HMAC Signature Bypass in Thumbor

Executive Summary

Thumbor before version 7.8.0 has a security flaw in its HMAC validation. The issue occurs because the code uses Python's .replace() method to remove a signature from a URL before checking it. Since .replace() removes all instances of the substring, attackers can insert the same signature multiple times in the URL. This lets them manipulate the final URL used for validation, causing the validated string to differ from the actual requested resource. As a result, attackers can load images from unintended domains or paths.

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Detection Guidance

This vulnerability can be detected by checking the Thumbor version in use. If the version is below 7.8.0, the system is vulnerable. No specific commands are provided in the context to detect exploitation attempts.

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Impact Analysis

An attacker could exploit this to access or serve unauthorized images from different domains or paths. This might lead to data leaks, unauthorized content exposure, or bypassing access controls. If you use Thumbor for image processing, an attacker could manipulate URLs to fetch images they shouldn't have access to.

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Compliance Impact

This vulnerability could lead to unauthorized access to sensitive data, which may violate GDPR (data protection) or HIPAA (health information privacy) requirements. If exploited, it might result in data breaches, triggering legal penalties, fines, or reputational damage due to non-compliance with these regulations.

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Mitigation Strategies

Upgrade Thumbor to version 7.8.0 or later to address the HMAC validation bypass issue. Ensure all instances are updated to prevent potential exploitation.

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