Microsoft Windows Privilege Escalation Vulnerability Leaked via Twitter

Microsoft Windows Privilege Escalation Vulnerability Leaked via Twitter

A security researcher who goes by the name “SandboxEscaper” leaked via Twitter an exploit code for a Microsoft Windows privilege escalation vulnerability.

In the now-deleted Twitter post, SandboxEscaper provided a link to a Github repository that contains the code necessary to exploit a Microsoft Windows privilege escalation vulnerability. Other security researchers have since verified the authenticity of the vulnerability exploit disclosed by SandboxEscaper.

The bug uncovered by SandboxEscaper lies in Microsoft Windows task scheduler service. Task scheduler allows users to schedule any program to run at a convenient time or when a specific event occurs. 

SandboxEscaper found that task scheduler uses unsecured API that allows an attacker, having access to a computer as a local user to gain system-level privileges, enabling the attacker to overwrite system files with malicious code to hijack Windows.

“The Microsoft Windows task scheduler SchRpcSetSecurity API contains a vulnerability in the handling of ALPC, which can allow an authenticated user to overwrite the contents of a file that should be protected by filesystem ACLs,” CERT Coordination Center (CERT/CC)described the uncovered flaw. “This can be leveraged to gain SYSTEM privileges.”

“The flaw is that the Task Scheduler API function SchRpcSetSecurity fails to check permissions,” security researcher Kevin Beaumont, for his part, noted. “So anybody  –  even a guest  –  can call it and set file permissions on anything locally.”

As a proof-of-concept, SandboxEscaper overwrites a file used by Windows' printing subsystem with a malicious code when an attempt is made to print.

According to CERT/CC, the exploit code leaked by SandboxEscaper works on 64-bit Windows 10, Windows Server 2016 systems, 32-bit Windows 10 with minor modifications and with other Windows versions with further modifications. CERT/CC said it’s currently unaware of a practical solution to this problem.

A Microsoft spokesperson told the Registerthat the company will “proactively update impacted devices as soon as possible.”

In another Twitter post, SandboxEscaper blamed depression for leaking the vulnerability exploit before Microsoft has time to issue a security update or a patch.

Exploits for privilege escalation vulnerabilities are rarely leaked to the public prior to a patch as many software vendors like Microsoft now offer financial rewards to security researchers who uncover and discreetly inform the concerned software vendors. This gives security vendors time to create a security fix to the reported problem.

Dangers of Privilege Escalation Attacks

In a privilege escalation attack, the attacker has to have local access to the computer or computer network that he or she wants to compromise. A local user needs the system administrator's password to complete certain tasks, such as overwriting system files. As such, this is given less priority by software vendors when it comes to patching.

Remote code execution attacks, on the other hand, are given high priority in terms of patching as these attacks don’t require that the attacker have local access to the target computer.

In a remote code execution attack, an attacker can install malicious code on a computer even when he or she has no local access, provided though that the computer is connected to the internet. An example of the remote code execution attack was the WannaCry attack. Hours after the WannaCry attack on May 12, 2017, Microsoft issued a security update for Windows platforms originally not covered by an earlier security patch, showing the importance of patching remote code execution attacks.

Privilege escalation attacks, however, aren’t given similar immediate attention. Privilege escalation vulnerabilities are typically patched during scheduled updates, like Microsoft’s regular security updates every second Tuesday of each month.

Client-side exploits, however, make privilege escalation attacks dangerous as attackers then effectively become local users and escalate their privileges to system administrators.

"If an attacker can gain access to a system through a client side exploit, they may then effectively become the local user, and escalate to local system,” SANS Technology Instituteinstructor Adrien de Beaupre wrote in a post "Privilege escalation, why should I care?" “Local system priv on a Windows computer is just a hop, skip, and jump away from being Domain administrator.”

Client-side exploits come in numerous and varied formats. Compared to remote execution attack like the WannaCry that has worm capability – meaning, it replicates itself without user interaction, client-side exploits need user interaction, such as clicking a malicious link or downloading a malicious email attachment.

Prevention

The fact that the exploit code is out and there’s no official patch from the software vendor should warrant some caution. However, unofficial patch has been posted by 0Patch.com

“Windows has a customer commitment to investigate reported security issues, and proactively update impacted devices as soon as possible,” a Microsoft spokesperson told Threatpost. “Our standard policy is to provide solutions via our current Update Tuesday schedule.”

That means that the next Windows update is still days away – this coming September 11^th. This gives attackers a window to exploit the flaw exposed by SandboxEscaper in the wild.

According to Kevin Beaumont, if you use Microsoft Sysmon, a sure way to find out whether a Microsoft Windows task scheduler exploit is being used is by looking for spoolsv.exe spawning abnormal processes.

Here are some general measures in preventing privilege escalation attacks like the one exposed by SandboxEscaper:

• Install software updates as soon as they’re made available.
• Use anti-virus protection.
• Educate staff about the dangers of clicking links and downloading email attachments from unknown sources.
• Practice network segmentation so that if one network is compromised the other network won’t be affected.
• Don’t allow untrusted users to install code.