WebRTC - VP9 Missing Frame Processing Out-of-Bounds Memory Access

EDB-ID: 44863
Author: Google Security Research
Published: 2018-06-08
CVE: CVE-2018-6129
Type: Dos
Platform: Multiple
Aliases: N/A
Advisory/Source: Link
Tags: N/A
Vulnerable App: N/A

In the file video_coding/rtp_frame_reference_finder.cc, the function RtpFrameReferenceFinder::MissingRequiredFrameVp9 contains the following code:

size_t temporal_idx = info.gof->temporal_idx[gof_idx];
for (size_t l = 0; l < temporal_idx; ++l) {
auto missing_frame_it = missing_frames_for_layer_[l].lower_bound(ref_pid);

missing_frames_for_layer_ is a std::array of length kMaxTemporalLayers which equals 5.

Meanwhile, values in the temporal_idx array are read in rtp_format_vp9.cc in the following code:

RETURN_FALSE_ON_ERROR(parser->ReadBits(&t, 3));
vp9->gof.temporal_idx[i] = t;

Reading three bits makes the maximum size of temporal_idx 7, which can go out of bounds of the missing_frames_for_layer_ array.

This issue causes a crash in Chrome. To reproduce the issue.

1) unzip the attached webrtc-from-chat.zip on a local webserver
2) fetch the webrtc source (https://webrtc.org/native-code/development/), and replace src/modules/rtp_rtcp/source/rtp_format_vp9.cc with the version attached to the code
3) build webrtc, including the examples
4) run the attached webrtcserver.py with python 3.6 or higher
5) start the peerconnection_client sample in the webrtc examples. Connect to the recommended server, and then select test2 as the peer to connect to
6) visit in chrome
7) Enter any username and hit "Log in"
8) Type anything into the chat window at the bottom and hit send

The attached file 'missingframe' contains the VP9 frame that causes this crash.

Though the attached PoC requires user interaction, it is not necessary to exercise this issue in a browser.

This issue affects any browser that supports VP9, and can be reached by loading a single webpage (though some browsers will prompt for permissions). It also affects native clients (such as mobile applications) that use webrtc and support VP9, though the user has to place or answer a video call for their client to be in the state where this issue is reachable.

Proof of Concept:

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