Heap Buffer Underread in GNU Wget

Compliance Impact

The provided information does not specify any direct impact of this vulnerability on compliance with common standards and regulations such as GDPR or HIPAA.

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Executive Summary

This vulnerability exists in GNU Wget versions up to 1.25.0 in the clean_metalink_string() function located in src/metalink.c. It is a heap buffer underread issue that occurs when a malicious server provides a Metalink document containing a URL made up entirely of whitespace characters. When processing such a URL, the function may decrement a pointer beyond the start of the allocated buffer, leading to memory corruption and potentially causing abnormal program behavior.

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Impact Analysis

The impact of this vulnerability is that an attacker controlling a malicious server can cause memory corruption in GNU Wget by serving a specially crafted Metalink document. This can lead to abnormal program behavior, which might include crashes or other unintended actions. Since the vulnerability does not require user interaction or privileges, it can be exploited remotely and silently.

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Detection Guidance

This vulnerability occurs when GNU Wget processes a Metalink document containing a whitespace-only URL, causing memory corruption. Detection involves monitoring for abnormal behavior or crashes in Wget when handling Metalink URLs.

Since the vulnerability is triggered by a Metalink document with a whitespace-only URL, you can detect attempts by inspecting network traffic for Metalink files and checking if any URLs contain only whitespace.

Suggested commands to detect suspicious Metalink URLs include:

• Use a network capture tool like tcpdump or Wireshark to capture Metalink files: tcpdump -i <interface> -s 0 -w capture.pcap 'tcp port 80 or tcp port 443'
• Extract Metalink files from captures and search for whitespace-only URLs using grep or similar tools: grep -Pzo '<url>\s*</url>' metalink_file
• Monitor Wget logs for crashes or abnormal termination when processing Metalink files.

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Mitigation Strategies

The immediate mitigation step is to update GNU Wget to a version that includes the fix from commit 37a40fc, which addresses the heap buffer underread vulnerability in the clean_metalink_string() function.

If updating is not immediately possible, avoid processing Metalink documents from untrusted or unknown sources, especially those that might contain whitespace-only URLs.

Additionally, monitor systems running vulnerable versions of Wget for crashes or abnormal behavior and restrict network access to potentially malicious servers serving crafted Metalink files.

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