Hello Tor Community!

As you know, different teams inside the Tor Project are working on the Arti Relay project where we hope to be able to begin the upgrade of the network towards our Rust implementation of Tor in the near future. To support this work, we would like to announce that we intend to actively stop compatibility for 0.4.8 and earlier C Tor versions soon. This means that these versions will no longer work on the network at all after our target date, which is currently September 1st, 2026.

If you’re a Tor Browser user running an up-to-date version of Tor Browser, this won't impact you. If you're running an older, perhaps not-so-well-maintained, Onion Service somewhere, or you’re building an app that integrates C Tor, you may want to read along here.

Tor 0.4.8 reached End of Life on the 1st of June, and there will not be any more updates to this release series. We highly encourage people to upgrade to the Tor 0.4.9 series (or later).

Usually, we try not to break existing releases, even if they are unsupported, unless we have a pretty good reason. In this case, we have several reasons. With the work towards Arti on both the client and relay, the Network Team has identified a couple of features we would like to remove from the Tor ecosystem. Removing support for 0.4.8 will help us facilitate a smooth transition, and reduce effort associated with difficult to maintain features that provide very little value. Unfortunately, because Tor’s Directory Protocol layer works the way it does, we cannot remove these features without affecting older clients.

The most important reason is this: in 0.4.9, we have made some former fields in our directory data obsolete -- specifically, TAP onion keys and family lines. Removing these fields will let us save a great deal of client directory bandwidth for everyone. This, in turn, will make all Tor clients bootstrap a little faster, especially those on slow connections. But when we remove these fields, clients and relays running earlier versions of Tor will no longer work, since they expect the TAP onion keys to be present. Therefore, in order to deliver improved performance faster, we need to accelerate the date on which 0.4.8 will stop working.

The secondary reason for sunsetting 0.4.8: Our Arti directory authority implementation needs network integration soon, and it will be easier to write if it doesn’t support deprecated fields.

With this blog post out, we will begin reaching out to the downstreams of Tor we identified as shipping older versions and try to get them to upgrade. We appreciate community help here, too. If you identify that your favorite project that bundles Tor uses an outdated version of Tor, please reach out to them and (politely!) encourage them to upgrade. We are tracking some of this outreach in network-health/team#460. If you have a very good reason for needing a longer time with 0.4.8 support than 1 September 2026, please let us know by leaving a comment on that ticket.

Thank you!