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I found an interesting blog post A Deep Analysis of the Microsoft Outlook Vulnerability CVE-2018-8587 about Microsoft Outlook heap buffer overflow vulnerability where is described how Microsoft Outlook can be exploited by using specially crafted mail classification rules file (RWZ).

To reproduce this vulnerability, we need to run Microsoft Outlook, then click "Rules => Manage Rules&Alerts => Options => Import Rules" and select the PoC file which causes Outlook to crash.

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In the end they are writing:

Applying this patch is critical since an attacker who successfully exploits this vulnerability could use a specially crafted file to perform actions in the security context of the current user.

But how likely is that someone could exploit this vulnerability? I mean an attacker needs to send this malicious file to a user who needs to actively import this file which exploits this heap buffer overflow bug. It seems to me completely different from such attacks where an attacker sends a malicious PDF document which exploits some vulnerability in Adobe Reader. Here you need to actively hack yourself (similar to self-XSS in web security).

Even Microsoft states:

To exploit the vulnerability, a user must open a specially crafted file with an affected version of Microsoft Outlook software. In an email attack scenario, an attacker could exploit the vulnerability by sending the specially crafted file to the user and convincing the user to open the file. In a web-based attack scenario, an attacker could host a website (or leverage a compromised website that accepts or hosts user-provided content) that contains a specially crafted file that is designed to exploit the vulnerability. However, an attacker would have no way to force the user to visit the website. Instead, an attacker would have to convince the user to click a link, typically by way of an enticement in an email or Instant Messenger message, and then convince the user to open the specially crafted file.

So this bug seems to be somewhat useless and very unlikely to exploit?

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    Even if it's very unlikely to exploit you need to fix it. There is a possibility as you stated. – CD Rohling Jul 20 at 15:14
  • The likelihood is entirely up to the social engineering part of the equation. I'm not sure that there is going to be an answer to the question you asked. You have asked, "how likely is it that an attacker can trick someone into opening a file?" Uh, that's surprisingly easy: [Spoof the IT dept email addr] "IMPORTANT ANNOUNCEMENT: we require that you update your Outlook to prevent phishing and ransomware. Open and import these Outlook rules in the following process ...." As a specific example. There are others. "Is it somewhat useless?" is a value judgement and is an odd question to ask. – schroeder♦ 7 hours ago
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Getting a victim to open a file is a proven effective way to attack an organisation - phishing and social engineering work, very well. So assume that part of the attack is low effort if you or your organisation is a target.

As regards anything that sounds like complex crafting of a payload, if it can be created it will. The market in attacks flows down from the very high end (nation state, organised crime etc) to lower level attackers. It's a commodity.

So don't think of the difficulty of the attack mechanism as important when assessing likelihood. Instead start with possible attacks that will take your organisation down, look at who might want to commit those attacks, and think about what you would need to mitigate them.

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  • I will accept it as an answer, but I'm still not 100% convinced that someone would import some "mail rule" files from a stranger. But, ok, maybe it's not a stranger but an employee who knows how to exploit software and sends this malicious file to a co-worker. Sounds a bit complicated and there might be easier ways to get RCE rather than this. But who knows. I know that social engineering is a real thing, so theoretically it might be a valid attack scenario. – notyourdolly Jul 21 at 10:38
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    +1 but it would be great if the answer could expand on what "opening a file" means. In the Microsoft advisory it sounds as if opening the file directly from eg an email program (eg double click on it) can exploit the issue (likelihood of success here seems high). In the linked article, it sounds as if the whole "import rules" step is necessary (and I agree with OP, that seems far less likely). But is it really necessary, or is that just for the purpose of the POC? – tim Jul 21 at 11:19

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