matrix-synapse vulnerable to temporary storage of plaintext passwords during password changes

Impact

When users update their passwords, the new credentials may be briefly held in the server database. While this doesn't grant the server any added capabilities—it already learns the users' passwords as part of the authentication process—it does disrupt the expectation that passwords won't be stored in the database. As a result, these passwords could inadvertently be captured in database backups for a longer duration.

These temporarily stored passwords are automatically erased after a 48-hour window.

Patches

https://github.com/matrix-org/synapse/pull/16272

References

This bug was due to a regression in https://github.com/matrix-org/synapse/pull/13188.