high CVSS 7.5

CVE-2025-67725·Python vulnerability

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.

Severity
high
Software
Python
Fixed in
6.5.3
Published
2025-12-12

Affected versions

Until: 6.5.3

Fixed in: 6.5.3

How to fix this CVE

Update Tornado to version 6.5.3 or later to resolve a quadratic time complexity vulnerability in HTTP header processing. This vulnerability allows attackers to craft malicious HTTP requests with repeated headers that consume excessive server resources, degrading performance. Apply the patch immediately if you are running Tornado 6.5.2 or earlier in a production environment.

sudo dnf update python3-tornado

Defensia detects this vulnerability

What an exploitation attempt looks like

Sample log line indicative of exploitation attempts:

HTTP requests containing multiple instances of the same header name with increasing or large cumulative values (e.g., repeated 'X-Custom-Header' fields totaling near or exceeding max_header_size limit)

WAF mitigation (if patching is not yet possible)

Add this rule to your WAF to block exploitation attempts while you schedule the patch. 

Configure WAF rules to rate-limit or block HTTP requests containing excessive repeated header fields or unusually large header sections. Set max_header_size appropriately in Tornado configuration and enforce header count limits at the ingress layer.

How to check if you are affected

  1. Check the installed Tornado version by running: python3 -c "import tornado; print(tornado.version)"
  2. Verify if Tornado is actively running as part of your application stack by checking running processes: ps aux | grep -i tornado
  3. Search application logs for repeated HTTP header patterns or request processing delays: grep -i 'header' /var/log/application/*.log | head -100
  4. Confirm the fix by re-running the version check after updating and verifying the output shows 6.5.3 or higher

FAQ

What is CVE-2025-67725?

CVE-2025-67725 is a denial-of-service vulnerability in Tornado's HTTPHeaders.add method that causes O(n²) time complexity when processing repeated header names. An attacker can craft HTTP requests with duplicate headers to exhaust server CPU cycles and block the event loop.

Is CVE-2025-67725 being actively exploited?

No, this vulnerability is not currently listed in the CISA Known Exploited Vulnerabilities (KEV) catalog and no public exploits are available.

What versions of Python/Tornado are affected by CVE-2025-67725?

Tornado versions 6.5.2 and earlier are affected. The vulnerability is fixed in Tornado 6.5.3. Note that this affects the Tornado framework, not Python itself, though Python 3 environments are the primary deployment target.

How do I check if my server is vulnerable to CVE-2025-67725?

Run 'python3 -c "import tornado; print(tornado.version)"' and compare the output to 6.5.3. If your version is 6.5.2 or lower, your server is vulnerable.

Does Defensia detect CVE-2025-67725?

Yes — Defensia's CVE advisory scanner compares installed package versions against the NVD database. If Tornado is installed on a monitored server, CVE-2025-67725 will appear in your dashboard with remediation steps.

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References

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