WordPress Plugin Caldera Forms 1.5.9.1 – Cross-Site Scripting

Authors:Federico Scalco      Risk:High

CVE:CVE-2018-7747           0day:Cross-Site Scripting

0day-id:0DAY-7747            Date:2018-04-19

Description

“Caldera Form is a free and powerful WordPress plugin that creates
responsive forms with a simple drag and drop editor.”
It is reported to have 100,000+ active installations at the moment of this
writing.

Overview

The application fails to validate user-supplied input, hence it stores the
unsanitized buffer in the database.
The vulnerabilities reported here will be exploitable ONLY if certain
conditions are met, which is not the case in a CF’s default configuration
(although still being vulnerable).

A note on buffers containing strings:
single (‘) and double (“) quotes are correctly escaped, backticks (`)
are not.

Details

3.a) Stored XSS – public
When submitting a CF form, the plugin will show a greeting message to
notify the user that everything went ok.
This message is editable by the site’s admin and can contain part of the
user-supplied data (e.g. they’re first name). In this case, simply inject
HTML code into the parameter which gets returned in the greeting message
and submit the POST request. A JSON response will follow, containing, among
other data:

- the greeting message ("html", which contains the malicious payload that
gets executed right away)
- form's ID ("form_id")
- data's ID ("cf_id")
 
{
    "data":{"cf_id":"<DATA_ID>"},
    "html":"<GREETING MESSAGE>",
    "type":"...",
    "form_id":"<FORM_ID>",
    "form_name":"...",
    "status":"..."
}

At this point, to reach the stored XSS, simply build a GET request using
the obtained data.
The malicious payload will be found at

http(s)://<target>/cf-api/<FORM_ID>/?cf_su=1&cf_id=<DATA_ID>

Vulnerable config:

– form > form settings > capture entries > checked (ON by default)
– form > form settings > success message > add some of the user
supplied fields (absent by default)

To replicate this on a fresh install

– Create a new, default, contact form
– Go to “Form Settings” tab and edit the success message to include,
for example, the user’s first name.
e.g.: Form has been successfully submitted. Thank you %first_name%.
– Save & publish
– As an unauthenticated user, submit the contact form injecting HTML
code in first name’s parameter. XSS will be triggered right away
– To recall the payload as a stored XSS, read the POST’s response and
point your browser to

<target>/cf-api/<FORM_ID>/?cf_su=1&cf_id=<DATA_ID>

3.b) Stored XSS – admin interface

CalderaForms gives the ability to notify the admin via email everytime a
form gets submitted.
Furthermore, an admin can choose to enable an “email transacion log” for
debugging purposes (disabled by default).
If this configuration is in place, a copy of the malicious payload
described above will be shown in the administration panel, when visiting
that form’s malicious entry’s details.

Vulnerable config:

– form > form settings > capture entries > checked (ON by default)
– form > email > debug mailer > checked (OFF by default)

To replicate this on a fresh install:

– Enable the transaction log (form -> edit -> email tab -> check
“Enable email send transaction log”)
– Replicate the injection described at 3.a (all fields can be used this
time) as an unauthenticated user
– Back again in the admin interface, visit form’s entries, identify the
malicious one and click on the “view” button

This will pop a details window and trigger the XSS.

3.c) Importing a weaponized form – admin interface

CalderaForms gives the ability to import a form (JSON format).
A malicious form field can be crafted which will trigger an XSS when said
field gets displayed/edited after the import.

It’s worth noting that this flaw does not depend on custom configurations,
although it’s not “remotely” and “automatically” exploitable. The problem
here arise, for example, when an admin imports a malicious JSON.

To replicate this on a fresh install

- Create a form and export it (JSON format)
    - Edit the json and inject HTML code. "label" and "slug" parameters
were tested, others may be vulnerable too.
    e.g.:
    {
        ...
            "label":"First<script>alert(1);</script> Name",
            "slug":"first_name\"/><script>alert(2);</script>"
        ...
    }

– Import the malicious form to trigger the XSS in the administration
interface

REMEDIATION

Update to the latest version available.

If any personalized configuration is found exploitable, the following steps
can be followed, as a temporary mitigation strategy, if no update is
available or updating is not an option, for whatever reason:
– for every form, under “Form Settings”, prune every variable that gets
returned to the user as a success message
– for every form, under the “Email” tab, un-check “Enable email send
transaction log”
– for every form that gets imported perform a thorough review

TIMELINE & FINAL NOTES

02-03-18 > vendor gets notified
06-03-18 > vendor replies
07-03-18 > CVE requested and assigned
27-03-18 > patch released
27-03-18 > vulnerability disclosed

Special thanks go to Josh Pollock and his team, from Caldera, who invested
passion and energy in understanding and patching these issues.

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