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Date: Wed, 21 Jul 2021 09:15:24 +0200 (CEST)
From: Daniel Stenberg <>
To: curl security announcements -- curl users <>,, libcurl hacking <>,
Subject: [SECURITY ADVISORY] curl: TELNET stack contents disclosure again

TELNET stack contents disclosure again

Project curl Security Advisory, July 21st 2021 -


curl supports the `-t` command line option, known as `CURLOPT_TELNETOPTIONS`
in libcurl. This rarely used option is used to send variable=content pairs to
TELNET servers.

Due to flaw in the option parser for sending `NEW_ENV` variables, libcurl
could be made to pass on uninitialized data from a stack based buffer to the
server. Therefore potentially revealing sensitive internal information to the
server using a clear-text network protocol.

This could happen because curl did not call and use sscanf() correctly when
parsing the string provided by the application.

The previous curl security vulnerability
[CVE-2021-22898]( is almost identical
to this one but the fix was insufficient so this security vulnerability

We are not aware of any exploit of this flaw.


This flaw has existed in curl since commit
[a1d6ad2610]( in libcurl 7.7,
released on March 22, 2001. There was a previous attempt to fix this issue in
curl 7.77.0 but it was not done proper.

The Common Vulnerabilities and Exposures (CVE) project has assigned the name
CVE-2021-22925 to this issue.

CWE-457: Use of Uninitialized Variable

Severity: Medium


- Affected versions: curl 7.7 to and including 7.77.0
- Not affected versions: curl < 7.7 and curl >= 7.78.0

Also note that libcurl is used by many applications, and not always advertised
as such.


Use sscanf() properly and only use properly filled-in buffers.

A [fix for CVE-2021-22925](


  A - Upgrade curl to version 7.78.0

  B - Apply the patch to your local version



This issue was reported to the curl project on June 11, 2021.

This advisory was posted on July 21, 2021.


This issue was reported and patched by Red Hat Product Security.

Thanks a lot!


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