CVE-2025-67725 | Tornado is Vulnerable to Quadratic DoS via Repeated Header Coalescing

Tornado is a Python web framework and asynchronous networking library. In versions 6.5.2 and below, a single maliciously crafted HTTP request can block the server's event loop for an extended period, caused by the HTTPHeaders.add method. The function accumulates values using string concatenation when the same header name is repeated, causing a Denial of Service (DoS). Due to Python string immutability, each concatenation copies the entire string, resulting in O(n²) time complexity. The severity can vary from high if max_header_size has been increased from its default, to low if it has its default value of 64KB. This issue is fixed in version 6.5.3.

Published: 2025-12-12 Last update: 2025-12-22 Assigner: [email protected] Source: [email protected]
NVD Status：Analyzed，CVE State: published
View at NVD, CVE.org, EUVD

CVE-2025-67725 is rated Moderate Risk (45.2/100): CVSS High severity, with low exploitation likelihood (EPSS 0.21%). Review affected assets and schedule remediation.

Risk is dynamic; we continuously reassess and refresh what is shown on this page as upstream context changes.

Exploit prediction scoring system (EPSS) score for CVE-2025-67725

EPSS lead: Daily EPSS estimates relative likelihood of exploitation; percentile ranks this CVE among scored vulnerabilities (higher = more severe relative rank).

# Date Old EPSS score New EPSS score Delta (New - Old)
1 2026-05-22 0.27% 0.21% -0.05%
2 2026-05-10 0.40% 0.27% -0.14%
3 2026-04-13 — 0.40% —

Full EPSS history (9 records total)

Common vulnerability scoring system (CVSS) metrics for CVE-2025-67725

CVSS metrics for this CVE.

Base score Version Severity Vector Exploitability Impact Score source
7.5 3.1 HIGH
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H Click to expand
Attack vector (AV:N)
Could be attacked over the internet or any normal routed network—not just someone sitting at the machine.
Attack complexity (AC:L)
Once they can reach the bug, pulling it off is straightforward—no weird race conditions or rare setup.
Privileges required (PR:N)
No account or special rights needed—anonymous or random user is enough.
User interaction (UI:N)
Nobody has to click “OK” or open a trap file; it can work without a victim helping.
Scope (S:U)
Damage stays in the same “trust bubble” as the broken component—no big spill into unrelated systems.
Confidentiality (C:N)
Doesn’t really leak secrets in a meaningful way.
Integrity (I:N)
Data isn’t meaningfully altered or forged.
Availability (A:H)
Could take the service down hard or make it unusable for people who depend on it.
 3.9 3.6 [email protected]

Weakness enumeration for CVE-2025-67725

OS Trackers for CVE-2025-67725

vendor priority summary link
alpine high CVE-2025-67725: 1 source package rows (py3-tornado); 1 state rows across 1 repos (edge-community); fixed 1, open 0. https://security.alpinelinux.org/vuln/CVE-2025-67725
debian not yet assigned CVE-2025-67725 not yet assigned priority: Debian including 1 source packages (python-tornado), 5 status rows across 5 suites (bookworm, bullseye, forky, sid, trixie): resolved 5. https://security-tracker.debian.org/tracker/CVE-2025-67725
redhat medium — https://access.redhat.com/security/cve/CVE-2025-67725
suse high — https://www.suse.com/security/cve/CVE-2025-67725/
ubuntu medium CVE-2025-67725 medium priority: Ubuntu including 1 source packages (python-tornado), 8 status rows across 8 suites (bionic, focal, jammy, noble, plucky, questing, upstream, xenial): released 7, needs-triage 1. https://ubuntu.com/security/CVE-2025-67725

Affected software / configurations for CVE-2025-67725

Vendor Product Version Raw CPE
tornadoweb tornado < 6.5.3 cpe:2.3:a:tornadoweb:tornado:*:*:*:*:*:*:*:*

References for CVE-2025-67725

cvelogic Threat Intelligence