CVE-2026-116577 in Tornado
Summary
by MITRE • 10/26/2026
Tornado is a Python web framework and asynchronous networking library. Prior to 6.6, Tornado's HTTP header parser accepts obsolete line folding and repeatedly concatenates short continuation lines, causing quadratic CPU consumption on the IOLoop thread. An unauthenticated client can submit a header block containing many obsolete line-folding continuations before application code runs, blocking other connections served by the process. The cost grows with max_header_size, and only availability is affected. This issue is fixed in version 6.6.
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Analysis
by VulDB Data Team • 10/26/2026
Tornado is a widely utilized Python web framework designed for high-performance asynchronous networking applications. It handles thousands of simultaneous connections efficiently under normal conditions by leveraging non-blocking I/O operations within an event loop architecture. However, prior to the release of version 6.6, the HTTP header parsing component contained a significant performance vulnerability related to how it processed obsolete line folding in HTTP headers. This flaw allows for a specific type of resource exhaustion attack that targets the availability of services built on this framework.
The technical root cause lies in Tornado's handling of HTTP/1.0 style header continuation lines, also known as obsolete line folding. In older HTTP specifications, long header values could be split across multiple lines by starting subsequent lines with whitespace or a tab character. While modern best practices discourage this practice due to ambiguity and security risks, legacy clients may still send such headers. Tornado's parser was designed to accept these folded headers for backward compatibility but implemented the concatenation logic inefficiently. Specifically, when processing a header block containing many short continuation lines, the framework repeatedly concatenated strings in a manner that resulted in quadratic time complexity relative to the number of folding continuations. This means that as the number of folded lines increases, the CPU time required to parse them grows exponentially rather than linearly.
This vulnerability enables an unauthenticated attacker to craft malicious HTTP requests with header blocks containing numerous obsolete line-folding continuations. Because Tornado processes these headers on its main IOLoop thread before passing control to application-level code, a single such request can consume excessive CPU resources for an extended period. This effectively creates a Denial of Service condition by blocking the event loop from processing other legitimate connections. The severity and impact of this attack are directly proportional to the configured max_header_size setting; larger limits allow attackers to include more folding lines before the parser rejects them, thereby increasing the computational cost and duration of the blockage.
The operational impact is strictly limited to availability rather than confidentiality or integrity. An attacker does not need authentication credentials to exploit this flaw, making it particularly dangerous for public-facing services. By saturating the IOLoop thread with CPU-intensive header parsing, the service becomes unresponsive to other users, leading to a complete outage of the application's functionality until the malicious request is processed and the loop resumes normal operation. This represents a classic resource exhaustion vulnerability where an attacker leverages asymmetric computational costs to degrade system performance significantly.
From a classification perspective, this issue aligns with CWE-400: Uncontrolled Resource Consumption, specifically manifesting as CPU exhaustion through inefficient algorithmic complexity in input parsing. In the context of the MITRE ATT&CK framework, this vulnerability facilitates Denial of Service (T1499) techniques that target service availability by overwhelming computational resources. It highlights the importance of validating and sanitizing legacy protocol features even when they are technically permitted but rarely used correctly or safely.
To mitigate this risk, organizations running Tornado versions prior to 6.6 must upgrade immediately to version 6.6 or later, where the header parsing logic has been optimized to prevent quadratic CPU consumption during line folding processing. For environments that cannot update immediately due to dependency constraints, applying network-level filtering rules can help block requests with excessively long headers or those containing patterns indicative of folded continuations before they reach the application server. Additionally, configuring strict limits on max_header_size and implementing rate limiting at a reverse proxy layer such as Nginx or Apache can provide an additional defense-in-depth measure to absorb malformed traffic and protect the underlying Tornado processes from being overwhelmed by malicious header blocks.