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CVE

CVE-2025-67725

Tornado is Vulnerable to Quadratic DoS via Repeated Header Coalescing
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CVE

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 maxheadersize 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.

Package Versions Affected

Package Version
patch Availability
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CVSS Version

Severity
Base Score
CVSS Version
Score Vector
C
H
U
7.5
-
3.1
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
C
H
U
0
-
3.1
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
C
H
U
-

Related Resources

No items found.

References

https://github.com/CVEProject/cvelistV5/tree/main/cves/2025/67xxx/CVE-2025-67725.json, https://github.com/tornadoweb/tornado/commit/771472cfdaeebc0d89a9cc46e249f8891a6b29cd, https://github.com/tornadoweb/tornado/releases/tag/v6.5.3, https://github.com/tornadoweb/tornado/security/advisories/GHSA-c98p-7wgm-6p64, https://nvd.nist.gov/vuln/detail/CVE-2025-67725

Severity

7.5

CVSS Score
0
10

Basic Information

Ecosystem
Base CVSS
7.5
EPSS Probability
0.00282%
EPSS Percentile
0.51115%
Introduced Version
0
Fix Available
38014ddb51520ff7762c1d55535925dba2cdbe31,0:6.4.0-1ubuntu0.4,0:4.5.3-1ubuntu0.2+esm2,0:6.0.3+really5.1.1-3ubuntu0.1~esm3,0:6.1.0-3ubuntu0.1~esm4,0:6.1.0-2.amzn2023.0.6,0:4.2.1-3.amzn2.0.3,0:5.0.2-4.amzn2.0.6

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