CVE-2026-91991

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.

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.

Summary

The CVE-2026-35536 fix added a validation loop that rejects [\x00-\x20\x3b\x7f], but only for the hardcoded lowercase keys name/domain/path/samesite. The still-live deprecated **kwargs path writes attacker-supplied attribute values straight into the Morsel with no validation, and because Morsel.__setitem__ is case-insensitive, a capitalized kwarg (Domain=, Path=, SameSite=, Max-Age=) routes to the same reserved attribute while bypassing the loop — re-opening ;-delimited attribute injection.

self.set_cookie("sid", "abc", Domain="evil.com; Secure; SameSite=None")
#  -> Set-Cookie: sid=abc; Domain=evil.com; Secure; SameSite=None; Path=/
# Sanity (the canonical lowercase named arg IS blocked):
self.set_cookie("sid", "abc", domain="evil.com; Secure")   # -> http.cookies.CookieError

The patch's regression test (SetCookieForbiddenCharHandler) only exercises the four named params, never the **kwargs path, so the gap is not regression-covered.

Affected code

• tornado/web.py → RequestHandler.set_cookie: the validation loop covers only the lowercase named args; the trailing if kwargs: loop does morsel[k] = v with no character validation.

Steps to reproduce

GET /upper (uses Domain= kwarg) emits Set-Cookie: c_upper=v; Domain=evil.com; Secure; SameSite=None; Path=/; GET /lower (uses lowercase domain=) returns a CookieError.

Impact

Injection of independent cookie attributes (force/drop Secure/HttpOnly/SameSite, rebind Domain/Path) — the same impact CVE-2026-35536 closed, via the sibling path the patch missed. Conditional on the app using a capitalized/legacy keyword.

Suggested remediation

Apply the same [\x00-\x20\x3b\x7f] validation to every entry in the **kwargs loop (after normalizing the key case), or remove the deprecated kwargs path; add a regression test for capitalized kwargs.

Credit

Reported as part of an incomplete-patch measurement study (responsible disclosure).

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 6.5.8 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.

This update for python-tornado6 fixes the following issues:

• CVE-2026-82397: an unauthenticated request body containing millions of separator-delimited fields can synchronously stall the single-threaded event loop and delay every connection (bsc#1277725).
• CVE-2026-91990: splitting of multipart data before validation of the max_parts limit allows for resource exhaustion and a denial of service (bsc#1281439).
• CVE-2026-91991: incomplete fix for cookie attribute injection allows for cookie validation bypass via semicolon- delimited data in capitalized parameters (bsc#1281440).

This update for python-tornado6 fixes the following issues:

• CVE-2026-82397: an unauthenticated request body containing millions of separator-delimited fields can synchronously stall the single-threaded event loop and delay every connection (bsc#1277725).
• CVE-2026-91990: splitting of multipart data before validation of the max_parts limit allows for resource exhaustion and a denial of service (bsc#1281439).
• CVE-2026-91991: incomplete fix for cookie attribute injection allows for cookie validation bypass via semicolon- delimited data in capitalized parameters (bsc#1281440).

This update for python-tornado6 fixes the following issues:

• CVE-2026-82397: an unauthenticated request body containing millions of separator-delimited fields can synchronously stall the single-threaded event loop and delay every connection (bsc#1277725).
• CVE-2026-91990: splitting of multipart data before validation of the max_parts limit allows for resource exhaustion and a denial of service (bsc#1281439).
• CVE-2026-91991: incomplete fix for cookie attribute injection allows for cookie validation bypass via semicolon- delimited data in capitalized parameters (bsc#1281440).

This update for python-tornado6 fixes the following issues:

• CVE-2023-54397: Tornado before 6.3.3 contains an HTTP request smuggling vulnerability due to improper parsing of Content-Length headers accepting non-standard characters. At (bsc#1280689).
• CVE-2024-58384: CRLF injection in CurlAsyncHTTPClient headers (bsc#1281059).
• CVE-2026-91990: splitting of multipart data before validation of the max_parts limit allows for resource exhaustion and a denial of service (bsc#1281439).
• CVE-2026-91991: incomplete fix for cookie attribute injection allows for cookie validation bypass via semicolon- delimited data in capitalized parameters (bsc#1281440).

This update for python-tornado fixes the following issues:

• CVE-2023-54397: Tornado before 6.3.3 contains an HTTP request smuggling vulnerability due to improper parsing of Content-Length headers accepting non-standard characters. At (bsc#1280689).
• CVE-2024-58384: CRLF injection in CurlAsyncHTTPClient headers (bsc#1281059).
• CVE-2026-91990: splitting of multipart data before validation of the max_parts limit allows for resource exhaustion and a denial of service (bsc#1281439).
• CVE-2026-91991: incomplete fix for cookie attribute injection allows for cookie validation bypass via semicolon- delimited data in capitalized parameters (bsc#1281440).