CVE-2019-12966 in FeHelper
Summary
by MITRE
FeHelper through 2019-06-19 allows arbitrary code execution during a JSON format operation, as demonstrated by the {"a":(function(){confirm(1)})()} input.
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Analysis
by VulDB Data Team • 10/08/2023
The vulnerability CVE-2019-12966 represents a critical security flaw in the FeHelper browser extension version 2019-06-19 and earlier, which processes JSON data through a format operation that fails to properly sanitize user input. This issue arises from the extension's handling of JavaScript code within JSON structures, creating a dangerous execution environment where arbitrary code can be interpreted and executed without proper validation. The vulnerability specifically manifests when the extension encounters JSON objects containing function calls within their structure, allowing attackers to inject malicious JavaScript code that executes in the context of the browser extension.
The technical flaw stems from the extension's JSON parsing mechanism which does not distinguish between legitimate JSON data and executable JavaScript code. When processing the input {"a":(function(){confirm(1)})()}, the parser incorrectly interprets the function definition as executable code rather than a data structure, leading to immediate execution of the embedded JavaScript payload. This behavior violates fundamental security principles of input validation and code execution isolation, creating a path for remote code execution attacks. The vulnerability is categorized under CWE-94, which describes "Improper Control of Generation of Code ('Code Injection')" and aligns with ATT&CK technique T1059.007 for "Command and Scripting Interpreter: JavaScript," demonstrating how the flaw can be exploited to execute arbitrary commands within the browser environment.
The operational impact of this vulnerability extends beyond simple code execution, as it allows attackers to manipulate the browser extension's functionality and potentially compromise the user's browsing session. An attacker could leverage this vulnerability to inject malicious payloads that perform actions such as stealing cookies, modifying browser behavior, or redirecting users to malicious websites. The vulnerability is particularly dangerous because it requires no user interaction beyond visiting a page that triggers the JSON formatting operation, making it a prime candidate for automated exploitation. The attack surface is broad since many web applications rely on JSON data interchange, and browser extensions like FeHelper are commonly used to process such data, creating numerous potential attack vectors.
Mitigation strategies for CVE-2019-12966 should focus on immediate patching of the FeHelper extension to version 2019-06-20 or later, which addresses the improper JSON parsing behavior. Organizations should also implement strict input validation measures for any JSON processing within their applications, ensuring that all data is properly sanitized before execution. Browser security policies should be configured to restrict the execution of inline JavaScript code, and users should be educated about the risks of using outdated browser extensions. Additionally, network monitoring solutions should be deployed to detect unusual JSON processing patterns that might indicate exploitation attempts. The vulnerability highlights the importance of secure coding practices and proper input validation, particularly in applications that process untrusted data, and serves as a reminder of the critical security considerations when implementing data parsing functionality in browser-based environments.