matrix_sydent is vulnerable to server-side request forgery (SSRF). An attacker is able to send HTTP GET requests on behalf of the server to internal systems due to lack of parameter validation or IP address blacklisting.
github.com/matrix-org/sydent/commit/0f00412017f25619bc36c264b29ea96808bf310a
github.com/matrix-org/sydent/commit/3d531ed50d2fd41ac387f36d44d3fb2c62dd22d3
github.com/matrix-org/sydent/commit/8936925f561b0c352c2fa922d5097d7245aad00a
github.com/matrix-org/sydent/commit/9e573348d81df8191bbe8c266c01999c9d57cd5f
github.com/matrix-org/sydent/releases/tag/v2.3.0
github.com/matrix-org/sydent/security/advisories/GHSA-9jhm-8m8c-c3f4
pypi.org/project/matrix-sydent/