Mozilla: Add-on security vulnerability announcement (Avoid Mozilla Sniffer Add-on for Firefox)

One malicious add-on and another add-on with a serious security vulnerability were discovered recently on the Mozilla Add-ons site. Both issues have been dealt with, and the details are described below.

  • Mozilla Sniffer

An add-on called “Mozilla Sniffer” was uploaded on June 6th to It was discovered that this add-on contains code that intercepts login data submitted to any website, and sends this data to a remote location. Upon discovery on July 12th, the add-on was disabled and added to the blocklist, which will prompt the add-on to be uninstalled for all current users.

Impact to users
If a user installs this add-on and submits a login form with a password field, all form data will be submitted to a remote location. Uninstalling the add-on stops this behavior. Anybody who has installed this add-on should change their passwords as soon as possible.

Mozilla Sniffer has been downloaded approximately 1,800 times since its submission and currently reports 334 active daily users. All current users should receive an uninstall notification within a day or so. The site this add-on sends data to seems to be down at the moment, so it is unknown if data is still being collected.

Mozilla Sniffer was not developed by Mozilla, and it was not reviewed by Mozilla. The add-on was in an experimental state, and all users that installed it should have seen a warning indicating it is unreviewed. Unreviewed add-ons are scanned for known viruses, trojans, and other malware, but some types of malicious behavior can only be detected in a code review.

  • CoolPreviews

A security escalation vulnerability was discovered in version 3.0.1 of the CoolPreviews add-on. The vulnerability can be triggered using a specially crafted hyperlink. If the user hovers the cursor over this link, the preview function executes remote JavaScript code with local chrome privileges, giving the attacking script control over the host computer. Version 3.0.1 and all older versions have been disabled on, and a fixed version was uploaded and reviewed within a day of the developer being notified.

Impact to users
Proof of concept code for this vulnerability was posted on this blog, but no known malicious exploits have been reported so far. If a user has a vulnerable version installed and clicks on a malicious link that targets the add-on, the code in the malicious link will run with local privileges, potentially gaining access to the file system and allowing code download and execution.

All users of CoolPreviews should update to the latest version as soon as possible in order to avoid exposure.

Currently, 177,000 users have a vulnerable version installed. This is less than 25% of the current install base and it will continue to decrease as more users are prompted to update to a new version. Vulnerable versions will also be blocklisted very soon.


Mozilla Blog

Also see:  Firefox User? Be Aware Of Two More Malicious Add-Ons

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