Mumble Security Advisory 2014-003
Date: February 2014
CVE Reference: Not assigned
Product: MumbleKit, Mumble for iOS
Mumble Website: http://mumble.info
Last Updated: 05-02-2014
A malformed Opus voice packet sent to a MumbleKit client (such as Mumble for iOS) could trigger a NULL pointer dereference. This causes a client crash (Denial of Service).
This can be triggered remotely by an entity participating in a Mumble voice chat.
Affected versions and configurations
All versions of MumbleKit with Opus support are affected unless they include the fix for this issue, which is available in the MumbleKit Git repository as commit fd190328a9b24d37382b269a5674b0c0c7a7e36d.
Mumble for iOS version 1.1 through 1.2.2 are vulnerable, as they use vulnerable versions of MumbleKit.
No known mitigation strategy for this issue exists.
Version 1.2.3 and prior of Mumble’s server component (‘Murmur’ or ‘mumble-server’) does not allow the transmission of Opus packets, and as such a vulnerable client connected to a stock Murmur server that runs version 1.2.3 or prior should not be affected by this issue.
Note however that since Mumble is a centralized VoIP system, a modified server could potentially also trigger malformed Opus packets to be sent to clients of its choosing, thus triggering this issue.
Mumble’s Opus voice packets are serialized as a buffer with a length-prefix using Mumble’s internal PacketDataStream serialization format.
MumbleKit failed to properly validate the length prefix of received Opus voice packets.
If an Opus packet with an invalid length prefix was received, MumbleKit would attempt to extract it using an MKPacketDataStream object’s copyDataBlock: method.
When the copyDataBlock: method is successful, it returns a valid NSData object. When an error occurs, it instead returns nil, and sets an error flag in the MKPacketDataStream object (which can be queired using the ‘valid’ method.)
Instead of performing proper error checking, MumbleKit used the returned NSData object as-is, without first checking whether it was nil, or whether the MKPacketDataStream object had its error flag set.
When the returned NSData object is nil, MumbleKit calls the Opus functions opus_packet_get_nb_frames() and opus_packet_get_samples_per_frame() with a NULL pointer as the packet buffer. This causes the functions to dereference the NULL pointer.
This issue was discovered by the Mumble team after a reproducible crash that happened when transmitting audio was reported by Wesley Wolfe on January 25, 2014.
A fix for this issue has beeen released in Mumble for iOS 1.2.3.
Other users of MumbleKit should make sure they are using a version of MumbleKit that includes the fixed commit, which is fd190328a9b24d37382b269a5674b0c0c7a7e36d.