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WinRAR Absolute Path Traversal Vulnerability Leads to Remote Code Execution (CVE-2018-20250)

A 19-year-old vulnerability in WinRAR’s ACE file format support (CVE-2018-20250) has been identified as part of an attack in the wild.

Background

On February 20, researchers at Check Point Research (CPR) published a blog detailing their discovery of multiple vulnerabilities within a library used by WinRAR, a popular file compression tool, to extract ACE archives. When exploited, these vulnerabilities can lead to remote code execution. An exploit script was published to Github one day after CPR’s blog post. The 360 Threat Intelligence Center (TIC) has reportedly identified an in-the-wild sample that attempts to exploit this vulnerability.

Analysis

CPR disclosed a total of four CVEs: CVE-2018-20250, CVE-2018-20251, CVE-2018-20252, CVE-2018-20253.

CVE-2018-20250 is an absolute path traversal vulnerability in unacev2.dll, the DLL file used by WinRAR to parse ACE archives that has not been updated since 2005 (14 years ago). A specially crafted ACE archive can exploit this vulnerability to extract a file to an arbitrary path and bypass the actual destination folder. In its example, CPR is able to extract a malicious file into the Windows Startup folder.

CVE-2018-20251 is a vulnerability in how WinRAR calls a validation function when handling ACE archives. The validation function is designed to prevent the extraction of files that contain path traversal patterns. However, the value from the validation function is not returned until after files or folders have been created.

Both CVE-2018-20252 and CVE-2018-20253 are out-of-bounds write vulnerabilities during the parsing of crafted archive formats. Successful exploitation of these CVEs could lead to arbitrary code execution.

Proof of concept

CPR created a proof of concept video, included in its blog post, that showcases how an ACE archive can extract a malicious file into the Windows Startup folder.

A proof of concept was also published to Github.

Solution

WinRAR has decided to drop support for unpacking ACE archives in WinRAR 5.70 Beta 1. The current beta version is 5.70 Beta 2. WinRAR users are encouraged to upgrade to the latest beta version as soon as possible.

Identifying affected systems

A list of Nessus plugins to identify these vulnerabilities will appear here as they’re released.

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