A couple of days ago, it became clear that Nokia is decrypting HTTPS traffic originating from its Xpress smartphone browser. It becomes more common for mobile browsers to use proxy servers to optimise websites, such that the browsing experience on a mobile device becomes much faster. However, the encryption used for HTTPS communication foils this, which caused Nokia to perform what effectively is a man-in-the-middle attack on your mobile traffic.

Previously, it became known that Opera Mini is already doing the same. This led to an official statement saying: “if you do not trust Opera, do not use our browser.” Nevertheless, these warning are omitted when downloading the application from the appstore.

Rerouting Secured Traffic
When you normally visit a website using HTTPS, the server sends a certificate that links the domain to a public key. Using this key, the traffic will be encrypted during the full session. This ensures no third party has access to the confidential information in transmission.

The so-called optimising browsers work slightly different: they reroute the traffic through a proxy server that is responsible for optimizations. However, this proxy server is not able to optimise traffic that is encrypted. Therefore, the Xpress browser internally changes all requests to go to a Nokia-owned domain. This means that communication between the browser and the proxy server is encrypted, but using a key that belongs to Nokia. Afterwards, the proxy communicates with the site of choice and performs the encrypted communication with the website.

Malicious Behaviour
The way these optimising browsers handle encrypted traffic is very suspicious. Simply put, malware build for eavesdropping would behave in a comparable way. From this point of view, it is rather curious that the browsers are programmed to decrypt secured traffic.

Due to the intermediate decryption of the HTTPS traffic, the browser manufacturers gain access to your private data. Even worse, end-to-end encryption is broken, making the proxy server a very interesting target for attack. For example, it may be possible to manipulate e-banking traffic of many citizens when one of the proxies is compromised.

The Man-in-the-Client Attack
Due to the intermediate decryption, the Nokia Xpress browser and Opera Mini handle confidential traffic in an insecure manner. Additionally, the way these browsers behave constitutes an eavesdropping attack. Such behaviour is not only less secure, but also breaks down existing awareness users have. Therefore, it is understandable that the security community was rather surprised by this architecture.

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