Experts found a critical remote buffer overflow vulnerability in Skype

Pierluigi Paganini June 28, 2017

The security expert Benjamin Kunz-Mejri from security firm Vulnerability Lab discovered a remote zero-day stack buffer overflow vulnerability in Skype.

The security expert Benjamin Kunz-Mejri from security firm Vulnerability Lab discovered a Skype zero-day stack buffer overflow vulnerability, tracked as CVE-2017-9948, that could be exploited by a remote attacker to execute malicious code.

Vulnerability Lab reported the flaw to Microsoft on 16th May, Microsoft released a patch on 8 June in Skype version 7.37.178.

The flaw resides in Skype Web messaging and call service and could be exploited during a team conference call.

“The buffer overflow vulnerability can be exploited by local and remote attackers without user interaction and with low privileged Skype user account.” states the security advisory published by Vulnerability Lab.

The flaw was ranked as a high-security risk and received a 7.2 CVSS score and affects Skype versions 7.2, 7.35, and 7.36 on Windows XP, Windows 7 and Windows 8.

“The problem is located in the print clipboard format & cache transmit via remote session on Windows XP, Windows 7, Windows 8 and Windows 10. In Skype v7.37 the vulnerability is patched,” continues the advisory.

A remote attacker can crash the application triggering an unexpected exception error, to overwrite the active process registers, or even execute malicious code on a vulnerable Sky instance.

The problem is caused by the way Skype uses the ‘MSFTEDIT.DLL’ file in case of a copy request on local systems.

“The security vulnerability allows to crash the software application with an unexpected exception error, to overwrite the active process registers to execute own malicious codes.” continues the advisory.

“The security vulnerability is located in the `clipboard format` function of the Skype software. Attackers are able to use a remote computer system with a shared clipboard, to provoke a stack buffer overflow on transmission to Skype. The issue affects the `MSFTEDIT.DLL` dynamic link library of the windows8 (x86) operating system”

The advisory details how to exploit the flaw, an attacker can craft a malicious image file and then copy and paste it from a clipboard of a computer system into a conversation window in the Skype software.

When the image is hosted on a clipboard on both the remote and the local systems, Skype triggers a stack buffer overflow.

“The limitation of the transmitted size and count for images via print of the remote session clipboard has no secure limitations or restrictions. Attackers [can] crash the software with one request to overwrite the EIP register of the active software process,” said researchers from Vulnerability Lab.

“Thus allows local or remote attackers to execute own codes on the affected and connected computer systems via the Skype software,” they added.

The researchers published the proof-of-concept (PoC) exploit code.

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Pierluigi Paganini

(Security Affairs – Petwrap ransomware , malware)

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