CVE-2025-53643 is a vulnerability identified in the aiohttp library, an asynchronous HTTP client/server framework widely used in Python applications leveraging asyncio. The vulnerability is classified under CWE-444, which pertains to inconsistent interpretation of HTTP requests, commonly known as HTTP request/response smuggling. Specifically, versions of aiohttp prior to 3.12.14 are affected when the pure Python implementation is used (i.e., without the usual C extensions) or when the environment variable AIOHTTP_NO_EXTENSIONS is enabled. The root cause lies in the Python parser's failure to properly parse the trailer sections of HTTP requests. This parsing deficiency allows an attacker to craft malicious HTTP requests that can be interpreted differently by intermediary devices such as firewalls or proxies and the backend server, enabling the attacker to smuggle requests past security controls. Such request smuggling can lead to bypassing firewall or proxy protections, potentially allowing unauthorized access or manipulation of HTTP traffic. The vulnerability has a CVSS 4.0 base score of 1.7, indicating a low severity level, primarily because exploitation does not require privileges or user interaction, and the impact on confidentiality, integrity, and availability is limited. No known exploits are currently reported in the wild. The issue was addressed in aiohttp version 3.12.14, which includes a patch to correctly parse HTTP trailer sections, mitigating the risk of request smuggling attacks.

For European organizations, the impact of this vulnerability depends largely on the deployment context of aiohttp. Organizations using aiohttp in web services, APIs, or microservices that handle HTTP traffic asynchronously and rely on the pure Python implementation or have disabled C extensions are at risk. Successful exploitation could allow attackers to bypass perimeter security controls such as firewalls or proxies, potentially enabling unauthorized access to internal services or manipulation of HTTP requests. This could lead to data leakage, unauthorized command execution, or session hijacking if combined with other vulnerabilities. However, given the low CVSS score and the requirement for specific configurations (pure Python parser or disabled extensions), the overall risk is moderate to low for most organizations. Nonetheless, sectors with high reliance on Python-based asynchronous web frameworks, such as fintech, healthcare, and critical infrastructure in Europe, should consider the risk carefully, as even low-severity vulnerabilities can be leveraged in complex attack chains.

European organizations should ensure that all aiohttp deployments are updated to version 3.12.14 or later, where the vulnerability is patched. For environments where upgrading is not immediately feasible, organizations should verify that the C extensions for aiohttp are enabled and that the environment variable AIOHTTP_NO_EXTENSIONS is not set, as the vulnerability only manifests when the pure Python parser is used. Additionally, network security teams should review firewall and proxy configurations to detect and block anomalous HTTP requests that could be indicative of request smuggling attempts. Implementing strict input validation and HTTP header normalization at the application and proxy layers can further reduce risk. Monitoring HTTP traffic for irregularities and employing Web Application Firewalls (WAFs) with updated signatures can help detect exploitation attempts. Finally, organizations should conduct security assessments and penetration tests focusing on HTTP request smuggling to identify any exploitable configurations.