Cross-site Scripting (XSS)

Affecting haml gem, versions <5.0.0.beta.2

medium severity


haml is a layer on top of HTML or XML that's designed to express the structure of documents in an elegant way.

Affected versions of this package are vulnerable to Cross-site Scripting (XSS). When using user input to perform tasks on the server, characters like \< > \" \' must be escaped properly. In this case, the \' character was missed. An attacker can manipulate the input to introduce additional attributes, potentially executing code.

engine ="%p&= foo")
engine.render(,:foo => 'h"i"')
# => "&lt;p&gt;h&amp;quot;i&amp;quot;&lt;/p&gt;"
engine.render(,:foo => "h'i'")
# => "&lt;p&gt;h'i'&lt;/p&gt;\n"

Disclosure Timeline

  • February 6th, 2017 - Reported the issue to package owner.
  • February 8th, 2017 - Issue fixed.
  • February 26th, 2017 - Version v5.0.0.beta.2 released.


Cross-Site Scripting (XSS) attacks occur when an attacker tricks a user’s browser to execute malicious JavaScript code in the context of a victim’s domain. Such scripts can steal the user’s session cookies for the domain, scrape or modify its content, and perform or modify actions on the user’s behalf, actions typically blocked by the browser’s Same Origin Policy.

These attacks are possible by escaping the context of the web application and injecting malicious scripts in an otherwise trusted website. These scripts can introduce additional attributes (say, a "new" option in a dropdown list or a new link to a malicious site) and can potentially execute code on the clients side, unbeknown to the victim. This occurs when characters like < > " ' are not escaped properly.

There are a few types of XSS:

  • Persistent XSS is an attack in which the malicious code persists into the web app’s database.
  • Reflected XSS is an which the website echoes back a portion of the request. The attacker needs to trick the user into clicking a malicious link (for instance through a phishing email or malicious JS on another page), which triggers the XSS attack.
  • DOM-based XSS is an that occurs purely in the browser when client-side JavaScript echoes back a portion of the URL onto the page. DOM-Based XSS is notoriously hard to detect, as the server never gets a chance to see the attack taking place.


Upgrade haml to version 5.0.0 or higher.


Do your applications use this vulnerable package?

Snyk Security Research Team
Snyk ID
07 Feb, 2017
08 May, 2017