CVE-2026-116577 — Tornado: Quadratic CPU usage when parsing HTTP headers with line folding

Affected: tornadoweb tornado

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.

GHSA-pcg2-g6cj-q92c — Tornado: Quadratic CPU usage when parsing HTTP headers with line folding

Affected: PyPI: tornado

### Summary

Tornado's HTTP header parser takes quadratic time (O(n²)) when a header value is continued across many "obsolete line folding" (obs-fold) lines. An unauthenticated attacker can send a request whose header block is mostly short continuation lines. Parsing it uses a large amount of CPU on the IOLoop thread and blocks every other connection that process is serving.

This issue is closely related to CVE-2025-67725 (GHSA-c98p-7wgm-6p64). The fix for that advisory covered repeated headers but not line-folded continuations.

### Impact

All Tornado HTTP servers are affected. Header parsing runs before any application code, so no authentication or particular handler is required.

How bad it gets depends on `max_header_size`:

- **Default (64 KB):** each malicious request blocks the event loop for tens of milliseconds. Repeated requests can degrade service. - **Raised `max_header_size`:** the cost grows with the square of the header size. A single 1 MB request can block the process for several seconds.

Only availability is affected. Confidentiality and integrity are not.

### Patches

Fixed in Tornado 6.6 by removing support for obsolete line folding completely; header lines that begin with whitespace are now rejected.

### Workarounds

- Keep `max_header_size` at its default value or lower it. This limits the impact but doesn't remove it. - Run Tornado behind a reverse proxy that rejects or normalizes obsolete line folding before forwarding requests.

### Credit

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CVE-2026-116577 — Tornado: Quadratic CPU usage when parsing HTTP headers with line folding

GHSA-pcg2-g6cj-q92c — Tornado: Quadratic CPU usage when parsing HTTP headers with line folding