Bug 2209958 (CVE-2023-32323)

Summary: CVE-2023-32323 matrix-synapse: Synapse Outgoing federation to specific hosts can be disabled by sending malicious invites
Product: [Other] Security Response Reporter: Avinash Hanwate <ahanwate>
Component: vulnerabilityAssignee: Red Hat Product Security <security-response-team>
Status: CLOSED UPSTREAM QA Contact:
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Priority: medium    
Version: unspecifiedKeywords: Security
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Hardware: All   
OS: Linux   
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Fixed In Version: matrix-synapse 1.74.0 Doc Type: If docs needed, set a value
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Last Closed: 2023-05-25 15:41:02 UTC Type: ---
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Bug Depends On: 2209959    
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Description Avinash Hanwate 2023-05-25 10:32:19 UTC 
A malicious user on a Synapse homeserver X with permission to create certain state events can disable outbound federation from X to an arbitrary homeserver Y.

Synapse instances with federation disabled are not affected.

The Matrix protocol allows homeservers to provide an invite_room_state field on a room invite containing a summary of room state. In versions of Synapse up to and including v1.73.0, Synapse did not limit the size of invite_room_state, meaning that it was possible to create an arbitrarily large invite event.

An attacker with an account on a vulnerable Synapse homeserver X could exploit this by having X create an over-sized invite event in a room with a user from another homeserver Y. Once acknowledged by the invitee's homeserver, the invite event would be sent in a batch of events to Y. If the malicious invite is so large that the entire batch is rejected as too large, X's outgoing traffic to Y would become "stuck", meaning that messages and state events created by X would remain unseen by Y.
Comment 1 Avinash Hanwate 2023-05-25 10:32:36 UTC 
Created matrix-synapse tracking bugs for this issue:

Affects: fedora-all [bug 2209959]
Comment 2 Product Security DevOps Team 2023-05-25 15:41:01 UTC 
This CVE Bugzilla entry is for community support informational purposes only as it does not affect a package in a commercially supported Red Hat product. Refer to the dependent bugs for status of those individual community products.