Advisory ID: SYSS-2019-016  
Product: SquirrelMail  
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 [1]):  
"SquirrelMail is a standards-based webmail package written in PHP.  
It includes built-in pure PHP support for the IMAP and SMTP protocols,  
and all pages render in pure HTML 4.0 (with no JavaScript required)  
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.  
Vulnerability Details:  
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  
agent behavior. Exploiting these differences JavaScript code can be  
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,  
including ones containing malicious JavaScript code.  
Proof of Concept (PoC):  
Send a HTML email to a victim user:  
attacker> mail -a "Content-type: text/html" -s "My little pony" \ <<EOD  
<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,  
and the embedded JavaScript code is executed.  
<!-- 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.  
Disclosure Timeline:  
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  
[1] Product website for SquirrelMail  
[2] SySS Security Advisory SYSS-2019-016  
[3] SySS Responsible Disclosure Policy  
This security vulnerability was found by Moritz Bechler of SySS GmbH.  
Public Key:  
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  
Creative Commons - Attribution (by) - Version 3.0