New HTTP Request Smuggling Attacks Target Web Browsers

Smuggling target web browsers

Smuggling target web browsers


BLACK HAT USA – LAS VEGAS – A security researcher who previously demonstrated how attackers can abuse weaknesses in the way websites handle HTTP requests warned that the same issues can be used in damaging browser-based attacks against users.  

James Kettle, director of PortSwigger, described his research as shedding new light on so-called desync attacks that exploit disagreements in how a website’s back-end and front-end servers interpret HTTP requests. Previously, at Black Hat USA 2019, Kettle showed how attackers could trigger these disagreements — over things like message length, for instance — to route HTTP requests to a back-end component of their choice, steal credentials, and invoke unexpected responses from an application and other malicious actions. Kettle also has previously shown how HTTP/2 implementation errors can put websites at risk of compromise.


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Kettle’s new research focuses on how threat actors can exploit the same improper HTTP request handling issues to also attack website users and steal credentials, install backdoors, and compromise their systems in other ways. Kettle said he had identified HTTP handling anomalies that enabled such client-side desync attacks on sites such as Amazon.com, those using the AWS Application Load Balancer, Cisco ASA WebVPN, Akamai, Varnish Cache servers, and Apache HTTP Server 2.4.52 and earlier.

The main difference between server-side desync attacks and client-side desync is that the former requires attacker-controlled systems with a reverse proxy front end and at least partly malformed requests, Kettle said in a conversation with Dark Reading following his presentation. A browser-powered attack takes place within the victim’s Web browser, using legitimate requests, he said. Kettle showed a proof-of-concept where he was able to store information such as authentication tokens of random users on Amazon in his shopping list as an example of what an attacker would be able to do. Kettle discovered he could have gotten each infected victim on Amazon’s site to relaunch the attack to others.

“This would have released a desync worm — a self-replicating attack which exploits victims to infect others with no user interaction, rapidly exploiting every active user on Amazon,” Kettle said. Amazon has since fixed the issue.

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Smuggling target web browsers

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