libcurl: wrong connect-only connection
Project curl Security Advisory, August 19th 2020 - Permalink
An application that performs multiple requests with libcurl's multi API and
CURLOPT_CONNECT_ONLY option, might in rare circumstances experience
that when subsequently using the setup connect-only transfer, libcurl will
pick and use the wrong connection - and instead pick another one the
application has created since then.
CURLOPT_CONNECT_ONLY is the option to tell libcurl to not perform an actual
transfer, only connect. When that operation is completed, libcurl remembers
which connection it used for that transfer and "easy handle". It remembers the
connection using a pointer to the internal
connectdata struct in memory.
If more transfers are then done with the same multi handle before the connect-only connection is used, leading to the initial connect-only connection to get closed (for example due to idle time-out) while also new transfers (and connections) are setup, such a new connection might end up getting the exact same memory address as the now closed connect-only connection.
If after those operations, the application then wants to use the original
transfer's connect-only setup to for example use
curl_easy_send() to send
raw data over that connection, libcurl could erroneously find an existing
connection still being alive at the address it remembered since before even
though this is now a new and different connection.
The application could then accidentally send data over that connection which wasn't at all intended for that recipient, entirely unknowingly.
We are not aware of any exploit of this flaw.
This flaw cannot trigger for users of the curl tool but only for applications
using libcurl and the
The flaw only happens if the exact same memory address is re-used again for the new connection as for the original connect-only connection.
The Common Vulnerabilities and Exposures (CVE) project has assigned the name CVE-2020-8231 to this issue.
CWE-825: Expired Pointer Dereference
- Affected versions: libcurl 7.29.0 to and including 7.71.1
- Not affected versions: libcurl < 7.29.0 and libcurl >= 7.72.0
We suggest you take one of the following actions immediately, in order of preference:
A - Upgrade curl to version 7.72.0
B - Apply the patch on your curl version and rebuild
C - Do not use
This issue was first reported to the curl project on July 31, 2020.
This advisory was posted on August 19th 2020.
This issue was reported by Marc Aldorasi. Patched by Daniel Stenberg.
Thanks a lot!