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Date: Wed, 21 Dec 2022 08:23:58 +0100 (CET)
From: Daniel Stenberg <>
To: curl security announcements -- curl users <>,, libcurl hacking <>,
Subject: curl: CVE-2022-43551: Another HSTS bypass via IDN

CVE-2022-43551: Another HSTS bypass via IDN

Project curl Security Advisory, December 21 2022 -


curl's HSTS check could be bypassed to trick it to keep using HTTP.

Using its HSTS support, curl can be instructed to use HTTPS instead of using
an insecure clear-text HTTP step even when HTTP is provided in the URL.

The HSTS mechanism could be bypassed if the host name in the given URL first
uses IDN characters that get replaced to ASCII counterparts as part of the IDN
conversion. Like using the character UTF-8 U+3002 (IDEOGRAPHIC FULL STOP)
instead of the common ASCII full stop (U+002E). Then in a subsequent request,
it does not detect the HSTS state and makes a clear text transfer. Because it
would store the info IDN encoded but look for it IDN decoded.

Reproducible like this:

     curl --hsts hsts.txt https://curl%E3%80%82se
     curl --hsts hsts.txt http://curl%E3%80%82se

We are not aware of any exploit of this flaw.


This flaw was introduced in [commit
7385610d0c7](, which was
shipped enabled by default from [commit
d71ff2b9db566b3f]( in
curl 7.77.0.

This issue is similar to both the previous issues
[CVE-2022-42916]( and

This became a new separate vulnerability simply because we did not properly
test and research related side-issues while we worked on fixing the previous

The Common Vulnerabilities and Exposures (CVE) project has assigned the name
CVE-2022-43551 to this issue.

CWE-319: Cleartext Transmission of Sensitive Information

Severity: Medium


- Affected versions: curl 7.77.0 to and including 7.86.0
- Not affected versions: curl < 7.77.0 and curl >= 7.86.0

curl built without IDN support is not vulnerable.

libcurl is used by many applications, but not always advertised as such!


A [fix for CVE-2022-43551](


  A - Upgrade curl to version 7.87.0

  B - Apply the patch to your local version

  C - Stick to always using `HTTPS://` in URLs


This issue was reported to the curl project on October 29, 2022. We contacted
distros@...nwall on December 12, 2022.

curl 7.87.0 was released on December 21 2022, coordinated with the publication
of this advisory.


- Reported-by: Hiroki Kurosawa
- Patched-by: Daniel Stenberg

Thanks a lot!


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