Pierluigi Paganini June 23, 2019

Security researcher from F5 Networks that released more details and proof-of-concept for the recently addressed flaw in Outlook for Android.

Microsoft has recently addressed an important vulnerability, tracked as CVE-2019-1105, in Outlook for Android, that potentially affected over 100 million users.

The vulnerability is a stored cross-site scripting issue that is related to the way the app parses incoming email messages.

“A spoofing vulnerability exists in the way Microsoft Outlook for Android software parses specifically crafted email messages. An authenticated attacker could exploit the vulnerability by sending a specially crafted email message to a victim.” reads the security advisory published by Microsoft.

“The attacker who successfully exploited this vulnerability could then perform cross-site scripting attacks on the affected systems and run scripts in the security context of the current user.”

An attacker could exploit the vulnerability by sending a specially crafted message to the victim and run malicious code in the context of the current user. The vulnerability could potentially lead to spoofing attacks.

The spoofing issue in Outlook was separately reported by multiple security researchers, including the researcher Bryan Appleby from F5 Networks that now released more details and proof-of-concept for the flaw. Appleby reported the vulnerability to Microsoft six months ago and explained in a post how he discovered the issue.
The expert was sending some JavaScript code to his friends via email when he accidentally discovered the XSS issue that could allow an attacker to embed an iframe into the email.

“the ability to embed an iframe into an email is already a vulnerability. Even worse, as the iframe was not affected by the block external images setting that prevents tracking pixels and web beacons. But if an attacker could gain the ability to run JavaScript in an email, there could be a much more dangerous attack vector.” explained the expert.

“With this in mind I tried inserting a script tag instead of an iframe into an email. It failed, which is good. However, I was able to circumvent this by using a JavaScript URL in my iframe.”

The experts pointed out that the flaw affected the way email server parses HTML entities included in the email messages.

Normally a JavaScript running in an iframe can only access the content within it, but Appleby discovered it was possible to execute JavaScript code inside the injected iframe to access data in the rest of the page. In Outlook on the Android, the iframe JavaScript had full access to cookies, tokens and even some emails. 

The bug allowed the expert to steal data from the app, he exploited it to read and extract the HTML.

“This kind of vulnerability could be exploited by an attacker sending an email with JavaScript in it. The server escapes that JavaScript and does not see it because it’s within an iframe. When delivered, the mail client automatically undoes the escaping and the JavaScript runs on the client device. Bingo – remote code execution.” added the expert.

“This code can do whatever the attacker desires, up to and including stealing information and/or sending data back out. An attacker can send you an email and just by you reading it, they could steal the contents of your inbox. Weaponized, this can turn into a very nasty piece of malware.”

Below the timeline of the flaw:

• 10 December 2018 – First disclosed to MSRC
• 16 January 2019 – Video screen capture of vulnerability sent. Microsoft cannot repro. Case went cold.
• 26 March 2019 – Universal POC sent to MSRC
• 26 March 2019 – Microsoft confirms repro
• 20 June 2019 – Fix released

Appleby also shared a PoC with the tech giant.

Pierluigi Paganini

(SecurityAffairs – Outllok Android, hacking)

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