CVE-2020-8927: Google Brotli Buffer Overflow Vulnerability

CVE-2020-8927 Overview

A buffer overflow vulnerability exists in the Google Brotli compression library versions prior to 1.0.8. This flaw allows an attacker who controls the input length of a "one-shot" decompression request to trigger a crash by copying data chunks larger than 2 GiB. The vulnerability affects the widely-used Brotli compression algorithm, which is commonly integrated into web browsers, web servers, and various development frameworks including Microsoft .NET, PowerShell, and Visual Studio.

Critical Impact

Remote attackers can exploit this buffer overflow to cause denial of service conditions through application crashes, with potential for limited integrity impact on systems using vulnerable Brotli library versions.

Affected Products

• Google Brotli (versions prior to 1.0.8)
• Microsoft .NET and .NET Core
• Microsoft PowerShell
• Microsoft Visual Studio 2019 and 2022
• Debian Linux 9.0 and 10.0
• Fedora 31, 32, 33, 34, 35, and 36
• Canonical Ubuntu Linux 16.04 ESM, 18.04 LTS, and 20.04 LTS
• openSUSE Leap 15.2

Discovery Timeline

• September 15, 2020 - CVE-2020-8927 published to NVD
• November 21, 2024 - Last updated in NVD database

Technical Details for CVE-2020-8927

Vulnerability Analysis

This buffer overflow vulnerability (CWE-120, CWE-130) occurs within the Brotli decompression library when handling specially crafted input through the "one-shot" API. The vulnerability is triggered when processing decompression requests where the input length exceeds 2 GiB, causing the library to improperly copy data beyond allocated buffer boundaries.

The flaw specifically manifests during the decompression routine when chunked data transfers exceed expected size limits. While the primary consequence is a crash leading to denial of service, the nature of buffer overflow vulnerabilities means there is also potential for limited integrity impact through memory corruption.

The vulnerability is remotely exploitable over the network without requiring authentication or user interaction, making it particularly concerning for web-facing applications and services that utilize Brotli compression for content delivery.

Root Cause

The root cause stems from improper length calculation when processing decompression requests exceeding 2 GiB through the "one-shot" decompression API. The Brotli library fails to properly validate and handle chunk sizes during the copy operation, leading to a buffer overflow condition when extremely large data chunks are processed. This represents a classic improper handling of length parameters (CWE-130) combined with buffer overflow (CWE-120) vulnerability pattern.

Attack Vector

An attacker can exploit this vulnerability by crafting a malicious compressed payload that, when decompressed using the vulnerable "one-shot" API, causes the library to attempt copying data chunks larger than 2 GiB. The attack requires network access to deliver the malicious payload to an application using the vulnerable Brotli library.

The exploitation mechanism involves:

1. Creating a specially crafted Brotli-compressed payload with manipulated length parameters
2. Delivering this payload to a target application that uses the "one-shot" decompression API
3. When the application attempts to decompress the payload, the oversized chunk copy operation triggers a buffer overflow
4. This results in application crash and potential memory corruption

Detection Methods for CVE-2020-8927

Indicators of Compromise

• Application crashes or service interruptions during Brotli decompression operations
• Unexpected memory consumption spikes when processing compressed content
• Segmentation faults or access violations in processes utilizing the Brotli library
• Log entries indicating failed decompression of abnormally large payloads

Detection Strategies

• Monitor for abnormal application behavior during content decompression, particularly crashes in components using Brotli
• Implement application-level logging to capture decompression failures and anomalous input sizes
• Use software composition analysis tools to identify applications and services running vulnerable Brotli library versions (prior to 1.0.8)
• Deploy network monitoring to detect unusually large compressed payloads targeting web services

Monitoring Recommendations

• Configure application performance monitoring to alert on unexpected crashes in services using Brotli compression
• Implement memory usage monitoring for services handling compressed content to detect potential exploitation attempts
• Review web server and application logs for patterns indicating malformed or oversized compressed content delivery
• Establish baseline metrics for normal decompression operations to identify anomalies

How to Mitigate CVE-2020-8927

Immediate Actions Required

• Update the Brotli library to version 1.0.8 or later immediately across all affected systems
• If immediate patching is not possible, switch from the "one-shot" API to the "streaming" API for decompression operations
• Implement input size limits on decompression requests to prevent processing of oversized data chunks
• Audit applications and dependencies to identify all instances of the vulnerable Brotli library

Patch Information

Google has released Brotli version 1.0.9 which addresses this vulnerability. The patch is available through the GitHub Brotli Release v1.0.9. Multiple Linux distributions have also released security updates:

• Debian Security Advisory DSA-4801
• Debian LTS Security Notice
• Ubuntu Security Notice USN-4568-1
• openSUSE Security Announcement

Microsoft products including .NET, .NET Core, PowerShell, and Visual Studio should be updated through their respective update channels.

Workarounds

• Use the "streaming" API instead of the "one-shot" API for Brotli decompression to avoid triggering the vulnerable code path
• Implement chunk size limits on data being processed by the decompression routines, keeping sizes below 2 GiB
• Deploy web application firewalls configured to inspect and limit compressed content sizes
• Consider temporarily disabling Brotli content encoding in web servers until patching can be completed

# Example: Check installed Brotli version on Linux systems
dpkg -l | grep brotli
# or
rpm -qa | grep brotli

# Update Brotli on Debian/Ubuntu
sudo apt-get update && sudo apt-get upgrade libbrotli1

# Update Brotli on Fedora/RHEL
sudo dnf update brotli