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TELNET stack contents disclosure

Project curl Security Advisory, May 26th 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 check the return code from a sscanf(command, "%127[^,],%127s") function invoke correctly, and would leave the piece of the send buffer uninitialized for the value part if it was provided longer than 127 bytes. The buffer used for this is 2048 bytes big and the variable part of the variable=content pairs would be stored correctly in the send buffer, making curl sending "interleaved" bytes sequences of stack contents. A single curl TELNET handshake could then be made to send off a total of around 1800 bytes of (non-contiguous) stack contents in this style:

[control byte]name[control byte]
stack contents
[control byte]name[control byte]
stack contents

An easy proof of concept command line looks like this:

curl telnet:// -tNEW_ENV=a,bbbbbb (256 'b's)


The Common Vulnerabilities and Exposures (CVE) project has assigned the name CVE-2021-22898 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 - Upgrade curl to version 7.77.0

B - Apply the patch to your local version



This issue was reported to the curl project on April 27, 2021.

This advisory was posted on May 26, 2021.


Thanks a lot!