Synapse Outgoing federation to specific hosts can be disabled by sending malicious invitesGHSA-f3wc-3vxv-xmvr
MEDIUMFix: matrix-org/synapse#14642GHSA-f3wc-3vxv-xmvr is a medium-severity (CVSS 5) Improper Input Validation vulnerability in matrix-synapse. 1 public exploit or proof-of-concept reference is indexed, so weaponization risk is real. A fix is available for matrix-synapse — see the affected versions and patch details below.
Exploitation Status
Proof-of-concept exploit code exists
- CISA’s SSVC triage found public proof-of-concept exploit code for this CVE, though no confirmed active exploitation.
Exploitation and automatability from CISA’s SSVC triage for GHSA-f3wc-3vxv-xmvr.
EPSS Exploitation Probability
Probability of exploitation in the next 30 days, from FIRST.org EPSS.
How urgent is this, really
Where this sits among everything scored
Real-World Exposure
matrix-synapseReal-time download stats are indexed for npm and PyPI packages. This vulnerability affects PyPI packages — download data is not available via public APIs for these ecosystems.
Description
Impact
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.
Details
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.
Patches
Synapse 1.74 refuses to create oversized invite_room_state fields. Server operators should upgrade to Synapse 1.74 or newer urgently.
Workarounds
There are no robust workarounds.
This attack needs an account on Synapse homeserver X to deny federation from X to another homeserver Y. As a partial mitigation, Synapse operators can disable open registration to limit the ability of attackers to create new accounts on homeserver X.
If homeserver X has been attacked in this way, restarting it will resume outgoing federation by entering "catchup mode". For catchup mode to ignore the oversized invites, every attacked room must have a correctly-sized event sent by X which is newer than any oversized invite. This is difficult to arrange, and does not prevent the attacker from repeating their attack.
References
- https://github.com/matrix-org/synapse/issues/14492 was caused by this issue.
- https://github.com/matrix-org/synapse/issues/14642 includes the patch described above.
For more information
If you have any questions or comments about this advisory, e-mail us at [email protected].
Affected Packages
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| 🐍PyPI | matrix-synapse | all versions | 1.74.0pip install --upgrade 'matrix-synapse==1.74.0' |
Affected Products
synapsematrixResearch use only. For defensive security, authorized penetration testing, and academic research only. Never execute exploit code against systems without explicit written authorization.
Detection & mitigation playbook
Open-source dependencyDetect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for matrix-synapse, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
Fix
Update matrix-synapse to 1.74.0 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms GHSA-f3wc-3vxv-xmvr is resolved across your whole dependency graph.
Workarounds
If you can't upgrade right away: gate or disable the affected feature, validate untrusted input at the boundary, and avoid passing attacker-controlled data into the vulnerable path. O3's runtime protection blocks exploitation in production as an interim safeguard until the upgrade lands.
Frequently Asked Questions
Is GHSA-f3wc-3vxv-xmvr in your dependencies?
Find it across PyPI, including transitive dependencies.