CVE-2026-91992

Tornado before 6.5.7 contains a credential leak vulnerability in CurlAsyncHTTPClient where pycurl handles are reused across requests without proper state clearing. Attackers can obtain sensitive credentials by issuing requests through the same client instance, allowing TLS certificates or proxy authentication to persist across unintended requests.

Tornado before 6.5.7 contains a credential leak vulnerability in CurlAsyncHTTPClient where pycurl handles are reused across requests without proper state clearing. Attackers can obtain sensitive credentials by issuing requests through the same client instance, allowing TLS certificates or proxy authentication to persist across unintended requests.

CurlAsyncHTTPClient leaks per-request credentials on handle reuse

Summary

CurlAsyncHTTPClient pools and reuses pycurl handles across requests but does not reset them between requests, and several per-request options are applied with no clearing branch. As a result, sensitive state set by one request persists onto a later request on the same client that does not set it. Two credential vectors are demonstrated below — a client TLS certificate (SSLCERT/SSLKEY) and proxy basic-auth credentials (PROXYUSERPWD) — both leaking to a different, unintended host. This affects all released versions through 6.5.6.

Details

In tornado/curl_httpclient.py, handles are created once and returned to a free list for reuse (_process_queue pops the handle at line 200, _finish re-appends it at line 245), and _curl_setup_request is never preceded by curl.reset(). The function clears some carried-over state on the reused handle — unsetopt(PROXYUSERPWD) in the no-proxy branch (line 394), unsetopt(USERPWD) when no auth is set (line 495), and the HTTP-method flag reset (lines 428-432) — but other options have no equivalent clearing path and persist until a later request sets them again.

Vector A — client TLS certificate (SSLCERT/SSLKEY). Set-only, no clearing branch:

# tornado/curl_httpclient.py (v6.5.6), lines 498-502
if request.client_cert is not None:
    curl.setopt(pycurl.SSLCERT, request.client_cert)

if request.client_key is not None:
    curl.setopt(pycurl.SSLKEY, request.client_key)

A request that sets client_cert leaves the certificate on the handle; a later request without client_cert presents it during its TLS handshake.

Vector B — proxy credentials (PROXYUSERPWD). PROXYUSERPWD is set only inside the credentials branch and unset only in the no-proxy else branch:

# tornado/curl_httpclient.py (v6.5.6), lines 371-394
if request.proxy_host and request.proxy_port:
    curl.setopt(pycurl.PROXY, request.proxy_host)
    curl.setopt(pycurl.PROXYPORT, request.proxy_port)
    if request.proxy_username:                 # only place PROXYUSERPWD is set
        ...
        curl.setopt(pycurl.PROXYUSERPWD, credentials)
    ...
else:
    try:
        curl.unsetopt(pycurl.PROXY)
    except TypeError:
        curl.setopt(pycurl.PROXY, "")
    curl.unsetopt(pycurl.PROXYUSERPWD)         # only place it is unset

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():
   …

Tornado is a Python web framework and asynchronous networking library, originally developed at FriendFeed. By using non-blocking network I/O, Tornado can scale to tens of thousands of open connections, making it ideal for long polling, WebSockets, and other applications that require a long-lived connection to each user.

Security Fix(es):

Tornado before 6.3.3 contains an HTTP request smuggling vulnerability due to improper parsing of Content-Length headers accepting non-standard characters. Attackers can send crafted HTTP requests with these characters to bypass proxy validation and smuggle requests when deployed behind certain proxies.(CVE-2023-54397)

Tornado before 6.4.1 ignores duplicate Transfer-Encoding: chunked headers, treating requests as having no message body and parsing the chunked body as a subsequent request. Attackers can exploit this inconsistency when Tornado is deployed behind proxies to perform HTTP request smuggling, enabling access control bypass, cache poisoning, or connection desynchronization.(CVE-2024-14029)

Tornado before 6.4.1 contains a CRLF injection vulnerability in CurlAsyncHTTPClient that fails to reject carriage return and line feed characters in request headers. Attackers can inject CRLF sequences into header values to inject arbitrary headers or construct entirely new HTTP requests.(CVE-2024-58384)

Tornado before 6.5.8 contains a memory amplification vulnerability in parse_multipart_form_data that splits multipart data before validating the max_parts limit. Attackers can send crafted multipart requests with many parts to create large transient lists, exhausting server memory and causing denial of service.(CVE-2026-91990)

Tornado before 6.5.8 contains an incomplete fix for cookie attribute injection that allows attackers to inject arbitrary cookie attributes by passing capitalized or legacy keyword arguments to set_cookie. Attackers can embed semicolon-delimited data in capitalized parameters like Domain, Path, or SameSite to bypass validation and modify cookie security attributes.(CVE-2026-91991)

Tornado before 6.5.7 contains a credential leak vulnerability in CurlAsyncHTTPClient where pycurl handles are reused across requests without proper state clearing. Attackers can obtain sensitive credentials by issuing requests through the same client instance, allowing TLS certificates or proxy authentication to persist across unintended requests.(CVE-2026-91992)

Tornado is a Python web framework and asynchronous networking library, originally developed at FriendFeed. By using non-blocking network I/O, Tornado can scale to tens of thousands of open connections, making it ideal for long polling, WebSockets, and other applications that require a long-lived connection to each user.

Security Fix(es):

Tornado before 6.3.3 contains an HTTP request smuggling vulnerability due to improper parsing of Content-Length headers accepting non-standard characters. Attackers can send crafted HTTP requests with these characters to bypass proxy validation and smuggle requests when deployed behind certain proxies.(CVE-2023-54397)

Tornado before 6.4.1 ignores duplicate Transfer-Encoding: chunked headers, treating requests as having no message body and parsing the chunked body as a subsequent request. Attackers can exploit this inconsistency when Tornado is deployed behind proxies to perform HTTP request smuggling, enabling access control bypass, cache poisoning, or connection desynchronization.(CVE-2024-14029)

Tornado before 6.4.1 contains a CRLF injection vulnerability in CurlAsyncHTTPClient that fails to reject carriage return and line feed characters in request headers. Attackers can inject CRLF sequences into header values to inject arbitrary headers or construct entirely new HTTP requests.(CVE-2024-58384)

Tornado before 6.5.8 contains a memory amplification vulnerability in parse_multipart_form_data that splits multipart data before validating the max_parts limit. Attackers can send crafted multipart requests with many parts to create large transient lists, exhausting server memory and causing denial of service.(CVE-2026-91990)

Tornado before 6.5.8 contains an incomplete fix for cookie attribute injection that allows attackers to inject arbitrary cookie attributes by passing capitalized or legacy keyword arguments to set_cookie. Attackers can embed semicolon-delimited data in capitalized parameters like Domain, Path, or SameSite to bypass validation and modify cookie security attributes.(CVE-2026-91991)

Tornado before 6.5.7 contains a credential leak vulnerability in CurlAsyncHTTPClient where pycurl handles are reused across requests without proper state clearing. Attackers can obtain sensitive credentials by issuing requests through the same client instance, allowing TLS certificates or proxy authentication to persist across unintended requests.(CVE-2026-91992)

Tornado is a Python web framework and asynchronous networking library, originally developed at FriendFeed. By using non-blocking network I/O, Tornado can scale to tens of thousands of open connections, making it ideal for long polling, WebSockets, and other applications that require a long-lived connection to each user.

Security Fix(es):

Tornado before 6.3.3 contains an HTTP request smuggling vulnerability due to improper parsing of Content-Length headers accepting non-standard characters. Attackers can send crafted HTTP requests with these characters to bypass proxy validation and smuggle requests when deployed behind certain proxies.(CVE-2023-54397)

Tornado before 6.4.1 ignores duplicate Transfer-Encoding: chunked headers, treating requests as having no message body and parsing the chunked body as a subsequent request. Attackers can exploit this inconsistency when Tornado is deployed behind proxies to perform HTTP request smuggling, enabling access control bypass, cache poisoning, or connection desynchronization.(CVE-2024-14029)

Tornado before 6.4.1 contains a CRLF injection vulnerability in CurlAsyncHTTPClient that fails to reject carriage return and line feed characters in request headers. Attackers can inject CRLF sequences into header values to inject arbitrary headers or construct entirely new HTTP requests.(CVE-2024-58384)

Tornado before 6.5.8 contains a memory amplification vulnerability in parse_multipart_form_data that splits multipart data before validating the max_parts limit. Attackers can send crafted multipart requests with many parts to create large transient lists, exhausting server memory and causing denial of service.(CVE-2026-91990)

Tornado before 6.5.8 contains an incomplete fix for cookie attribute injection that allows attackers to inject arbitrary cookie attributes by passing capitalized or legacy keyword arguments to set_cookie. Attackers can embed semicolon-delimited data in capitalized parameters like Domain, Path, or SameSite to bypass validation and modify cookie security attributes.(CVE-2026-91991)

Tornado before 6.5.7 contains a credential leak vulnerability in CurlAsyncHTTPClient where pycurl handles are reused across requests without proper state clearing. Attackers can obtain sensitive credentials by issuing requests through the same client instance, allowing TLS certificates or proxy authentication to persist across unintended requests.(CVE-2026-91992)

Tornado is a Python web framework and asynchronous networking library, originally developed at FriendFeed. By using non-blocking network I/O, Tornado can scale to tens of thousands of open connections, making it ideal for long polling, WebSockets, and other applications that require a long-lived connection to each user.

Security Fix(es):

Tornado before 6.3.3 contains an HTTP request smuggling vulnerability due to improper parsing of Content-Length headers accepting non-standard characters. Attackers can send crafted HTTP requests with these characters to bypass proxy validation and smuggle requests when deployed behind certain proxies.(CVE-2023-54397)

Tornado before 6.4.1 ignores duplicate Transfer-Encoding: chunked headers, treating requests as having no message body and parsing the chunked body as a subsequent request. Attackers can exploit this inconsistency when Tornado is deployed behind proxies to perform HTTP request smuggling, enabling access control bypass, cache poisoning, or connection desynchronization.(CVE-2024-14029)

Tornado before 6.4.1 contains a CRLF injection vulnerability in CurlAsyncHTTPClient that fails to reject carriage return and line feed characters in request headers. Attackers can inject CRLF sequences into header values to inject arbitrary headers or construct entirely new HTTP requests.(CVE-2024-58384)

When SimpleAsyncHTTPClient follows a 3xx redirect, it shallow-copies the original HTTPRequest, rewrites the URL, decrements max_redirects, and removes only the Host header. It does not clear Authorization, auth_username, auth_password, or auth_mode when the redirect target changes origin. As a result, credentials intended for one origin can be forwarded to a different origin when follow_redirects=True, which is the default. Beginning in Tornado 6.5.6, SimpleAsyncHTTPClient matches the default behavior of libcurl (and therefore CurlAsyncHTTPClient): When a redirect changes the scheme, host, or port of the url, the Authorization and Cookie headers will be removed when following the redirect.(CVE-2026-49853)

SummaryTornado's optional native extension tornado.speedups implements websocket_mask without validating that the mask argument is exactly four bytes long. The C function reads four bytes from mask unconditionally, even when Python passes a shorter byte string. This can read beyond the provided buffer, exposing up to 3 bytes of uninitialized memory.The behavior is reachable from Tornado's XSRF token decoder when xsrf_cookies=True and the native extension is active. ### MitigationsThis bug is fixed in Tornado 6.5.6. Prior to upgrading to this version, setting the environment variable TORNADO_EXTENSION=0 will disable the vulnerable code (at the expense of reducing websocket performance).(CVE-2026-49854)

Tornado's gzip decompression routines work in limited-size chunks, but have no overall limit for the total size of decompressed chunks that they will accumulate (There has always been a limit for the total compressed size). This allows a malicious server to consume effectively unlimited amounts of memory if it is accessed via SimpleAsyncHTTPClient in its default configuration. HTTPServer is not affected in its default configuration, but it is if decompress_request=True is set. This bug is fixed in Tornado 6.5.6. max_body_size is now checked both for the compressed and cumulative decompressed size of the response.(CVE-2026-49855)

Tornado is a Python web framework and asynchronous networking library. Prior to 6.5.8, Tornado parses application/x-www-form-urlencoded request bodies with urllib.parse.parse_qs in tornado/escape.py without passing max_num_fields. RequestHandler._execute in tornado/web.py parses the body before handler dispatch through HTTPServerRequest._parse_body and parse_body_arguments in tornado/httputil.py, so an unauthenticated request body containing millions of separator-delimited fields can synchronously stall the single-threaded event loop and…

Tornado before 6.5.7 contains a credential leak vulnerability in CurlAsyncHTTPClient where pycurl handles are reused across requests without proper state clearing. Attackers can obtain sensitive credentials by issuing requests through the same client instance, allowing TLS certificates or proxy authentication to persist across unintended requests.

Tornado before 6.5.7 contains a credential leak vulnerability in CurlAsyncHTTPClient where pycurl handles are reused across requests without proper state clearing. Attackers can obtain sensitive credentials by issuing requests through the same client instance, allowing TLS certificates or proxy authentication to persist across unintended requests.

TuxCare rebuilt tornado to address 3 CVEs. Fixed in 6.1.0.post4+tuxcare.

TuxCare rebuilt tornado to address 15 CVEs. Fixed in 5.1.1.post4+tuxcare.

Root has patched GHSA-pw6j-qg29-8w7f in the tornado package for Root:PyPI. Multiple fixed versions available.

It was discovered that Tornado did not properly handle certain HTTP requests and network inputs. An attacker could possibly use these issues to cause a denial of service, expose sensitive information, or bypass access controls.

(CVE-2023-54397, CVE-2024-14029, CVE-2026-91990, CVE-2026-91992, CVE-2026-103262, CVE-2026-103263)