CVE-2025-53643 in aiohttp
Summary
by MITRE • 07/15/2025
AIOHTTP is an asynchronous HTTP client/server framework for asyncio and Python. Prior to version 3.12.14, the Python parser is vulnerable to a request smuggling vulnerability due to not parsing trailer sections of an HTTP request. If a pure Python version of aiohttp is installed (i.e. without the usual C extensions) or AIOHTTP_NO_EXTENSIONS is enabled, then an attacker may be able to execute a request smuggling attack to bypass certain firewalls or proxy protections. Version 3.12.14 contains a patch for this issue.
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Analysis
by VulDB Data Team • 01/10/2026
The vulnerability identified as CVE-2025-53643 affects the aiohttp asynchronous HTTP client/server framework for Python, specifically exposing a request smuggling flaw in versions prior to 3.12.14. This issue stems from the framework's inadequate handling of HTTP trailer sections within requests, creating a potential security gap that could be exploited by malicious actors. The vulnerability manifests when the pure Python implementation of aiohttp is utilized, which occurs when the C extensions are not installed or when the AIOHTTP_NO_EXTENSIONS environment variable is set, making the framework more susceptible to attack vectors that would otherwise be mitigated by the optimized C components.
The technical flaw resides in the HTTP parser's inability to properly process trailer sections of HTTP requests, which are part of the HTTP/1.1 specification designed to carry additional metadata after the main request body. When these trailer sections are improperly handled, they create opportunities for attackers to manipulate request boundaries and potentially bypass security controls implemented at network perimeters or through proxy servers. This parsing deficiency creates a scenario where the framework may misinterpret the structure of incoming HTTP requests, leading to inconsistent request handling that could be exploited for various malicious purposes including cache poisoning, authentication bypass, or data exfiltration.
The operational impact of this vulnerability extends beyond simple protocol parsing errors, as it specifically targets the fundamental HTTP request processing capabilities that underpin web applications built on aiohttp. Attackers leveraging this flaw could potentially bypass firewall rules, proxy configurations, and other network security controls that rely on proper HTTP request parsing for their effectiveness. The vulnerability is particularly concerning because it affects environments where the pure Python implementation is used, which may be common in development, testing, or containerized deployments where C extensions are not available or desired. This makes the attack surface broader than initially apparent, as many deployments may unknowingly use the vulnerable pure Python implementation.
Organizations using aiohttp should immediately upgrade to version 3.12.14 or later to remediate this vulnerability, as the patch addresses the core parsing issue that enables request smuggling attacks. The mitigation strategy should also include monitoring for any unusual HTTP request patterns that might indicate exploitation attempts, particularly around trailer section handling. Security teams should also consider implementing additional network-level controls and request validation mechanisms to detect and prevent potential smuggling attempts. This vulnerability aligns with CWE-444, which describes improper handling of HTTP requests and responses, and could be categorized under ATT&CK technique T1190 for exploit via proxy, highlighting the attack surface that exists through proxy server configurations and the potential for bypassing security controls through malformed HTTP requests.