CVE-2025-67725

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.

Database specific

{
    "cwe_ids": [
        "CWE-400"
    ],
    "osv_generated_from": "https://github.com/CVEProject/cvelistV5/tree/main/cves/2025/67xxx/CVE-2025-67725.json",
    "cna_assigner": "GitHub_M"
}

References

Affected packages

Git / github.com/tornadoweb/tornado

Affected ranges

Type: GIT
Repo: https://github.com/tornadoweb/tornado
Events: Introduced

0 Unknown introduced commit / All previous commits are affected

Fixed

38014ddb51520ff7762c1d55535925dba2cdbe31

Affected versions

v1.*

v1.0.0

v1.1.0

v1.1.1

v1.2.0

v1.2.1

v2.*

v2.0.0

v2.1.0

v2.1.1

v2.2.0

v2.2.1

v2.3.0

v2.4.0

v2.4.1

v3.*

v3.0.0

v3.0.1

v3.0.2

v3.1.0

v3.1.1

v3.2.0

v3.2.0b1

v3.2.0b2

v3.2.1

v3.2.2

v4.*

v4.0.0

v4.0.0b1

v4.0.0b2

v4.0.0b3

v4.0.1

v4.0.2

v4.1.0

v4.1.0b1

v4.1.0b2

v4.2.0

v4.2.0b1

v4.2.1

v4.3.0

v4.3.0b1

v4.3.0b2

v4.4.0

v4.4.0b1

v4.4.1

v4.4.2

v4.4.3

v4.5.0

v4.5.1

v4.5.2

v4.5.3

v5.*

v5.0.0

v5.0.1

v5.1.0

v5.1.0b1

v6.*

v6.0.0

v6.0.0b1

v6.1.0

v6.1.0b1

v6.1.0b2

v6.2.0

v6.2.0b1

v6.2.0b2

v6.3.0

v6.3.0b1

v6.3.1

v6.4.0

v6.4.0b1

v6.4.1

v6.5.0

v6.5.0b1

v6.5.1

v6.5.2

Database specific

source

"https://storage.googleapis.com/cve-osv-conversion/osv-output/CVE-2025-67725.json"