parse-server's endpoint `/loginAs` allows `readOnlyMasterKey` to gain full read and write access as any user

Impact

The readOnlyMasterKey can call POST /loginAs to obtain a valid session token for any user. This allows a read-only credential to impersonate arbitrary users with full read and write access to their data. Any Parse Server deployment that uses readOnlyMasterKey is affected.

Patches

The fix adds a check to the /logInAs handler.

Workarounds

There is no workaround other than not using readOnlyMasterKey.

References

• GitHub security advisory: https://github.com/parse-community/parse-server/security/advisories/GHSA-79wj-8rqv-jvp5
• Fix for Parse Server 9: https://github.com/parse-community/parse-server/releases/tag/9.5.0-alpha.4
• Fix for Parse Server 8: https://github.com/parse-community/parse-server/releases/tag/8.6.6