CVE-2025-67725: Tornado · Quadratic DoS via Repeated Header Coalescing
Affects nvcheckerSource GCVE Database
Not known to be exploited · CVSS 7.5 High : upgrade to 2.20 soon.
Data as of 30 Oct 2026, 07:30 UTC
- Patch
- Available – fixed in 2.20
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:HExploitation evidence
Disclosed: evidence found.
CVE published 12 Dec 2025
Detectable: no evidence on record.
No public nuclei template indexed
Weaponized: no evidence on record.
No public exploit code indexed in Metasploit or Exploit-DB
Exploited in the wildExploited: no evidence on record.
Not in CISA KEV and no source reports exploitation
Observed by OffSeq MirageSeen by Mirage: no evidence on record.
No attack traffic recorded by OffSeq Mirage sensors
A missing step is not evidence of safety. Sources: cve.org (opens in a new tab), the Metasploit, Exploit-DB and nuclei-templates index as of 29 Oct 2026.
What to do
Upgrade nvchecker to 2.20.
Affected software
| Product | Affected | Fixed in |
|---|---|---|
Homebrewnvcheckerpkg:brew/nvcheckerOther threats | Affected
| Fixed in
|
Check your own hosts for these packages with threat-finder (opens in a new tab), our free open-source scanner.
Description
## Summary The `HTTPHeaders.add` method in Tornado accumulates values using string concatenation when the same header name is repeated. Due to Python string immutability, each concatenation copies the entire string, resulting in O(n²) time complexity. Given Tornado's single event loop architecture, a single maliciously crafted HTTP request can block the server's event loop for an extended period, causing a Denial of Service (DoS). **Severity:** **High** if `max_header_size` has been increased from its default, **low** if it has its default value of 64KB.
References
- github.com/tornadoweb/tornado/security/advisories/GHSA-c98p-7wgm-6p64 (opens in a new tab)
- nvd.nist.gov/vuln/detail/CVE-2025-67725 (opens in a new tab)
- github.com/tornadoweb/tornado/commit/771472cfdaeebc0d89a9cc46e249f8891a6b29cd (opens in a new tab)
- github.com/pypa/advisory-database/tree/main/vulns/tornado/PYSEC-2025-266.yaml (opens in a new tab)
- github.com/tornadoweb/tornado (opens in a new tab)
- github.com/tornadoweb/tornado/releases/tag/v6.5.3 (opens in a new tab)