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TELNET stack contents disclosure again

Project curl Security Advisory, July 21st 2021 - Permalink


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 remained.

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


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.


Thanks a lot!