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Clarify that arbitrary unicode is allowed in user/room IDs and room aliases #1506

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Clarify that arbitrary unicode is allowed in user/room IDs and room aliases #1506

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6215072
Clarify that arbitrary unicode is allowed in user/room IDs and room a…
tulir May 1, 2023
ec55c38
Merge branch 'main' into clarify-identifier-localparts
turt2live Aug 9, 2023
459634f
Clarify historical ID set further
turt2live Aug 9, 2023
97381fd
Apply suggestions from code review
tulir Feb 27, 2024
9b75333
Clarify allowed control characters
tulir Feb 27, 2024
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1 change: 1 addition & 0 deletions changelogs/appendices/newsfragments/1506.clarification
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Clarify that arbitrary unicode is allowed in user/room IDs and room aliases.
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Strongly disagree on this.

The original vintage 2014 Synapse implementation only allowed non URL quotable characters in user localparts when registering for an account. Unfortunately, federation did not apply the same check, alluded to by other comments here:

Arbitrary unicode ids exist in the wild too, but it has never been possible to create them without modifying the server.

If you are playing silly games (removing validation checks on your server) we should NOT set the precedent that we are just going to accept your games imo, otherwise where do you draw the line? Some comments in this proposal mention about excluding the null byte, but why? Sure, postgres cannot represent it when used with TEXT columns, but hey I am using a modified Synapse which doesn't have this problem, so why are we allowing some silly games but not others?

The result is an inconsistent mess, and it was never designed to be that way. This isn't like other cases where "hey this is what synapse does, in the spec it goes" because in order to get the failure mode of unicode characters you need a malicious and/or buggy actor.

The consequences of making this the rule in the specification, and hence removing these checks in Dendrite, Conduit, et al is an increased risk of homograph attacks. I cannot and will not support increasing the attack surface of Matrix just because a few people back in 2014 removed validation and sent unicode user IDs into a room, which synapse accepted.

It is worth emphasising that room versions ARE NOT a get-out-of-jail-free card here, as user IDs are outside the scope of rooms. For example, the sliding sync proxy recently had an issue with unicode user IDs in device list changes. It's not hard to see how this can also be an issue with the user directory and to-device msgs, both of which sit outside of rooms.

Counter proposal: sorry folks with smiley poos as user localparts, you're going to be broken in the next release of Synapse, and we remove / subsequently ignore events with malformed user IDs aka what we should have done in 2014.

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While I agree that this only really happens because of people modifying their servers, the result of enforcing those checks in one implementation and not in another can end up being outright disastrous for innocent users.

For example, if Dendrite enforces these checks but Synapse doesn't, then all it takes is for a Synapse user with an emoji localpart to make a power level change or perform any kind of power action to break the room for Dendrite users probably irreversibly.

Hell, they might not even have to perform a power action, because we might drop auth events (such as the user joining to begin with) or refuse to pull in prev events (from normal timeline events), which in turn causes us to run state resolution with a different set of input events (which can result in a different output state set or a complete state reset) or to accidentally propagate broken state to other servers when they ask us for /state_ids or /state.

This situation 100% sucks but Matrix only functions if implementations agree to handle these things in the same way, otherwise different implementations will never arrive at a consistent state. We already see this happen due to other corner cases and it just ends up feeling terrible for all users involved.

As for room versions, it is true that it's not a great solution to the problem due to the fact that it still leaks into device updates or other areas, but at least it's possible in a future room version to make sure that users with invalid localparts can't join the rooms to begin with. That is a huge step towards stamping out invalid localparts.

25 changes: 21 additions & 4 deletions content/appendices.md
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Expand Up @@ -611,11 +611,20 @@ characters permitted in user ID localparts. There are currently active
users whose user IDs do not conform to the permitted character set, and
a number of rooms whose history includes events with a `sender` which
does not conform. In order to handle these rooms successfully, clients
and servers MUST accept user IDs with localparts from the expanded
character set:
and servers MUST accept user IDs with localparts consisting of any legal
unicode codepoint except for `:` and `NUL` (U+0000), including other control
characters and the empty string. Localparts MUST be valid UTF-8 sequences.

Servers SHOULD NOT produce user IDs with localparts outside of the following
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character set, and SHOULD NOT forward such user IDs to clients when referenced
outside the context of an event. For example, device list updates from "invalid"
user IDs would be dropped by the receiving server.
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This is worse than just removing events by these users, as things will only half work (E2EE will entirely break for them in weird and wonderful ways).

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If you set your localpart to a smiley poo, E2EE will break for you. That's fine by me. What I care about is you not being able to DoS a room by setting your localpart to a smiley poo, which is why we have to accept events over federation whose senders are invalid; we don't have to do much else to help such users.

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How about this:

Suggested change
Servers SHOULD NOT produce user IDs with localparts outside of the following
character set, and SHOULD NOT forward such user IDs to clients when referenced
outside the context of an event. For example, device list updates from "invalid"
user IDs would be dropped by the receiving server.
User IDs with localparts containing characters outside the range U+0021 to U+007E, or with
an empty localpart, are considered non-compliant. For current room versions, servers must
still accept events using such user IDs over federation; however they SHOULD NOT forward
such user IDs to clients when referenced outside the context of an event. For example,
device list updates from non-compliant user IDs would be dropped by the receiving server.


extended_user_id_char = %x21-39 / %x3B-7E ; all ASCII printing chars except :

A future room version may prevent users using a historical character set
from participating. Use of the historical character set is *deprecated*.
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##### Mapping from other character sets

In certain circumstances it will be desirable to map from a wider
Expand Down Expand Up @@ -663,6 +672,11 @@ Room IDs are case-sensitive. They are not meant to be
human-readable. They are intended to be treated as fully opaque strings
by clients.

The localpart of a room ID (`opaque_id` above) may contain any valid
unicode codepoints, including control characters, except `:` and `NUL`
(U+0000), but it is recommended to only include ASCII letters and
digits (`A-Z`, `a-z`, `0-9`) when generating them.

#### Room Aliases

A room may have zero or more aliases. A room alias has the format:
Expand All @@ -673,8 +687,11 @@ The `domain` of a room alias is the [server name](#server-name) of the
homeserver which created the alias. Other servers may contact this
homeserver to look up the alias.

Room aliases MUST NOT exceed 255 bytes (including the `#` sigil and the
domain).
The localpart of a room alias may contain any valid unicode codepoints
except `:`.

Room aliases MUST NOT exceed 255 bytes as UTF-8 (including the `#` sigil
and the domain).
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similar questions: do they really have to be valid UTF-8? It probably depends on where they are used, I suppose...

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I think we should assume yes until proven otherwise, when we can update the spec again.


#### Event IDs

Expand Down
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