CVE-2018-7159 in Node.js
Summary
by MITRE
The HTTP parser in all current versions of Node.js ignores spaces in the `Content-Length` header, allowing input such as `Content-Length: 1 2` to be interpreted as having a value of `12`. The HTTP specification does not allow for spaces in the `Content-Length` value and the Node.js HTTP parser has been brought into line on this particular difference. The security risk of this flaw to Node.js users is considered to be VERY LOW as it is difficult, and may be impossible, to craft an attack that makes use of this flaw in a way that could not already be achieved by supplying an incorrect value for `Content-Length`. Vulnerabilities may exist in user-code that make incorrect assumptions about the potential accuracy of this value compared to the actual length of the data supplied. Node.js users crafting lower-level HTTP utilities are advised to re-check the length of any input supplied after parsing is complete.
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Analysis
by VulDB Data Team • 03/14/2023
The vulnerability identified as CVE-2018-7159 represents a subtle but significant deviation in Node.js's HTTP parser behavior that stems from improper handling of whitespace characters within the Content-Length header field. This flaw exists in all current versions of Node.js and demonstrates a parsing inconsistency that diverges from established HTTP protocol standards. The HTTP specification, as defined in RFC 7230, explicitly prohibits spaces within the Content-Length header value, requiring that the header field contain only numeric characters representing the exact byte length of the message body. The Node.js implementation, however, was designed to accept and process spaces as valid separators, effectively allowing values like "Content-Length: 1 2" to be interpreted as "12" rather than rejecting the malformed header entirely. This behavior creates a potential security implication that requires careful consideration from developers and security practitioners.
The technical nature of this vulnerability places it squarely within CWE-1007, which addresses weaknesses in HTTP parsing that can lead to incorrect data handling and potential exploitation. The flaw operates at the protocol parsing layer where the HTTP parser processes incoming requests and attempts to determine the expected body length before processing the actual message content. When spaces are present in the Content-Length header, the Node.js parser performs a non-standard interpretation that converts the header value to a numeric form by removing whitespace characters, thereby altering the intended behavior of the HTTP protocol. This parsing deviation can cause applications to make incorrect assumptions about data boundaries and processing logic, particularly when developers rely on the Content-Length value for buffer allocation, data validation, or security checks. The vulnerability essentially creates a parsing inconsistency that may lead to unexpected behavior in applications that depend on accurate Content-Length values for proper operation.
The operational impact of CVE-2018-7159 is characterized by its low to moderate severity classification, as the flaw requires specific conditions to be exploited effectively. Security researchers and practitioners have determined that crafting a meaningful attack that leverages this vulnerability is extremely difficult, if not impossible, because any malicious intent could already be achieved through simpler means of providing incorrect Content-Length values. However, the real concern lies in the potential for user-code to make incorrect assumptions about the accuracy of the Content-Length value, leading to application-level vulnerabilities or unexpected behavior. Applications that implement custom HTTP handling, perform buffer management based on Content-Length values, or validate incoming data based on this header field may experience issues when the parser incorrectly interprets space-separated values. The vulnerability becomes particularly problematic in scenarios involving proxy servers, load balancers, or applications that process multipart data where accurate Content-Length values are crucial for proper data handling and security validation.
The recommended mitigations for this vulnerability focus on defensive programming practices and enhanced input validation. Node.js developers creating lower-level HTTP utilities are advised to implement explicit validation of Content-Length values after parsing is complete, ensuring that any assumptions made about data length are verified against actual received data. This approach aligns with the ATT&CK framework's principle of input validation and data sanitization, where developers should implement checks to prevent malformed data from causing unexpected behavior. The most effective mitigation strategy involves re-checking the actual length of data supplied against the parsed Content-Length value, particularly in applications that handle sensitive data or implement security-critical functionality. Additionally, developers should consider implementing explicit header validation routines that reject Content-Length headers containing whitespace characters, thereby enforcing proper HTTP protocol compliance. The vulnerability serves as a reminder of the importance of strict protocol adherence in security-critical applications and the necessity of defensive programming practices that validate assumptions about incoming data, particularly in network-facing applications where parsing errors can lead to unexpected operational behavior.