CVE-2021-39214 in mitmproxy
Summary
by MITRE • 09/16/2021
mitmproxy is an interactive, SSL/TLS-capable intercepting proxy. In mitmproxy 7.0.2 and below, a malicious client or server is able to perform HTTP request smuggling attacks through mitmproxy. This means that a malicious client/server could smuggle a request/response through mitmproxy as part of another request/response's HTTP message body. While a smuggled request is still captured as part of another request's body, it does not appear in the request list and does not go through the usual mitmproxy event hooks, where users may have implemented custom access control checks or input sanitization. Unless one uses mitmproxy to protect an HTTP/1 service, no action is required. The vulnerability has been fixed in mitmproxy 7.0.3 and above.
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Analysis
by VulDB Data Team • 09/19/2021
The CVE-2021-39214 vulnerability represents a critical HTTP request smuggling flaw in mitmproxy versions 7.0.2 and earlier, where malicious entities can exploit the proxy's handling of HTTP message bodies to inject unauthorized requests that bypass normal interception and processing mechanisms. This vulnerability specifically affects HTTP/1.1 services where the proxy does not properly validate or sanitize HTTP message boundaries, allowing attackers to smuggle requests through legitimate HTTP transactions. The flaw stems from improper parsing of HTTP message structures, where the proxy fails to correctly separate individual requests within a single HTTP message body, creating opportunities for malicious actors to inject hidden requests that remain undetected by standard proxy processing hooks.
The technical implementation of this vulnerability involves the manipulation of HTTP message boundaries and chunked transfer encoding in HTTP/1.1 communications. When mitmproxy processes HTTP messages, it fails to properly validate the integrity of message boundaries, allowing attackers to craft requests that appear as part of a legitimate message body while actually containing separate, smuggled requests. This occurs because the proxy's HTTP parser does not adequately enforce HTTP protocol compliance when processing HTTP/1.1 message structures, particularly when dealing with chunked encoding or content-length headers that may be manipulated by malicious parties. The vulnerability is classified under CWE-444 as an Inconsistent Interpretation of HTTP Requests, which directly maps to the core issue of improper HTTP message boundary handling.
The operational impact of CVE-2021-39214 is significant for organizations relying on mitmproxy for HTTP traffic interception and security monitoring, particularly in environments where custom access control policies or input sanitization are implemented through proxy event hooks. Since smuggled requests bypass the normal proxy processing pipeline, they avoid inspection by custom security rules, access control checks, and input validation mechanisms that administrators may have configured. This creates a potential vector for bypassing security controls and executing unauthorized actions that would otherwise be prevented by standard proxy security measures. The vulnerability essentially creates a blind spot in the proxy's monitoring capabilities, where malicious requests can be processed without triggering any security alerts or access control enforcement.
Organizations using mitmproxy for HTTP/1.1 services are particularly vulnerable to this attack vector, as the flaw specifically targets HTTP/1.1 message handling where the proxy's message boundary parsing is weakest. The vulnerability becomes exploitable when attackers can manipulate HTTP headers or message structures to create the conditions necessary for request smuggling, typically through manipulation of content-length headers, chunked encoding, or other HTTP/1.1 specific features that allow for message boundary confusion. The mitigation strategy requires immediate upgrade to mitmproxy version 7.0.3 or later, which includes fixes for the HTTP message boundary parsing logic and proper validation of HTTP message structures to prevent the smuggling of requests through legitimate HTTP transactions.
This vulnerability demonstrates the importance of proper HTTP protocol implementation in proxy systems and aligns with ATT&CK technique T1071.004 for Application Layer Protocol: DNS, where proxy systems must properly validate and sanitize all network traffic to prevent protocol-level attacks. The flaw also relates to ATT&CK technique T1566.001 for Phishing: Spearphishing Attachment, as attackers could potentially use this vulnerability to smuggle malicious requests that bypass security controls, though the primary attack vector is through direct HTTP request manipulation rather than traditional phishing techniques. Organizations should implement network monitoring to detect unusual HTTP traffic patterns that might indicate request smuggling attempts and ensure that all proxy systems are regularly updated to prevent exploitation of known vulnerabilities. The fix in mitmproxy 7.0.3 addresses the root cause by implementing stricter HTTP message boundary validation and ensuring that all HTTP messages are properly parsed and validated before being processed through the proxy's event hooks and security controls.