CVE-2018-20250: WinRAR Absolute Path Traversal Vulnerability

CVE-2018-20250 is an absolute path traversal vulnerability in RARLAB WinRAR that can lead to remote code execution. Attackers who can get a user to open a crafted archive may write files outside the intended extraction path, which in practice has been abused to run code on the victim system. CISA notes known ransomware use of this issue, so organizations that still run WinRAR should treat it as a priority for inventory and remediation.

Public detail is limited to the product, the CWE class, and the remote-code-execution outcome; exact affected builds, CVSS scores, and exploit mechanics must be confirmed against the vendor advisory. The required action is to apply updates per vendor instructions.

How it works

The weakness is CWE-36, absolute path traversal. In archive utilities this class of flaw occurs when the extractor does not properly constrain destination paths supplied inside the archive. An attacker crafts an archive whose internal paths resolve to absolute locations on the target filesystem rather than staying inside the user-chosen extraction folder.

When a user opens or extracts that archive with a vulnerable WinRAR instance, the product can write attacker-controlled content to unintended locations. Combined with the ability to place executable content or overwrite startup artifacts, this path-control failure can escalate to remote code execution under the privileges of the user who opened the file. No further exploit specifics are provided in the available facts; defenders should treat any untrusted archive as a potential delivery vehicle and validate behavior against the vendor advisory.

Am I affected? How to find it in your systems

WinRAR is a common desktop archive tool on Windows endpoints, often installed by users or packaged in software images for help-desk and developer workstations. It may also appear on jump hosts, forensic workstations, or any system where staff routinely unpack compressed files from email or file shares.

• Inventory: query software inventory (SCCM/MECM, Intune, Tanium, bigfix agents, or simple WMI/registry scans) for RARLAB WinRAR installations and record the installed version string.
• Version check: compare each discovered version against the fixed releases listed in the vendor advisory; do not assume a build is safe without that confirmation.
• Configuration: note whether WinRAR is associated with archive file extensions and whether users extract content with elevated rights or into sensitive directories.
• Telemetry signs: look for unexpected file writes outside normal user extraction folders shortly after archive open events, new executables or scripts appearing in startup locations, and process-creation chains that begin with WinRAR and spawn unusual children. Endpoint detection and response (EDR) file-write and process trees are the primary signals; archive-related phishing or malware lures in mail gateways are secondary indicators.

Because the flaw is triggered by opening a malicious archive, any host that can receive and open such files is in scope until patched or mitigated.

How to remediate

Patch first. Apply the WinRAR updates published by RARLAB exactly as directed in the vendor advisory and in line with CISA’s required action to apply updates per vendor instructions. After installation, verify the new version string on a sample of endpoints and confirm that the previous vulnerable builds are no longer present.

• Redeploy or push the updated package through your standard software-distribution channel and block reinstallation of older builds via application control or package blacklisting where feasible.
• For this path-traversal class, harden extraction defaults: encourage extraction only to dedicated, low-privilege directories; avoid running the archiver as administrator; and restrict write access to system and startup locations for standard users.
• Reduce the attack surface by limiting which users need WinRAR at all and by preferring built-in OS archive handling where it meets business needs.
• Update security baselines and golden images so newly provisioned systems receive only the fixed version.

If you can't patch immediately

Until the vendor update can be deployed everywhere, reduce exposure with compensating controls that address both the delivery and the impact of absolute path traversal.

• Segmentation and least privilege: keep high-risk user populations (those who open external archives) off sensitive servers; ensure standard users cannot write to system directories or common persistence locations.
• Application control: allow-list only the approved WinRAR binary and block execution from user-writable paths that an exploit might target.
• Virtual patching / content inspection: where mail or web gateways can inspect archives, quarantine or rewrite suspicious path constructs; endpoint policies that warn or block extraction of archives containing absolute or traversal sequences provide partial protection.
• Feature and association hardening: temporarily remove file-type associations that auto-launch WinRAR, or instruct users to inspect archives inside a sandboxed or isolated environment.
• Monitoring: heighten EDR alerts on WinRAR process trees, anomalous file creates outside the extraction folder, and ransomware-like behaviors (mass encryption, shadow-copy deletion), given the known ransomware use of this CVE.

These measures lower likelihood and blast radius but do not replace the vendor patch.

If your data may have been exposed

Actively exploited vulnerabilities, especially those with documented ransomware use, frequently precede broader compromise and data theft. If you have evidence that vulnerable WinRAR hosts opened untrusted archives, or if ransomware activity has already been observed, follow your incident-response plan: isolate affected systems, preserve forensic images, rotate credentials, and assess what data those hosts could reach. As a quick personal check, individuals can run a free exposure scan of their email addresses against known breach data sets to see whether their credentials or personal information already appear in public breach corpora, then force password changes and enable multi-factor authentication where relevant.