Adobe Flash Player ActionScript 3 ByteArray Use-After-Free Remote Code Execution (CVE-2015-5119)

1. Vulnerability Overview & Root Cause

This vulnerability is a Use-After-Free (UAF) flaw residing in the ByteArray class implementation within ActionScript 3 (AS3) in Adobe Flash Player. The root cause lies in the improper handling of memory reallocation when the ByteArray capacity is modified. By overriding the default valueOf method of a custom object passed during the resizing operation, an attacker can execute arbitrary ActionScript code mid-operation, leading to premature memory deallocation while retaining active references.

2. Exploitation Mechanics & Attack Scenario

An attacker exploits this vulnerability by hosting a malicious Flash (.swf) file on a website or embedding it within an email. The attack sequence proceeds as follows:

• The malicious Flash script instantiates a ByteArray and allocates a specific memory buffer.
• It triggers a resizing operation on the ByteArray, passing an object with an overridden valueOf method as an argument.
• During the execution of the overridden valueOf method, the script forces the release or reallocation of the original ByteArray buffer.
• Once valueOf returns, the Flash Player engine attempts to write to the previously freed memory address, resulting in a Use-After-Free condition that allows the attacker to overwrite critical pointers and hijack the execution flow.

3. Infrastructure & Business Impact

This vulnerability was actively exploited in the wild as a zero-day exploit, notably leaked during the Hacking Team breach in July 2015. Successful exploitation allows complete compromise of the host operating system under the context of the running browser process. This can lead to unauthorized data exfiltration, deployment of ransomware, and lateral movement within the corporate network, posing a severe threat to enterprise infrastructure.