CVE-2026-111750 in aiohttp
Summary
by MITRE • 10/19/2026
AIOHTTP is an asynchronous HTTP client/server framework for asyncio and Python. Prior to 3.14.4, the pure Python parser accepts ambiguous HTTP request framing. An unauthenticated attacker can consequently smuggle requests when an application uses AIOHTTP_NO_EXTENSIONS or otherwise runs without a standard wheel and an intermediary interprets the framing differently. This issue is fixed in version 3.14.4.
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Analysis
by VulDB Data Team • 10/19/2026
The vulnerability identified in aiohttp versions prior to 3.14.4 represents a critical HTTP request smuggling flaw rooted in ambiguous input parsing within the framework's pure Python implementation. Aiohttp serves as a widely adopted asynchronous HTTP client and server library for Python, leveraging asyncio to handle high-concurrency network operations efficiently. The core technical deficiency lies in how the parser processes HTTP message framing when operating without C extensions or standard wheels. Specifically, the parser fails to strictly enforce unambiguous delimiters between headers and body content, allowing an attacker to craft requests that exploit discrepancies in how different components interpret these boundaries. This ambiguity enables a technique known as HTTP request smuggling, where maliciously formatted packets are designed to be interpreted differently by sequential network entities such as reverse proxies, load balancers, or the application server itself.
In this specific attack scenario, an unauthenticated attacker can inject additional headers or body content into legitimate requests by manipulating whitespace and delimiter sequences that the pure Python parser accepts but which downstream intermediaries might process according to different standards. For instance, if a front-end proxy strips certain characters or interprets chunked encoding differently than the backend application server running aiohttp without extensions, an attacker can smuggle their own request through the proxy undetected. This effectively allows the malicious payload to be associated with another user's session or administrative action because the backend processes it as part of a legitimate transaction. The reliance on AIOHTTP_NO_EXTENSIONS or environments lacking compiled C optimizations exacerbates this risk by forcing the system into the vulnerable pure Python parsing path, which lacks the rigorous validation present in optimized builds.
The operational impact of this vulnerability is severe, primarily facilitating unauthorized access and data exfiltration through session hijacking or privilege escalation. By successfully smuggling a request, an attacker can perform actions on behalf of other users, such as changing passwords, accessing sensitive financial records, or modifying application state without authorization. Furthermore, the ability to inject arbitrary headers may allow for cache poisoning attacks if the infrastructure relies on caching mechanisms that do not properly distinguish between smuggled and legitimate traffic. This undermines the integrity of web applications and can lead to significant data breaches, regulatory non-compliance, and loss of user trust. The lack of authentication requirements makes this vulnerability particularly dangerous as it does not require prior compromise of credentials to exploit.
Mitigation strategies must prioritize immediate version upgrades and architectural adjustments. Organizations utilizing aiohttp should upgrade to version 3.14.4 or later where the parsing logic has been hardened to reject ambiguous framing constructs that previously led to smuggling vulnerabilities. It is also advisable to avoid running in pure Python mode if performance allows, as compiled extensions often provide more robust validation mechanisms. Additionally, security teams should implement strict input validation at the network perimeter using Web Application Firewalls configured to detect and block HTTP request smuggling patterns, such as unusual combinations of Content-Length and Transfer-Encoding headers or malformed chunked encodings. Regular penetration testing focusing on protocol-level ambiguities will further ensure that any residual risks are identified and addressed before they can be exploited in production environments.