[matrix-synapse] Improper Verification of Cryptographic Signature in matrix-synapse

Matrix Synapse before 1.5.0 mishandles signature checking on some federation APIs. Events sent over /send_join, /send_leave, and /invite may not be correctly signed, or may not come from the expected servers.

References

• https://nvd.nist.gov/vuln/detail/CVE-2019-18835
• https://github.com/matrix-org/synapse/pull/6262
• https://github.com/matrix-org/synapse/releases/tag/v1.5.0
• https://github.com/advisories/GHSA-cppw-2mf8-qpm5