Monal before 4.9 does not implement proper sender verification on MAM and Message Carbon (XEP-0280) results. This allows a remote attacker (able to send stanzas to a victim) to inject arbitrary messages into the local history, with full control over the sender and receiver displayed to the victim.
Source: https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2020-26547
Reference (s):
- https://monal.im/blog/cve-2020-26547/
- https://github.com/anurodhp/Monal/commits/develop