[tor-dev] Cross-user TLS traffic mixing in snowflake-server until 2023-03-13

[David Fifield <david@xxxxxxxxxxxxxxx>]

Between 2022-10-01 and 2023-03-13, there was a bug in the software
deployed at Snowflake bridges that could cause parts of a user's stream
to be overwritten by parts of other users' streams. Though the Snowflake
team believes the privacy risks of the bug are minor, we are treating it
as a security bug and therefore making this announcement. The fix is
already deployed on the server, and no action is required by users.

## The cause

The source of the bug was issue https://bugs.torproject.org/tpo/anti-censorship/pluggable-transports/snowflake/40199
and specifically commit https://gitlab.torproject.org/tpo/anti-censorship/pluggable-transports/snowflake/-/commit/839d2218837dfbd1682ff39b375f45660b3974b5,
which was intended as a performance optimization to reduce memory
allocation during the large influx of Snowflake users in late
September 2022.

The analysis surrounding the performance improvement incorrectly
concluded that certain memory buffers were not reused and therefore did
not need to be copied. But in fact, the buffers were reused, with the
effect that data meant for one user could be unpredictably overwritten
by data for another user.

## The effect

There is analysis at issue https://bugs.torproject.org/tpo/anti-censorship/pluggable-transports/snowflake/40260.
If our understanding of the bug is correct, it did not reveal plaintext
data or IP addresses to other users. The Snowflake bridge is a Tor
bridge, not an exit relay, so even a catastrophic failure could not leak
users' plaintext data or what destinations they were accessing.
A Snowflake connection consists of a layered set of protocols, and the
bug was in a layer that does not have access to IP addresses or any
other persistent identifiers.

* IP
* TCP
* HTTPS (WebSocket)
* session/retransmission ← The bug was here
* Tor TLS
* user data

At most, users and proxies would see fragments of other users' Tor TLS
streams. We believe the degree of information disclosure is less than
what an eavesdropper listening at the Snowflake bridge's HTTPS port
would see, which we already believe to be safe. The practical effect,
more than a loss of privacy, was excessive retransmissions at the
session/retransmission layer, and corrupted TLS data resulting in a
terminated connection.

## Remediation

We reverted the erroneous commit and added a test to check for the
erroneous buffer reuse. We deployed the update to both Snowflake bridges
on 2023-03-13.

https://gitlab.torproject.org/tpo/anti-censorship/pluggable-transports/snowflake/-/merge_requests/140
https://gitlab.torproject.org/tpo/anti-censorship/pluggable-transports/snowflake/-/commit/b63d2272bfd262f5166c44f8f88330ed7a732009
https://bugs.torproject.org/tpo/anti-censorship/pluggable-transports/snowflake/40262

## Required action

No action is required by users. The bug was in code that was only used
by the Snowflake server, not proxies or clients.

## Thanks

The problem was discovered and first investigated by Haz Æ 41
(https://gitlab.torproject.org/hazae41) and reported on HackerOne:
https://hackerone.com/reports/1880610.
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