[SECURITY ADVISORY] curl: Automatic referer leaks credentials

[Daniel Stenberg via curl-users]

Automatic referer leaks credentials
===================================

Project curl Security Advisory, March 31st 2021 -
[Permalink](https://curl.se/docs/CVE-2021-22876.html)

VULNERABILITY
-------------

libcurl does not strip off user credentials from the URL when automatically
populating the `Referer:` HTTP request header field in outgoing HTTP requests,
and therefore risks leaking sensitive data to the server that is the target of
the second HTTP request.

libcurl automatically sets the `Referer:` HTTP request header field in
outgoing HTTP requests if the `CURLOPT_AUTOREFERER` option is set. With the
curl tool, it is enabled with `--referer ";auto"`.

We are not aware of any exploit of this flaw.

INFO
----

This flaw has existed in libcurl since commit
[f30ffef477](https://github.com/curl/curl/commit/f30ffef477) in libcurl 7.1.1,
released on August 21, 2000.

The Common Vulnerabilities and Exposures (CVE) project has assigned the name
CVE-2021-22876 to this issue.

CWE-359: Exposure of Private Personal Information to an Unauthorized Actor

Severity: Low

AFFECTED VERSIONS
-----------------

- Affected versions: curl 7.1.1 to and including 7.75.0
- Not affected versions: curl < 7.1.1 and curl >= 7.76.0

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

THE SOLUTION
------------

If a provided URL contains credentials, they will be blanked out before the
URL is used to populate the header field.

A [fix for CVE-2021-22876](https://github.com/curl/curl/commit/7214288898f5625a6cc196e22a74232eada7861c)

(The patch URL will change in the final published version of this advisory)

RECOMMENDATIONS
--------------

We suggest you take one of the following actions immediately, in order of
preference:

A - Upgrade libcurl to version 7.76.0

B - Apply the patch to your local version

C - Provide the credentials with `-u` or `CURLOPT_USERPWD`

D - Avoid `CURLOPT_AUTOREFERER` and `--referer ";auto"`,

TIMELINE
--------

This issue was reported to the curl project on February 12, 2021.

This advisory was posted on March 31st 2021.

CREDITS
-------

This issue was reported and patched by Viktor Szakats.

Thanks a lot!