CVE-2020-36846: Critical Buffer Overflow Vulnerability in Brotli Compression Library

Overview

The Common Vulnerabilities and Exposures (CVE) project has recently disclosed a critical security vulnerability, identified as CVE-2020-36846, that exists in versions of the IO::Compress::Brotli module prior to 0.007. The vulnerability stems from a buffer overflow condition in the embedded Brotli library, which can be exploited by an attacker to cause an application crash and potentially compromise the system. This vulnerability is of particular concern to organizations and individuals that rely on the Brotli compression library in their applications, as it poses a significant risk of data leakage and system compromise if left unpatched.

Vulnerability Summary

CVE ID: CVE-2020-36846
Severity: Critical (9.8 CVSS Score)
Attack Vector: Network
Privileges Required: None
User Interaction: None
Impact: Potential system compromise or data leakage

Affected Products

Product | Affected Versions

IO::Compress::Brotli | Versions prior to 0.007

How the Exploit Works

The vulnerability exists due to inadequate handling of data during the decompression process in the Brotli library. If an attacker can control the input length of a “one-shot” decompression request to a script, they can trigger a buffer overflow condition, causing the application to crash. This happens when copying over chunks of data larger than 2 GiB. In some scenarios, this could also lead to arbitrary code execution, allowing the attacker to compromise the system.

Conceptual Example Code

Below is a conceptual example of how the vulnerability might be exploited. This is a pseudo-code representation of a malicious payload designed to trigger the buffer overflow condition:

import brotli
def exploit(target):
payload = b"A" * (2**31 + 1)  # More than 2 GiB of data.
compressed = brotli.compress(payload)
target.decompress_one_shot(compressed)  # Trigger buffer overflow

Mitigation

To mitigate this vulnerability, it is recommended to update your IO::Compress::Brotli module to version 0.007 or later. If updating is not possible, consider using the “streaming” API instead of the “one-shot” API, and impose chunk size limits on decompression requests. As a temporary measure, a Web Application Firewall (WAF) or Intrusion Detection System (IDS) can be used to detect and block attempts to exploit this vulnerability.