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Security warning for users of Mailvelope in Firefox

Created at 04.May 2017, 12:00 | Category: Blog

Dear Mailvelope users,

We have a security notice for anyone who uses the encryption add-on Mailvelope with Firefox.

We have had a current security audit of Mailvelope undertaken, in which a critical vulnerability was found in the interaction between Mailvelope and Firefox. Under certain circumstances, Firefox’s security architecture allows attackers to access users’ private keys via compromised add-ons. We therefore ask all users of Mailvelope in Firefox to carefully read our security recommendations found in this article, below.

This also affects Mailvelope users with all other providers such as Gmail, Outlook.com, Yahoo!Mail, etc.

Firefox’s architecture does not sufficiently compartmentalise add-ons from each other – this has been known for years. The fact that a Mailvelope user’s private keys could be compromised via targeted attacks in Firefox was not proven until now, however. The security engineers that we engaged from Cure53 have now proved this. In the past, Cure53 had already audited Mailvelope for Chrome – on our assignment the engineers have now also investigated the plug-in’s interaction with Firefox. In their investigative report, they conclude that Firefox does not currently constitute a suitable environment for Mailvelope. They write,

“At the end of the day, the Cure53 testing team cannot in good conscience recommend the use of Mailvelope on Firefox.”

Weakness expected to last until November 2017

We informed Thomas Oberndörfer, the developer of Mailvelope, after the security audit. He is unable to fix the weakness, however, as it has to do with Firefox’s architecture. New architecture is already being developed at Firefox. Mozilla is planning to conclude this work with the release of Firefox 57 in November 2017. Oberndörfer is also working on a version of Mailvelope for the new and improved Firefox architecture. We would like to thank him for his development work.

Until Mozilla has updated the architecture, the following security recommendations apply:

Option 1.) In the interim, switch to different software. Either use Mailvelope in a different browser, or use PGP with a local email program. You can find various instructions for these options in the Posteo help section.

Option 2.) Alternatively, using an independent Firefox profile for Mailvelope minimises the risk in the interim. In the Posteo help section, we have published step-by-step instructions for the creation of Firefox profiles on Mac and on Windows. Mailvelope users with other providers can also follow these instructions. Please be sure to note the following security recommendations in order to effectively minimise the risk of a fruitful attack:

  • Do not install any further add-ons in the newly-created browser profile
  • Use the Firefox profile exclusively for your encrypted Mailvelope communication. Only access your provider’s webmail interface and never visit other websites using this profile.
  • In addition, use a password for your PGP key that is as secure as possible
  • Be careful not to accidentally install any add-ons via phishing, through which you could be attacked

Due to the problems with the Firefox architecture, we additionally recommend:

  • Restrict the use of add-ons in the Firefox browser to a minimum, until Mozilla has updated the architecture
  • You can further protect yourself from potential attackers by setting up an additional user on your operating system for end-to-end encrypted communication

Here are the recommendations from the Cure53 report once again, for transparency reasons:

“Two paths can be recommended for the users who rely on Mailvelope for encryption and decryption of highly sensitive data. First, they could use Mailvelope on a browser profile that hosts only and exclusively Mailvelope with no other extensions. Secondly, they would need to rely on a different software solution, for instance Thunderbird with Enigmail.”

“At present, any users working with Mailvelope on Firefox are encouraged to export their settings, delete the extension and migrate their setup to a Mailvelope installation running on Google Chrome. Alternatively, a separate browser profile running Mailvelope only could be used, with the caveat that one must not have any other extensions installed in order to minimize the risk of key material leakage.”

Security engineers engaged by Posteo found the weakness

In their daily activities, our customers use various devices, browsers and add-ons in their local environments. Our users’ communication security is very important to us – we therefore also continually have external standard components checked for weaknesses. Among others, we work together to this end with independent IT security experts at Cure53. They have now made a find with Mailvelope in Firefox.

Dr Mario Heiderich from Cure53 explains,

“the problem is currently located in the architecture. There is therefore no easy fix. Mozilla knows this, but also has to keep a difficult balance between radical changes and ones that are prudent but are often decisions that are slow to take effect. Things are going in the right direction, however, which is definitely something positive for more complex software.”

Thomas Oberndörfer of Mailvelope states,

“Mailvelope is naturally dependent on the security of the underlying browser. Weaknesses in Firefox’s add-on system have been known of for some time, so Mozilla’s improvement should be welcomed. Security audits such as the one undertaken by Posteo are important indicators for us to see how we can further improve Mailvelope.”

Report to be published after weakness is overcome

The weakness outlined above is expected to be overcome by Mozilla in November 2017. Out of consideration for security, we will therefore first publish the report at a later point. In it, the method of attack will be described in detail. The report is already available to Mailvelope and the BSI (German Federal Office for Information Security).

The security audit has also yielded some positive results for Mailvelope, which we would like to outline here: There was a check made as to whether email providers for which Mailvelope is used could access a Mailvelope user’s private keys saved in the browser – this was not possible. All other attempts made by the security engineers to access private keys saved in Mailvelope, such as operating third party websites or man-in-the-middle attacks, were also unsuccessful.

Weakness shows that open source increases security

For security reasons, we exclusively support open source components with transparent code – such as the encryption plug-in Mailvelope. In our view, transparent code is essential for the security and democratic control of the internet: Independent experts can at any time identify weaknesses or backdoors via code analysis, as happened here. A provider or developer’s security claims do not need to be trusted. With the security audits that we commission, we want to contribute to further increasing the security of established open source components and genuine end-to-end encryption.

Best regards,
The Posteo Team