CVE-2024-52815

What it actually does

Improper validation of federation invites allows a malicious remote server to submit a crafted invite event that corrupts the invited user's /sync response stream.

Why it matters to the business

Disrupted user synchronization can prevent Matrix clients from receiving messages or state updates, causing operational outages for homeserver users and increasing support load until the server is updated.

How an attack unfolds

An attacker running a malicious Matrix server fabricates an invite event containing malformed data, sends it over federation to the target Synapse instance, and the receiving server stores the invite without validation, causing subsequent /sync requests for the invited user to fail or return errors.

How to fix it

Upgrade element-hq/synapse to a build containing commit fe3d88b833f76742874642c119b14b788341c905 (version 1.120.1 or later). Restart the server after the upgrade and verify federation invite processing resumes normally.

How to confirm the fix worked

Confirm the running binary or container image contains the fixed commit hash, then send a test federation invite from a controlled server and verify the invited user's /sync completes without error.

Typical attack path

1. Deploy or control a Matrix homeserver that participates in federation
2. Craft an invite event with invalid or malicious content targeting a user on the victim server
3. Transmit the invite over the federation protocol
4. Trigger or wait for the victim user to perform a /sync that processes the stored invite

How to detect it

Review Synapse federation and sync logs for repeated errors or malformed invite events originating from remote servers; correlate with user reports of /sync failures.

What your security team should do

• Immediately schedule upgrade of all Synapse instances to 1.120.1 or newer
• Monitor federation logs for unusual invite volumes during the rollout window
• Test invite and sync functionality in a staging environment before production deployment