CVE-2021-0967: Google Android Information Disclosure Flaw

CVE-2021-0967 Overview

CVE-2021-0967 is an out-of-bounds write vulnerability affecting the vorbis_book_decodev_set function within the codebook.c file of Google Android. The vulnerability arises from a missing bounds check during Vorbis audio codec processing, which could allow an attacker to write data outside the intended memory boundaries. Successful exploitation could lead to remote information disclosure without requiring additional execution privileges, though user interaction is necessary for exploitation.

Critical Impact

This out-of-bounds write vulnerability in Android's Vorbis audio codec implementation could enable remote attackers to achieve information disclosure or potentially execute arbitrary code through maliciously crafted audio content.

Affected Products

• Google Android 9.0
• Google Android 10.0
• Google Android 11.0
• Google Android 12.0

Discovery Timeline

• December 15, 2021 - CVE-2021-0967 published to NVD
• November 21, 2024 - Last updated in NVD database

Technical Details for CVE-2021-0967

Vulnerability Analysis

This vulnerability is classified as CWE-787 (Out-of-Bounds Write) and resides in Android's native Vorbis audio codec implementation. The vorbis_book_decodev_set function in codebook.c fails to properly validate input boundaries before performing write operations. When processing specially crafted Vorbis audio data, the function can write beyond the allocated buffer boundaries, corrupting adjacent memory regions.

The vulnerability is exploitable over the network through malicious audio content delivered via web pages, media applications, or messaging platforms. While user interaction is required (such as opening a malicious audio file or visiting a webpage containing malicious media), no special privileges are needed for exploitation.

Root Cause

The root cause of CVE-2021-0967 is a missing bounds check in the vorbis_book_decodev_set function. The Vorbis codec implementation processes codebook data without adequately validating that write operations remain within allocated buffer boundaries. This oversight allows attackers to craft malicious Vorbis audio streams that trigger writes beyond the intended memory region, potentially overwriting critical data structures or control flow information.

Attack Vector

The attack vector for this vulnerability is network-based and requires user interaction. An attacker could exploit this vulnerability through several methods:

1. Hosting malicious audio content on a website that users visit
2. Sending malicious audio files via messaging applications
3. Embedding malicious Vorbis audio in multimedia content
4. Distributing applications containing malicious audio resources

When a victim opens or streams the malicious audio content, the Android media framework processes the Vorbis data, triggering the out-of-bounds write condition in the vulnerable function.

The vulnerability mechanism involves the Vorbis codec's codebook decoding logic. During audio stream processing, the vorbis_book_decodev_set function processes vector data from the audio stream. Without proper boundary validation, attacker-controlled values can cause the function to write data outside the allocated buffer. Technical details are available in the Android Security Bulletin December 2021.

Detection Methods for CVE-2021-0967

Indicators of Compromise

• Unusual crashes or unexpected behavior in media playback applications
• Memory corruption errors in system logs related to audio codec processing
• Abnormal audio file processing patterns in Android media services

Detection Strategies

• Monitor for anomalous behavior in Android media framework processes
• Implement application-level logging for media file processing events
• Deploy mobile threat defense solutions capable of detecting exploit attempts
• Review crash reports for patterns indicating memory corruption in codec libraries

Monitoring Recommendations

• Enable enhanced logging for Android media services
• Monitor device telemetry for signs of exploitation attempts
• Implement network-level filtering for suspicious audio content
• Establish baseline behavior patterns for media processing to detect anomalies

How to Mitigate CVE-2021-0967

Immediate Actions Required

• Update all affected Android devices to the December 2021 security patch level or later
• Restrict downloading and playing audio content from untrusted sources
• Enable automatic security updates on Android devices
• Consider deploying mobile device management (MDM) solutions to enforce patch compliance

Patch Information

Google addressed this vulnerability in the Android Security Bulletin December 2021. The fix adds proper bounds checking to the vorbis_book_decodev_set function to prevent out-of-bounds write operations. Organizations should ensure all Android devices are updated to security patch level 2021-12-01 or later. The security bulletin is available at the Android Security Bulletin December 2021.

Workarounds

• Avoid opening audio files from untrusted or unknown sources
• Disable automatic media playback in applications where possible
• Use mobile security applications that scan media files before processing
• Consider restricting access to media applications on sensitive devices until patching is complete

# Verify Android security patch level
adb shell getprop ro.build.version.security_patch
# Expected output: 2021-12-01 or later