CVE-2020-26281

Async-h1 request smuggling possible with long unread bodies

Workarounds

none

References

https://github.com/http-rs/async-h1/releases/tag/v2.3.0

For more information

If you have any questions or comments about this advisory:

• Open an issue in async-h1
• Contact a core team member on zulip or discord

This vulnerability affects any webserver that uses async-h1 behind a reverse proxy, including all such Tide applications.

If the server does not read the body of a request which is longer than some buffer length, async-h1 will attempt to read a subsequent request from the body content starting at that offset into the body.

One way to exploit this vulnerability would be for an adversary to craft a request such that the body contains a request that would not be noticed by a reverse proxy, allowing it to forge forwarded/x-forwarded headers. If an application trusted the authenticity of these headers, it could be misled by the smuggled request.

Another potential concern with this vulnerability is that if a reverse proxy is sending multiple http clients' requests along the same keep-alive connection, it would be possible for the smuggled request to specify a long content and capture another user's request in its body. This content could be captured in a post request to an endpoint that allows the content to be subsequently retrieved by the adversary.

CVE-2020-26281 has a CVSS score of 6.8 (Medium). The vector is network-reachable, no 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 (2.3.0); upgrading removes the vulnerable code path.

Kodem intelligence

Already deployed Kodem?

This has been addressed in async-h1 2.3.0 and previous versions have been yanked.

1. What is CVE-2020-26281? CVE-2020-26281 is a medium-severity security vulnerability in async-h1 (rust), affecting versions < 2.3.0. It is fixed in 2.3.0.
2. How severe is CVE-2020-26281? CVE-2020-26281 has a CVSS score of 6.8 (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.
3. Which versions of async-h1 are affected by CVE-2020-26281? async-h1 (rust) versions < 2.3.0 is affected.
4. Is there a fix for CVE-2020-26281? Yes. CVE-2020-26281 is fixed in 2.3.0. Upgrade to this version or later.
5. Is CVE-2020-26281 exploitable, and should I be worried? Whether CVE-2020-26281 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
6. What actually determines whether CVE-2020-26281 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.
7. How do I fix CVE-2020-26281? Upgrade async-h1 to 2.3.0 or later.

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