CVE-2021-22898
TELNET stack contents disclosure
Project curl Security Advisory, May 26th 2021 - Permalink
VULNERABILITY
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://example.com -tNEW_ENV=a,bbbbbb (256 'b's)INFO
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
AFFECTED VERSIONS
- Affected versions: curl 7.7 to and including 7.76.1
- Not affected versions: curl < 7.7 and curl >= 7.77.0
- Introduced-in: https://github.com/curl/curl/commit/a1d6ad2610
Also note that libcurl is used by many applications, and not always advertised as such.
SOLUTION
Use sscanf() properly and only use properly filled-in
buffers.
RECOMMENDATIONS
A - Upgrade curl to version 7.77.0
B - Apply the patch to your local version
C - Avoid using CURLOPT_TELNETOPTIONS
TIMELINE
This issue was reported to the curl project on April 27, 2021.
This advisory was posted on May 26, 2021.
CREDITS
- Reported-by: Harry Sintonen
- Patched-by: Harry Sintonen
Thanks a lot!