Advisory ID: SYSS-2019-016
Manufacturer: The SquirrelMail Project
Affected Version(s): 1.4.22, SVN
Tested Version(s): SVN
Vulnerability Type: Cross-Site Scripting (CWE-79)
Risk Level: Medium
Solution Status: Open
Manufacturer Notification: 2019-04-17
Solution Date: N/A
Public Disclosure: 2019-07-01
CVE Reference: CVE-2019-12970
Author of Advisory: Moritz Bechler, SySS GmbH
SquirrelMail is a open-source webmail package.
The manufacturer describes the product as follows (see ):
"SquirrelMail is a standards-based webmail package written in PHP.
It includes built-in pure PHP support for the IMAP and SMTP protocols,
for maximum compatibility across browsers.
It has very few requirements and is very easy to configure and install.
SquirrelMail has all the functionality you would want from an email
client, including strong MIME support, address books, and folder
Due to insufficient HTML sanitization SquirrelMail is vulnerable to
Cross-Site-Scripting when viewing HTML mails.
When viewing e-mails in HTML mode (not active by default) SquirrelMail
applies a custom sanitization step in an effort to remove possibly
malicious script and other content from the viewed e-mail.
Due to improper handling of RCDATA and RAWTEXT type elements, the HTML
parser used in this process shows differences compared to real user
introduced which is not removed.
Due to the blacklisting nature of this mechnanism multiple such element
types are allowed and not correctly handled, including the elements
noembed, noframes, noscript and textarea.
When using a construction like
<p title="</noembed><img src=x onerror=alert(1)>"></p>
SquirrelMail's parser will consider the included <img> tag as part
of the title attribute and therefore does not sanitize it's contents.
However, correctly parsing this, the <noembed> element ends at the
first closing tag and the <img> tag is actual content.
This allows having otherwise forbidden elements and attributes,
Proof of Concept (PoC):
Send a HTML email to a victim user:
attacker> mail -a "Content-type: text/html" -s "My little pony" \
<p title="</noscript><img src=x onerror=alert(1)>"></p>
Viewing the recieved e-mail in HTML format the following HTML code
is produced. This code is interpreted by common browsers in a way
that the <img> tag with it's onerror handler is a regular tag,
<!-- begin sanitized html -->
<div class="bodyclass"><noscript><p title="</noscript>
<img src="SquirrelMail_files/x.html" onerror="alert(1)">"><p></p>
<!-- end sanitized html -->
Avoid Squirrelmail, as it appears to be unmaintained.
Disable HTML viewing of messages.
2019-04-03: Vulnerability discovered
2019-04-17: Vulnerability reported to manufacturer
2019-05-03: No response, second attempt to contact manufacturer
2019-07-01: No response, public disclosure of vulnerability
 Product website for SquirrelMail
 SySS Security Advisory SYSS-2019-016
 SySS Responsible Disclosure Policy
This security vulnerability was found by Moritz Bechler of SySS GmbH.
Key ID: 0x768EFE2BB3E53DDA
Key Fingerprint: 2C8F F101 9D77 BDE6 465E CCC2 768E FE2B B3E5 3DDA
The information provided in this security advisory is provided "as is"
and without warranty of any kind. Details of this security advisory may
be updated in order to provide as accurate information as possible. The
latest version of this security advisory is available on the SySS Web
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