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Avast AntiTrack Flaw Allows MitM Attacks on HTTPS Traffic

A vulnerability in Avast’s anti-tracking solution could allow malicious actors to perform man-in-the-middle (MitM) attacks on HTTPS traffic, a security researcher has discovered.

The security flaw, which impacts both Avast and AVG AntiTrack, as they share underlying code, resides in the manner in which the software filters HTTPS traffic.

Moreover, with the feature enabled, a browser could successfully connect using TLS 1.0, even if the old security protocol had been explicitly disabled in the browser.

The bug could be triggered without performing any special action, using AntiTrack in its default configuration, web designer and security consultant David Eade reveals.

Eade discovered that AntiTrack proxies traffic to HTTPS sites using its own certificates, after adding its own certificate (named "AvastAntiTrack 2") to the Windows "Trusted Root Certification Authorities" store at installation.

The solution presents the browser with a certificate of its own for each website visited, and the user is shown a secure padlock icon when navigating to the site, although traffic is not protected to the end web server, the researcher says.

According to Eade, Avast AntiTrack fails to check the validity of web server certificates, which makes it easy for an MitM attacker to serve a fake site using a self-signed certificate. While the attack should not work under normal circumstances, AntiTrack's proxy ignores the certificate problem and presents to the victim its own certificate, which is trusted by the victim's browser.

The anti-tracking solution also downgrades the browser's security protocol to TLS 1.0, even if the application was configured to use only TLS 1.2 or higher — as is the case with Internet Explorer and Microsoft Edge.

On top of that, the researcher discovered that AntiTrack does not honor browser cipher suites and ignores them in favor of older, weaker ciphers. Furthermore, the solution does not support Forward Secrecy.

Tracked as CVE-2020-8987, the issues were reported to Avast in August last year and were fixed with the release of Avast AntiTrack version 1.5.1.172 and AVG AntiTrack version 2.0.0.178 in late February and early March 2020, respectively.

Related: Vulnerabilities Disclosed in Kaspersky, Trend Micro Products

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Related: Avast Discloses New Supply-Chain Attack Attempt

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