssl_ctx_set_session_id_context in OpenSSL (see, e.g.,
https://linux.die.net/man/3/ssl_ctx_set_session_id_context): session id context is not set.

h2. Bugfix
To fix this, this context has to be set in  tcptls.c:__ssl_setup, e.g., directly before setting up the TLS options, right after setting up the general SSL-contect. There, add the following line:
SSL_CTX_set_session_id_context(cfg->ssl_ctx, (const unsigned char *) "asterisk", 8);

As an alternative, you can also disable the usage of RFC5077 by setting the SSL option SSL_OP_NO_TICKET. With this, Asterisk will no longer send the NewSessionTicket extension message, hence for the second TLS setup a SessionTicket is not included in the ClientHello.




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