Disclosing CVE-2021-40823 and CVE-2021-40824: E2EE vulnerability in multiple Matrix clients Matrix.org
https://matrix.org/blog/2021/09/13/vulnerability-disclosure-key-sharing

>Today we are disclosing a critical security issue affecting multiple Matrix clients and libraries including Element (Web/Desktop/Android), FluffyChat, Nheko, Cinny, and SchildiChat. Element on iOS is not affected.
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>Specifically, in certain circumstances it may be possible to trick vulnerable clients into disclosing encryption keys for messages previously sent by that client to user accounts later compromised by an attacker.
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>Exploiting this vulnerability to read encrypted messages requires gaining control over the recipient’s account. This requires either compromising their credentials directly or compromising their homeserver.
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>Thus, the greatest risk is to users who are in encrypted rooms containing malicious servers. Admins of malicious servers could attempt to impersonate their users' devices in order to spy on messages sent by vulnerable clients in that room.
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>This is not a vulnerability in the Matrix or Olm/Megolm protocols, nor the libolm implementation. It is an implementation bug in certain Matrix clients and SDKs which support end-to-end encryption (“E2EE”).
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>We have no evidence of the vulnerability being exploited in the wild.
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>This issue was discovered during an internal audit by Denis Kasak, a security researcher at Element.
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