CVE-2019-16786 in Waitress
Summary
by MITRE
Waitress through version 1.3.1 would parse the Transfer-Encoding header and only look for a single string value, if that value was not chunked it would fall through and use the Content-Length header instead. According to the HTTP standard Transfer-Encoding should be a comma separated list, with the inner-most encoding first, followed by any further transfer codings, ending with chunked. Requests sent with: "Transfer-Encoding: gzip, chunked" would incorrectly get ignored, and the request would use a Content-Length header instead to determine the body size of the HTTP message. This could allow for Waitress to treat a single request as multiple requests in the case of HTTP pipelining. This issue is fixed in Waitress 1.4.0.
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Analysis
by VulDB Data Team • 03/16/2024
The vulnerability described in CVE-2019-16786 affects the Waitress web server implementation through version 1.3.1, representing a critical parsing flaw in HTTP request handling that directly impacts the server's ability to properly process incoming requests. This issue stems from improper interpretation of the Transfer-Encoding HTTP header, which according to the HTTP/1.1 specification defined in RFC 7230 should be processed as a comma-separated list of encoding values. The vulnerability specifically manifests when Waitress encounters a Transfer-Encoding header containing multiple values, such as "Transfer-Encoding: gzip, chunked" where the chunked encoding is not the first value in the sequence. The server's parser only examines the first string value in the header and, if that value is not "chunked", it falls back to using the Content-Length header for determining request body size. This behavior violates the HTTP standard where the inner-most encoding should be processed first, with chunked encoding typically appearing as the final element in the sequence. The flaw creates a significant security risk because it allows an attacker to manipulate HTTP request parsing through carefully crafted Transfer-Encoding headers, potentially enabling request smuggling or other HTTP-based attacks that exploit the server's incorrect handling of request boundaries.
The technical implementation of this vulnerability occurs at the HTTP request parsing layer within Waitress, where the server's header processing logic fails to properly parse comma-separated values in the Transfer-Encoding header. This parsing error creates a fundamental misalignment between the server's interpretation of HTTP semantics and the actual HTTP specification requirements. When an attacker sends a request with Transfer-Encoding: gzip, chunked, the server incorrectly processes only the first value "gzip" and ignores the presence of "chunked" encoding. As a result, the server treats the request body size determination as if it were using Content-Length instead of chunked transfer encoding, which fundamentally changes how the HTTP message is interpreted and processed. The vulnerability becomes particularly dangerous in HTTP pipelining scenarios where multiple requests are sent sequentially, as the incorrect parsing can cause Waitress to misinterpret the boundaries between requests. This misinterpretation can lead to situations where what should be a single request is treated as multiple requests, potentially enabling various attack vectors including request smuggling, denial of service through malformed request handling, and other HTTP protocol violations that can compromise server integrity and availability.
The operational impact of CVE-2019-16786 extends beyond simple parsing errors to potentially enable sophisticated attack patterns that exploit HTTP protocol handling weaknesses. This vulnerability creates conditions where an attacker can manipulate server behavior through carefully crafted HTTP headers, potentially leading to request smuggling attacks where malicious requests can be injected between legitimate requests or where request boundaries become ambiguous. The issue is particularly concerning in environments where Waitress serves as a backend component or where HTTP pipelining is utilized, as these scenarios increase the likelihood of successful exploitation. The vulnerability affects the fundamental HTTP message processing capabilities of Waitress, potentially allowing for data corruption, unauthorized access, or service disruption. From a security perspective, this flaw aligns with CWE-1030, which covers "HTTP Header Parsing," and represents a specific instance of improper handling of HTTP transfer encodings that violates the HTTP specification. The impact is further exacerbated by the fact that the vulnerability can be exploited without requiring authentication, making it a significant concern for any system running vulnerable versions of Waitress.
Mitigation strategies for CVE-2019-16786 focus on upgrading to Waitress version 1.4.0 or later, which contains the necessary fixes to properly parse Transfer-Encoding headers according to HTTP standards. Organizations should prioritize immediate patching of all affected Waitress installations, particularly in production environments where HTTP pipelining or complex request handling scenarios may be in use. Additionally, network monitoring should be enhanced to detect suspicious HTTP header patterns that might indicate exploitation attempts, including unusual Transfer-Encoding header values. Security teams should implement proper HTTP request validation and ensure that all HTTP servers properly handle comma-separated header values according to RFC 7230 specifications. The fix in Waitress 1.4.0 addresses the core parsing logic to correctly process Transfer-Encoding headers as comma-separated lists, ensuring that the inner-most encoding is properly handled before falling back to Content-Length processing. This vulnerability demonstrates the importance of adhering to HTTP specifications and proper header parsing in web server implementations, and aligns with ATT&CK technique T1071.004 for application layer protocol manipulation. Organizations should also consider implementing web application firewalls or HTTP proxies that can validate HTTP header structures and detect malformed requests that attempt to exploit similar parsing vulnerabilities in other HTTP implementations.