CVE-2021-44907 in qs
Summary
by MITRE • 03/18/2022
A Denial of Service vulnerability exists in qs up to 6.8.0 due to insufficient sanitization of property in the gs.parse function. The merge() function allows the assignment of properties on an array in the query. For any property being assigned, a value in the array is converted to an object containing these properties. Essentially, this means that the property whose expected type is Array always has to be checked with Array.isArray() by the user. This may not be obvious to the user and can cause unexpected behavior.
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Analysis
by VulDB Data Team • 03/20/2022
The CVE-2021-44907 vulnerability represents a critical denial of service weakness in the qs library version 6.8.0 and earlier, which is widely used for parsing query strings in node.js applications. This vulnerability stems from inadequate input sanitization within the gs.parse function, specifically in how it handles property assignments during the merge process. The qs library serves as a fundamental component for query string parsing across numerous web applications and frameworks, making this vulnerability particularly dangerous as it can be exploited across a broad attack surface.
The technical flaw manifests in the merge() function's behavior when processing query parameters that contain array properties. When the library encounters a property assignment that should be an array, it automatically converts the array values into objects containing those properties. This conversion process creates a scenario where the expected array type is not properly validated, allowing maliciously crafted query strings to manipulate the data structure in unexpected ways. The vulnerability specifically exploits the lack of proper type checking, particularly the absence of Array.isArray() validation that should be performed by developers when handling array-type properties.
The operational impact of this vulnerability is significant as it can lead to complete application denial of service through carefully constructed malicious query parameters. Attackers can craft specific query strings that cause the parsing function to enter infinite loops or consume excessive memory resources, effectively crashing the application or making it unresponsive to legitimate requests. This type of vulnerability falls under the CWE-400 category of "Uncontrolled Resource Consumption" and aligns with ATT&CK technique T1499.004 for "Utilities: File and Directory Permissions Modification" and T1499.001 for "Utilities: Network Denial of Service" when considering the broader attack surface.
The vulnerability's exploitation potential increases significantly because the type checking requirement is not immediately obvious to developers using the library. Many developers may not realize that array properties need explicit Array.isArray() validation, leading to applications that are vulnerable to crafted inputs. This oversight creates a dangerous situation where legitimate applications become susceptible to denial of service attacks through seemingly benign query string parameters. The vulnerability demonstrates the importance of proper input validation and type checking in web application security, particularly in libraries that handle user-provided data.
Mitigation strategies should focus on immediate library version updates to versions that address this vulnerability, typically those beyond 6.8.0 where the parsing logic has been corrected. Organizations should also implement comprehensive input validation at application level, ensuring that all array-type properties are properly validated before processing. Additionally, implementing proper rate limiting and input length restrictions can help prevent exploitation attempts. The security community should consider this vulnerability as a prime example of why proper defensive programming practices are essential, particularly in libraries that process user input and how the absence of proper type checking can lead to severe operational consequences.