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Date: Wed, 31 Mar 2021 08:01:59 +0200 (CEST)
From: Daniel Stenberg <>
To: curl security announcements -- curl users <>,, libcurl hacking <>,
Subject: [SECURITY ADVISORY] curl: Automatic referer leaks credentials

Automatic referer leaks credentials

Project curl Security Advisory, March 31st 2021 -


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.


This flaw has existed in libcurl since 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: 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.


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](

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


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

  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"`,


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

This advisory was posted on March 31st 2021.


This issue was reported and patched by Viktor Szakats.

Thanks a lot!


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