1101528 - Firefox uses the same TLS session ticket and/or ID between normal and private browsing

Status: RESOLVED WORKSFORME

(Core :: Security: PSM, defect)

Description

[Frederik Braun [:freddy]]

In private browsing mode, Firefox keeps using TLS session tickets established previously in non-private browsing mode.

Comment 1

[Frederik Braun [:freddy]]

The Tor browser bundle deals with this by disabling caching session IDs altogether, setting noCache in the sslOptions to TRUE: https://lists.torproject.org/pipermail/tor-commits/2013-April/054489.html

Comment 2

[Tim Taubert [:ttaubert] (inactive)]

https://gitweb.torproject.org/torbrowser.git/blob/master:/src/current-patches/firefox/0007-Disable-SSL-Session-ID-tracking.patch

Comment 3

[Daniel Veditz [:dveditz]]

NSS has no concept of Private Browsing and it's going to be hard to teach it.

Comment 4

[Tim Taubert [:ttaubert] (inactive)]

Just found out that bug 967977 introduced a pref to disable the use of session identifiers.

Comment 7

[Brian Smith (:briansmith, :bsmith, use NEEDINFO?)]

(In reply to Daniel Veditz [:dveditz] from comment #3)
> NSS has no concept of Private Browsing and it's going to be hard to teach it.

NSS does have a way of partitioning the session cache and as far as I remember Firefox uses that. So, this may be just a straight-up bug in NSS (looking in the default session cache for session tickets) or in Firefox (not using the cache partitioning mechanism correctly.)

How did you test/verify this?

Comment 8

[Frederik Braun [:freddy]]

I did not test this and took the information provided on the tor mailing list for granted.

It would make sense to come up with an automated test, maybe Mike Perry (who authored the reference from comment 1) has one?

(Link in comment 3 has gone stale. See https://gitweb.torproject.org/torbrowser.git/tree/src/current-patches/firefox/0007-Disable-SSL-Session-ID-tracking.patch instead)

Comment 9

[Mike Perry]

In Tor browser, we completely disable TLS session tickets and SSL session identifiers via security.enable_tls_session_tickets and security.ssl.disable_session_identifiers (see also Bug 967977). 

We did this primarily to avoid third party tracking/linkability via TLS, as we were not aware of the NSS partitioning features Brian mentioned in comment 7. We may revisit this if these features are easy to make use of.

I don't recall if we explicitly tested private browsing versus non-private browsing cross-over of TLS tickets or SSL session ids. We definitely don't have a test for this case, anyway.

Comment 10

[Patrick McManus [:mcmanus]]

This is the PSM code that I believe partitions the session cache with the PB flag

https://mxr.mozilla.org/mozilla-central/source/security/manager/ssl/src/nsNSSIOLayer.cpp#2587

so someone should actually verify that this happens in firefox

Comment 11

[Dana Keeler (she/her) [:keeler]]

Firefox/NSS is doing the right thing here.