CVE-2022-2246
Summary
by MITRE • 06/29/2022
Prototype Pollution in GitHub repository clever/underscore.deep prior to 0.5.3.
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Analysis
by VulDB Data Team • 06/15/2026
Prototype pollution vulnerabilities occur when an application fails to properly sanitize user input before using it to modify object prototypes, allowing attackers to inject malicious properties into the prototype chain. The clever underscore.deep library before version 0.5.3 was susceptible to this type of attack due to inadequate handling of nested object properties during deep merge operations. When developers used this library to merge user-provided data with existing objects, maliciously crafted input could pollute the prototype of the base object, potentially affecting all instances of that object throughout the application's lifecycle.
The technical flaw in underscore.deep stemmed from the library's inability to distinguish between legitimate object properties and prototype pollution attempts during deep merge operations. Specifically, when processing nested objects with potentially malicious keys such as _proto_ or constructor, the library would blindly assign these values to the target object's prototype chain rather than rejecting or sanitizing them. This behavior aligns with CWE-471, which describes the vulnerability of an application's use of an object's prototype, and represents a classic example of prototype pollution as defined by the OWASP Top Ten. The vulnerability allowed attackers to manipulate the behavior of core JavaScript objects, potentially leading to remote code execution, denial of service, or other serious security implications.
The operational impact of this vulnerability extended beyond simple data corruption, as prototype pollution could enable attackers to modify the behavior of fundamental JavaScript objects like Object, Array, or Function. When an attacker successfully polluted a prototype, any subsequent use of these objects throughout the application could be affected, potentially causing unexpected behavior or enabling further exploitation techniques. This vulnerability particularly affected applications that relied on user input for configuration or data merging operations, creating a vector for attackers to gain elevated privileges or manipulate application logic. The attack surface was broad since many applications use deep merge operations to handle configuration data, user preferences, or API responses, making the exploitation of this vulnerability particularly dangerous in real-world scenarios.
Mitigation strategies for this prototype pollution vulnerability required immediate action to upgrade to version 0.5.3 or later of the underscore.deep library, which included proper sanitization of prototype properties during merge operations. Organizations should also implement input validation and sanitization practices to prevent malicious data from reaching merge operations, particularly when dealing with user-provided content. The ATT&CK framework categorizes this type of vulnerability under T1068, which describes the use of privilege escalation techniques, as prototype pollution can be leveraged to gain elevated privileges within an application. Additional defensive measures included implementing Content Security Policy headers, using safer alternative libraries that properly handle prototype pollution, and conducting regular security audits of third-party dependencies to identify similar vulnerabilities. Organizations should also consider implementing runtime protections and monitoring for unusual prototype modifications to detect potential exploitation attempts.