Quick answers
- Is CVE-2025-67725 exploited in the wild?
- No. Junglewise Threat Intelligence has no report of CVE-2025-67725 being exploited in the wild.
- How severe is CVE-2025-67725?
- High severity, with a CVSS score of 7.5, published 12 December 2025.
- Which products and versions are affected?
- tornado (PyPI) < 6.5.3, ECOSYSTEM:introduced=0,fixed=6.5.3.
- Is there a fix for CVE-2025-67725?
- Yes. tornado (PyPI): upgrade to 6.5.3 or later. Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.
- How is CVE-2025-67725 attacked?
- It is attacked over the network, in a software library.
- Am I affected by CVE-2025-67725?
- Check whether you run tornado (PyPI) or Tornadoweb Tornado at an affected version. Junglewise does that check for you around the clock: connect your code, cloud and domains, and it matches new vulnerabilities like this one against what you actually run and tells you when one reaches you.
How to fix
- tornado (PyPI): upgrade to 6.5.3 or later
pip install "tornado>=6.5.3"
Technical details
The HTTPHeaders.add method in Tornado's web server implementation uses string concatenation to coalesce values when the same header name is repeated in a request. Because Python strings are immutable, each concatenation requires copying the existing string, leading to O(n²) time complexity relative to the number of repeated headers. Since Tornado operates on a single-threaded event loop, an attacker can exploit this by sending a large number of repeated headers to block the loop for an extended period. The impact is significantly higher if the default 'max_header_size' (64KB) has been increased. This issue is fixed in version 6.5.3.
Affected products
- Tornadoweb Tornado < 6.5.3
Identifiers
- CVE-2025-67725
- GHSA-c98p-7wgm-6p64
- PYSEC-2025-266
Timeline
- 2025-12-11: disclosed
- 2025-12-12: advisory: NVD publication date
- 2026-07-20: patched: GitHub Advisory published/updated date