matrix-doc/proposals/2540-stricter-event-validation.md

MSC2540: Stricter event validation: JSON compliance

Background

There has been prior discussions about validating events more strictly. This MSC proposes fixing a small piece of this: JSON compliance.

The Canonical JSON specification requires that numbers that are serialized in JSON are integers in the inclusive range of [-(2^53) + 1, (2^53) - 1], which matches the requirements of section 6 of RFC 7159. Note that it is not explicit, but all floats are invalid.

It is worth mentioning that there are common extensions to JSON which produce invalid JSON according to the Matrix specification; some programming languages even support these by default. One common additional feature is handling "special" float values: Infinity, -Infinity, and NaN.

Proposal

In a future room version, homeserver implementations are to strictly enforce the JSON compliance of the Canonical JSON specification for events. Non-compliant events should be treated like any other malformed event, for example by rejecting the request with an HTTP 400 error with M_BAD_JSON, or by discarding the event.

The rationale for doing this in a future room version is to avoid a split brain room -- where some federated servers believe an event is valid and others reject it as invalid. Rooms will be able to opt into this behavior as part of a room version upgrade.

Homeserver implementations are not to strictly enforce this JSON compliance in room versions 1, 2, 3, 4, and 5. The rationale is essentially the same as why a future room version is necessary: this ensures that all federated servers treat the same events as valid.

Potential issues

Homeserver implementations might include JSON parsers which are stricter than others. It may not be worthwhile or reasonable to loosen those restrictions for stable room versions.

Alternatives

It could be argued that this MSC is unnecessary since it does not add any new requirements for handling of JSON data. Unfortunately starting to enforce these requirements in current rooms could cause federation to break as homeservers will disagree on whether events are valid.

Security considerations

N/A

Unstable prefix

A room version of org.matrix.strict_canonicaljson until a future room version is available. This room version will use room version 5 as base and include the above modifications.