Tim Cliffe - Blog

Making Your Thunderbird (or any other) E-mails More Secure

Published: 2021-01-08. Reading Time: 5.5 minutes.

01 Target Audience

(01.1) Anyone concerned with their on-line privacy and security.

02 Executive Summary

(02.1) When you send an e-mail, the software you use, in most cases, is giving away information about your computer and e-mail software.

(02.2) Unscrupulous people can use this information to help them to attack (e.g., viruses) and/or gain access to your computer.

(02.3) If you use the Thunderbird E-mail Client to send e-mails, this article will show you, in seven simple steps, how to stop every e-mail you send giving away such information and make you a little safer on-line.

03 Structure of This Article

(04) Introduction
(05) User-Agent - The Giver of Information
(06) Hiding your Thunderbird User-Agent Details
(07) Confirming User-Agent Details have been Removed

04 Introduction

(04.1) You will have heard about e-mails that have been sent, containing a virus of some sort, that have infected people's and company's computers.

(04.2) Viruses are Operating System (Linux, Mac OSX, Windows) specific. They are often software specific also. Have you ever wondered how people know which virus to send in an e-mail?

(04.3) One simple way is to have an e-mail, which has been sent from the target computer, and use it to find out which Operating System is being used and which software. This information is provided by something called the User-Agent string, which is contained in The Headers of the e-mail.

05 User-Agent - The Giver of Information

(05.1) Most people don't know The Headers exist, but they include, who sent the e-mail, the Subject, the time the e-mail was sent; however, most of the information in The Headers is not displayed, by default.

(05.2) If you know how, it is very easy to access all the information in The Headers of an e-mail, including something called the User-Agent string (string is another word for text).

(05.3) The User-Agent string takes the form
User-Agent: Mozilla/5.0 (<operating system information>) <software> (<software-details>) <extensions>

A Thunderbird User-Agent string could look like this:
Mozilla/5.0 (X11; Linux x86_64; rv:45.0) Gecko/20100101 Thunderbird/45.8.0

Fortunately, it is very easy to hide the User-Agent string.

06 Hiding your Thunderbird User-Agent Details

(06.1) The following instructions apply to the Thunderbird E-mail Client (software); however, it is easy to find how to make the same changes to other e-mail clients, with a little searching on the Internet, for example, you might search for, Hide Outlook User-Agent.

(06.2) If you use a web-based e-mail service, such as Gmail or Hotmail, I'm afraid you cannot change how the User-Agent is configured.

(06.3) To hide the User-Agent string:

1. Launch Thunderbird, then;

Image of the main Thunderbird window and Menu list. — 2. Select the Menu icon at the top right of the Thunderbird window, then select Preferences from the drop-down list;

Image of Thunderbird's Preferences - General screen. — 3. The Preferences - General screen will display;

Image of the Config Editor... button on Thunderbird's Preferences - General screen. — 4. Scroll to the bottom of the Preferences - General screen, then select the Config Editor... button;

Image of about:config > I Accept the Risk. — 5. The about:config window will display. Ensure the check-box is ticked, then select the 'I accept the risk!' button;

Image of about:config Search window. — 6. The about:config Search window will display. In the Search box enter...
general.useragent.override
Ensure there are no spaces before or after the text and enter the text **exactly as shown**;

Image of about:config Search > Modify > New > String. — 7. The entry, general.useragent.override, no longer appears under Preference Name by default. You need to add general.useragent.override manually.
To do this, right-click on the large white area. The Modify drop-down list will appear. Place your mouse over the New option, then select String from the secondary menu;

Image of the New String Value window. — 8. The New String Value dialogue will appear. Enter the text general.useragent.override exactly as shown, then select OK.
A second dialogue will appear. Leave the text field empty and select OK.

(06.4) Thunderbird is now configured so as not to include the User-Agent string in The Headers of your e-mails.

You can now close the Preferences window and return to the e-mail window. The last thing to do, is to check your new configuration is working.

07 Confirming User-Agent Details have been Removed

(07.1) Now, simply send an e-mail to yourself. It can be as short and simple as you like.

Image of the Thunderbird File > Print drop-down menu. — 1. When the e-mail, you sent to yourself, arrives, open it, then select File > Print;

Image of the Print window. — 2. The Print window will appear. Select PDF > Print as PDF...
You will be prompted to save the file in the usual way.

(07.2) Now open the file you have just saved, either in your PDF viewer or your browser. The top of the document (your e-mail) will look something like this...