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The vulnerability stems from a critical oversight in the message edit validation logic of the `matrix-sdk-ui` crate in versions prior to 0.16.1. During the processing of an event that replaces a previous message, the affected versions did not verify that the replacement event itself is encrypted. When the original event being replaced is encrypted, this validation step is mandatory to ensure data integrity and authenticity.
A malicious actor with the ability to control or manipulate a Matrix homeserver — typically a homeserver administrator or an attacker who has compromised administrative privileges — can exploit this missing encryption check. By sending an unencrypted replacement event, the attacker can impersonate any user within a room that utilizes end-to-end encryption. The receiving Matrix client, using a vulnerable version of the SDK, would incorrectly accept this unencrypted edit and display the spoofed message as if it originated from the legitimate target user, rather than rejecting it as an invalid edit.
The attack vector is network-based, with low complexity, but it requires high privileges (homeserver admin) and no user interaction. The primary security impact is the high loss of message authenticity and integrity, as the system no longer provides reliable guarantees about who sent a message, making it trivial for a rogue server admin to plant false statements or alter the chat history in a way that a recipient cannot detect. This flaw was patched in version 0.16.1, which now correctly enforces that any replacement for an encrypted event must itself be encrypted, aligning with the Matrix specification.
DailyCVE Form:
Platform: Rust crate
Version: <0.16.1
Vulnerability: Missing encryption check
Severity: Medium (4.9)
date: 2026-06-04
Prediction: 2026-06-18
What Undercode Say:
Check affected version in Cargo.toml grep "matrix-sdk-ui" Cargo.toml Update to patched version cargo update -p matrix-sdk-ui --precise 0.16.1 Verify lock file version cargo tree | grep matrix-sdk-ui
How Exploit:
A malicious homeserver admin can craft an `m.room.message` event of type `m.replace` relation that points to a victim’s original encrypted event. The replacement event is sent as an unencrypted `m.room.message` containing the spoofed content. The vulnerable client, upon receiving this replacement event, bypasses the encryption check and applies the edit, displaying the malicious content as if it were an authenticated edit by the victim.
Protection:
Update the `matrix-sdk-ui` dependency to version 0.16.1 or later immediately. No workarounds are available. All Matrix clients using the affected Rust SDK must rebuild against the patched version to enforce the correct validation rule: replacement events for encrypted messages must also be encrypted.
Impact:
Complete erosion of message authenticity in affected rooms. A malicious homeserver administrator can impersonate any user, forge arbitrary messages, and alter chat history without leaving detectable traces in the end-to-end encryption context. The integrity of conversations is fully compromised under the control of a hostile server.
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Sources:
Reported By: github.com
Extra Source Hub:
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