Incorrect parsing of access level in gomatrixserverlib and dendrite

Vulnerability Description

gomatrixserverlib is a Go library for matrix protocol federation. Dendrite is a Matrix homeserver written in Go, an alternative to Synapse. The power level parsing within gomatrixserverlib was failing to parse the `"events_default"` key of the `m.room.power_levels` event, defaulting the event default power level to zero in all cases. Power levels are the matrix terminology for user access level. In rooms where the `"events_default"` power level had been changed, this could result in events either being incorrectly authorised or rejected by Dendrite servers. gomatrixserverlib contains a fix as of commit `723fd49` and Dendrite 0.9.3 has been updated accordingly. Matrix rooms where the `"events_default"` power level has not been changed from the default of zero are not vulnerable. Users are advised to upgrade. There are no known workarounds for this issue.

References

• https://github.com/matrix-org/gomatrixserverlib/security/advisories/GHSA-grvv-h2f9-7v9c
• https://github.com/matrix-org/gomatrixserverlib/commit/723fd495dde835d078b9f2074b6b62c06dea4575
• https://matrix.org/docs/guides/moderation/#power-levels