Summary
HTTP Request Smuggling: LF vs CRLF handling in Waitress
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: [email protected] (if security related)
Impact
Waitress implemented a "MAY" part of the RFC7230 (https://tools.ietf.org/html/rfc7230#section-3.5) which states:
Although the line terminator for the start-line and header fields is
the sequence CRLF, a recipient MAY recognize a single LF as a line
terminator and ignore any preceding CR.
Unfortunately if a front-end server does not parse header fields with an LF the same way as it does those with a CRLF it can lead to the front-end and the back-end server parsing the same HTTP message in two different ways. This can lead to a potential for HTTP request smuggling/splitting whereby Waitress may see two requests while the front-end server only sees a single HTTP message.
Example:
Content-Length: 100[CRLF]
X-Header: x[LF]Content-Length: 0[CRLF]
Would get treated by Waitress as if it were:
Content-Length: 100
X-Header: x
Content-Length: 0
This could potentially get used by attackers to split the HTTP request and smuggle a second request in the body of the first.
CVE-2019-16785 has a CVSS score of 7.1 (Medium). The vector is network-reachable, low 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 (1.4.0); upgrading removes the vulnerable code path.
Affected versions
Security releases
Kodem intelligence
Severity tells you how bad this could be in the worst case. It does not tell you whether you are exposed. Exploitability and impact are functions of runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A vulnerable package can sit in your dependency tree and never run.
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. Kodem's runtime-powered SCA identifies whether this CVE is reachable in your applications.
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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 no longer implements the MAY part of the specification and instead requires that all lines are terminated correctly with CRLF. If any lines are found with a bare CR or LF a 400 Bad Request is sent back to the requesting entity.
The Pylons Project recommends upgrading as soon as possible, while validating that the changes in Waitress don't cause any changes in behavior.
Frequently Asked Questions
- What is CVE-2019-16785? CVE-2019-16785 is a medium-severity security vulnerability in waitress (pip), affecting versions < 1.4.0. It is fixed in 1.4.0.
- How severe is CVE-2019-16785? CVE-2019-16785 has a CVSS score of 7.1 (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.
- Which versions of waitress are affected by CVE-2019-16785? waitress (pip) versions < 1.4.0 is affected.
- Is there a fix for CVE-2019-16785? Yes. CVE-2019-16785 is fixed in 1.4.0. Upgrade to this version or later.
- Is CVE-2019-16785 exploitable, and should I be worried? Whether CVE-2019-16785 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
- What actually determines whether CVE-2019-16785 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.
- How do I fix CVE-2019-16785? Upgrade
waitressto 1.4.0 or later.