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CVE-2022-39255: Matrix iOS SDK vulnerable ton Olm/Megolm protocol confusion

• Severity: high (CVSS 7.5 v3.1)
• Exploited in the wild: not listed in CISA KEV
• EPSS: 1.0% probability of exploitation in the next 30 days
• Public exploit or detection template: none known
• Published: 2022-09-28 · updated 2026-06-17
• Weakness: CWE-287

Description

Matrix iOS SDK allows developers to build iOS apps compatible with Matrix. Prior to version 0.23.19, 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. These attacks are possible due to a protocol confusion vulnerability that accepts to-device messages encrypted with Megolm instead of Olm. matrix-ios-sdk version 0.23.19 has been modified to only accept Olm-encrypted to-device messages. 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. To avoid malicious backup attacks, one should not verify one's new logins using emoji/QR verifications methods until patched.

Affected products

• Matrix Software Development Kit: before 0.23.19 — fixed in 0.23.19

How to fix

Upgrade to a fixed version: Matrix Software Development Kit 0.23.19. Check the vendor advisory for mitigations.

References

• https://github.com/matrix-org/matrix-ios-sdk/commit/5ca86c328a5faaab429c240551cb9ca8f0f6262c
• https://github.com/matrix-org/matrix-ios-sdk/releases/tag/v0.23.19
• https://github.com/matrix-org/matrix-ios-sdk/security/advisories/GHSA-hw6g-j8v6-9hcm
• https://matrix.org/blog/2022/09/28/upgrade-now-to-address-encryption-vulns-in-matrix-sdks-and-clients
• https://github.com/matrix-org/matrix-ios-sdk/commit/5ca86c328a5faaab429c240551cb9ca8f0f6262c
• https://github.com/matrix-org/matrix-ios-sdk/releases/tag/v0.23.19
• https://github.com/matrix-org/matrix-ios-sdk/security/advisories/GHSA-hw6g-j8v6-9hcm
• https://matrix.org/blog/2022/09/28/upgrade-now-to-address-encryption-vulns-in-matrix-sdks-and-clients

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