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Date: Mon, 20 Mar 2023 08:26:17 +0100 (CET)
From: Daniel Stenberg <>
To: curl security announcements -- curl users <>,, libcurl hacking <>,
Subject: [SECURITY ADVISORY] curl: CVE-2023-27537: HSTS double-free

CVE-2023-27537: HSTS double-free

Project curl Security Advisory, March 20th 2023 -


libcurl supports sharing HSTS data between separate "handles". This sharing was
introduced without considerations for do this sharing across separate threads
but there was no indication of this fact in the documentation.

Due to missing mutexes or thread locks, two threads sharing the same HSTS data
could end up doing a double-free or use-after-free.

We are not aware of any exploit of this flaw.


This feature was not implemented to support sharing between threads. That is
still left for future improvements. The fix for this issue is therefore a
documentation update clarifying that sharing HSTS between threads is not
expected to work.

CVE-2023-27537 was introduced in [commit
076a2f629119222a](, shipped
in curl 7.88.0.

CWE-415: Double Free

Severity: Low

Severity is set to Low because

  - Not widely used functionality
  - The timing necessary to trigger this has to match fairly exact
  - Exploitation this for anything but denial of service is difficult


- Affected versions: curl 7.88.0 to and including 7.88.1
- Not affected versions: curl < 7.88.0 and curl >= 8.0.0

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


A [fix for CVE-2023-27537](


  A - Do not share HSTS data between threads


This issue was reported to the curl project on March 8 2023. We contacted
distros@...nwall on March 13, 2023.

curl 8.0.0 was released on March 20 2023, coordinated with the publication of
this advisory.


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

Thanks a lot!


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