Exploitation for Client Execution
Vulnerabilities can exist in software due to unsecure coding practices that can lead to unanticipated behavior. Adversaries can take advantage of certain vulnerabilities through targeted exploitation for the purpose of arbitrary code execution. Oftentimes the most valuable exploits to an offensive toolkit are those that can be used to obtain code execution on a remote system because they can be used to gain access to that system. Users will expect to see files related to the applications they commonly used to do work, so they are a useful target for exploit research and development because of their high utility.
Several types exist:
Web browsers are a common target through Drive-by Compromise and Spearphishing Link. Endpoint systems may be compromised through normal web browsing or from certain users being targeted by links in spearphishing emails to adversary controlled sites used to exploit the web browser. These often do not require an action by the user for the exploit to be executed.
Common office and productivity applications such as Microsoft Office are also targeted through Spearphishing Attachment, Spearphishing Link, and Spearphishing via Service. Malicious files will be transmitted directly as attachments or through links to download them. These require the user to open the document or file for the exploit to run.
Common Third-party Applications
Other applications that are commonly seen or are part of the software deployed in a target network may also be used for exploitation. Applications such as Adobe Reader and Flash, which are common in enterprise environments, have been routinely targeted by adversaries attempting to gain access to systems. Depending on the software and nature of the vulnerability, some may be exploited in the browser or require the user to open a file. For instance, some Flash exploits have been delivered as objects within Microsoft Office documents.
Cobalt Group had exploited multiple vulnerabilities for execution, including Microsoft’s Equation Editor (CVE-2017-11882), an Internet Explorer vulnerability (CVE-2018-8174), CVE-2017-8570, CVE-2017-0199, and CVE-2017-8759.
Patchwork uses malicious documents to deliver remote execution exploits as part of. The group has previously exploited CVE-2017-8570, CVE-2012-1856, CVE-2014-4114, CVE-2017-0199, and CVE-2015-1641.
SpeakUp attempts to exploit the following vulnerabilities in order to execute its malicious script: CVE-2012-0874, CVE-2010-1871, CVE-2017-10271, CVE-2018-2894, CVE-2016-3088, JBoss AS 3/4/5/6, and the Hadoop YARN ResourceManager.
Other types of virtualization and application microsegmentation may also mitigate the impact of client-side exploitation. The risks of additional exploits and weaknesses in implementation may still exist. 
Security applications that look for behavior used during exploitation such as Windows Defender Exploit Guard (WDEG) and the Enhanced Mitigation Experience Toolkit (EMET) can be used to mitigate some exploitation behavior.  Control flow integrity checking is another way to potentially identify and stop a software exploit from occurring.  Many of these protections depend on the architecture and target application binary for compatibility.
Detecting software exploitation may be difficult depending on the tools available. Also look for behavior on the endpoint system that might indicate successful compromise, such as abnormal behavior of the browser or Office processes. This could include suspicious files written to disk, evidence of Process Injection for attempts to hide execution, evidence of Discovery, or other unusual network traffic that may indicate additional tools transferred to the system.
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