CVE-2019-16786: HTTP Request Smuggling: Invalid Transfer-Encoding in Waitress
Impact
Waitress would parse the Transfer-Encoding header and only look for a single string value, if that value was not chunked it would fall through and use the Content-Length header instead.
According to the HTTP standard Transfer-Encoding should be a comma separated list, with the inner-most encoding first, followed by any further transfer codings, ending with chunked.
Requests sent with:
Transfer-Encoding: gzip, chunked
Would incorrectly get ignored, and the request would use a Content-Length header instead to determine the body size of the HTTP message.
This could allow for Waitress to treat a single request as multiple requests in the case of HTTP pipelining.
Patches
This issue is fixed in Waitress 1.4.0. This brings a range of changes to harden Waitress against potential HTTP request confusions, and may change the behaviour of Waitress behind non-conformist proxies.
Waitress will now return a 501 Not Implemented error if the Transfer-Encoding is not chunked or contains multiple elements. Waitress does not support any transfer codings such as gzip or deflate.
The Pylons Project recommends upgrading as soon as possible, while validating that the changes in Waitress don't cause any changes in behavior.
Workarounds
Various reverse proxies may have protections against sending potentially bad HTTP requests to the backend, and or hardening against potential issues like this. If the reverse proxy doesn't use HTTP/1.1 for connecting to the backend issues are also somewhat mitigated, as HTTP pipelining does not exist in HTTP/1.0 and Waitress will close the connection after every single request (unless the Keep Alive header is explicitly sent... so this is not a fool proof security method).
Issues/more security issues:
open an issue at https://github.com/Pylons/waitress/issues (if not sensitive or security related) email the Pylons Security mailing list: pylons-project-security@googlegroups.com (if security related)
Other sources
An HTTP-interpretation flaw was found in waitress which did not properly validate incoming HTTP headers. When parsing the Transfer-Encoding header, waitress would look only for a single string value. According to the HTTP standard, Transfer-Encoding should be a comma-separated list, with the inner-most encoding first, followed by any further transfer codings, ending with 'chunked'. Because of this flaw, requests sent with: "Transfer-Encoding: gzip, chunked" would get ignored, and waitress would use the Content-Length header instead to determine the body size of the HTTP message. A remote attacker could exploit this flaw to force waitress to accept potentially bad HTTP requests or treat a single request as multiple requests in the case of HTTP pipelining.
Waitress through version 1.3.1 would parse the Transfer-Encoding header and only look for a single string value, if that value was not chunked it would fall through and use the Content-Length header instead. According to the HTTP standard Transfer-Encoding should be a comma separated list, with the inner-most encoding first, followed by any further transfer codings, ending with chunked. Requests sent with: "Transfer-Encoding: gzip, chunked" would incorrectly get ignored, and the request would use a Content-Length header instead to determine the body size of the HTTP message. This could allow for Waitress to treat a single request as multiple requests in the case of HTTP pipelining. This issue is fixed in Waitress 1.4.0.
Affected Software
Remediation
Patch Available
Event History
Parent advisories
This vulnerability appears in the following advisories.
Frequently Asked Questions
What is CVE-2019-16786?
CVE-2019-16786 is a vulnerability that affects Waitress through version 1.3.1.
What is the impact of CVE-2019-16786?
The impact of CVE-2019-16786 is that Waitress would parse the Transfer-Encoding header and only look for a single string value instead of a comma separated list as required by the HTTP standard.
How can CVE-2019-16786 be exploited?
CVE-2019-16786 can be exploited by sending a malicious Transfer-Encoding header that does not contain the 'chunked' value.
What is the severity of CVE-2019-16786?
The severity of CVE-2019-16786 is high with a severity value of 7.
How can I fix CVE-2019-16786?
To fix CVE-2019-16786, upgrade Waitress to version 1.4.0 or later.