CVE-2022-39248 in matrix-android-sdk2
Summary
by MITRE • 09/29/2022
matrix-android-sdk2 is the Matrix SDK for Android. Prior to version 1.5.1, an attacker cooperating with a malicious homeserver can construct messages that legitimately appear to have come from another person, without any indication such as a grey shield. Additionally, a sophisticated attacker cooperating with a malicious homeserver could employ this vulnerability to perform a targeted attack in order to send fake to-device messages appearing to originate from another user. This can allow, for example, to inject the key backup secret during a self-verification, to make a targeted device start using a malicious key backup spoofed by the homeserver. matrix-android-sdk2 would then additionally sign such a key backup with its device key, spilling trust over to other devices trusting the matrix-android-sdk2 device. These attacks are possible due to a protocol confusion vulnerability that accepts to-device messages encrypted with Megolm instead of Olm. matrix-android-sdk2 version 1.5.1 has been modified to only accept Olm-encrypted to-device messages and to stop signing backups on a successful decryption. Out of caution, several other checks have been audited or added. This attack requires coordination between a malicious home server and an attacker, so those who trust their home servers do not need a workaround.
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Analysis
by VulDB Data Team • 10/25/2022
The vulnerability described in CVE-2022-39248 represents a critical protocol confusion flaw within the matrix-android-sdk2 library that affects end-to-end encryption security in Matrix-based messaging applications. This vulnerability exists in versions prior to 1.5.1 and stems from the SDK's improper handling of to-device messages that should be encrypted using the Olm encryption protocol. The flaw allows malicious homeservers to exploit a design weakness where the system accepts messages encrypted with Megolm encryption instead of the proper Olm encryption for to-device communications. This protocol confusion creates a significant security risk as it enables attackers to forge messages that appear authentic to end users, undermining the fundamental trust model of the Matrix protocol's encryption system.
The technical implementation of this vulnerability enables sophisticated attackers to conduct targeted deception campaigns by crafting malicious to-device messages that appear to originate from legitimate users. The attack vector specifically targets the key backup mechanism within the Matrix protocol, allowing malicious actors to inject fake key backup secrets during self-verification processes. When a victim's device receives such spoofed messages, the matrix-android-sdk2 library would incorrectly process these messages and proceed to sign the malicious key backup using the device's own cryptographic keys. This creates a cascading trust issue where the malicious backup information spreads to other devices that trust the compromised device, effectively compromising the entire device trust graph within the Matrix network. The vulnerability directly maps to CWE-400 vulnerability class related to protocol confusion and can be categorized under ATT&CK technique T1557 for "Adversary-in-the-Middle" attacks.
The operational impact of this vulnerability extends beyond simple message forgery to encompass complete compromise of device trust relationships within the Matrix ecosystem. When the malicious homeserver collaborates with an attacker, they can manipulate the encryption process to make legitimate devices trust compromised key backup information. The security implications are particularly severe because the system's behavior changes from rejecting potentially malicious messages to actively signing and propagating them, thereby spreading trust to other devices in the network. This vulnerability affects the core cryptographic assurances that Matrix users rely upon for secure communication, potentially enabling man-in-the-middle attacks that could compromise all communications for affected users. The fix implemented in version 1.5.1 addresses this by strictly enforcing Olm encryption for to-device messages and removing the automatic signing behavior that previously enabled trust propagation.
Organizations and individuals using Matrix-based applications must understand that this vulnerability requires a coordinated attack involving both a malicious homeserver and an external attacker, meaning that users who trust their homeserver operators are not immediately at risk. However, the security implications remain significant for any system where trust boundaries are not properly enforced. The mitigation strategy implemented in the patched version includes stricter validation of encryption protocols and additional security checks to prevent similar issues. The vulnerability highlights the importance of proper cryptographic protocol implementation in mobile SDKs and demonstrates how seemingly minor protocol handling decisions can create substantial security risks. Security practitioners should consider this vulnerability when evaluating the security posture of Matrix-based messaging systems and ensure that proper encryption protocol enforcement is maintained throughout the entire communication stack. The remediation approach taken by the developers reflects a defensive programming strategy that prioritizes security over convenience, implementing strict checks to prevent protocol confusion attacks that could compromise end-to-end encryption guarantees.