CVE-2021-39214: 
Python vulnerability analysis and mitigation

The vulnerability (CVE-2021-39214) affects mitmproxy versions 7.0.2 and earlier, allowing malicious clients or servers to perform HTTP request smuggling attacks. The vulnerability was discovered in September 2021 and patched in version 7.0.3. This security flaw enables attackers to smuggle requests or responses through mitmproxy as part of another request's HTTP message body (GitHub Advisory).

The vulnerability is classified as CWE-444 and received a Moderate severity rating. When exploited, mitmproxy would only see one request while the target server would see multiple requests. The smuggled request is captured as part of another request's body but doesn't appear in the request list and bypasses the usual mitmproxy event hooks, where users may have implemented custom access control checks or input sanitization (GitHub Advisory).

The vulnerability primarily affects systems using mitmproxy to protect HTTP/1 services. When exploited, it allows attackers to bypass security controls as smuggled requests do not go through mitmproxy's normal event hooks, potentially circumventing custom access control checks and input sanitization mechanisms (GitHub Advisory).

The vulnerability has been patched in mitmproxy version 7.0.3 and above. Users are advised to upgrade to the latest version to protect against HTTP request smuggling attacks. If using mitmproxy to protect an HTTP/1 service, immediate action is recommended (GitHub Advisory).