GHSA-c98p-7wgm-6p64: tornado (CVE-2025-67725)

Package	Ecosystem	Affected	Fixed in
tornado	PyPI	<6.5.3	6.5.3

Machine-written summary

Summary written by Claude Haiku from the source advisory. Not reviewed by a person.

Tornado versions before 6.5.3 are affected. The HTTPHeaders.add method joins repeated values of the same header name by string concatenation, which becomes quadratic in time as the number of repeats grows. An attacker who can send HTTP requests to the server can send one crafted request that blocks the event loop for an extended time, causing denial of service. The severity depends on max_header_size: it is rated high if that limit has been raised above its 64KB default, and low at the default. Upgrade to Tornado 6.5.3 or later.

Written 5 Nov 2026.

Not analysed yet

We haven’t published our analysis of this Advisory yet. Once a person has reviewed it, this page will say what’s vulnerable, what has to be true for your app to be affected, and how to check.

Severity and exploitation

Score	Rating	CVSS	Vector	Source
7.5	High	3.1	CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H	CNA (GitHub_M)

CISA KEV Not listed.
EPSS 0.0060, the 49th percentile, on 15 Nov 2026: FIRST’s estimate of the chance of exploitation in the next 30 days.

Timeline

31 Oct 2026, 14:42 UTC First seen by agentprime
31 Oct 2026, 14:42 UTC Confirmed by OSV

Sources

Facts on this page come from the GitHub Advisory Database (CC-BY 4.0), OSV and the other sources listed, as published. We don’t reproduce full advisory descriptions, which can contain exploit code.