CVE-2018-20250

WinRAR versions through 5.61 contain a path traversal flaw (CWE-22, also mapped to CWE-36) in the UNACEV2.dll library used to unpack ACE-format archives: when a crafted archive's filename field contains absolute-path patterns, WinRAR ignores the user-selected extraction destination and writes the file to any absolute path the attacker specifies. It is triggered by inducing a user to open a malicious ACE archive (user interaction required), allowing arbitrary file placement on the endpoint — for example, writing an executable to a Windows startup folder to achieve code execution at next logon, as demonstrated in the original Check Point research. Only RARLAB WinRAR is affected, and the issue was assigned by Check Point acting as CNA, with multiple public proofs of concept available. Exploitation is well established: the flaw is on CISA's Known Exploited Vulnerabilities catalog (added 2022-02-15) with known ransomware use (e.g., JNEC.a delivered via this exploit), has been linked to targeted-activity by threat actors, and EPSS currently assigns a 96.3% probability of exploitation within 30 days. Because WinRAR lacks an automatic update mechanism, stale installations remained exposed and under active attack long after the vendor fix was published.

What to do: Apply updates per vendor instructions by upgrading to a WinRAR release newer than 5.61, in which the vulnerable ACE handling is patched or replaced; ensure endpoints actually run the updated build given WinRAR does not auto-update. If patching cannot be done promptly, delete or neutralize UNACEV2.dll (disabling ACE extraction) or unpack ACE archives with a patched alternative, and scrutinize ACE attachments arriving via email. Hunt for unexpected files dropped into auto-run locations (e.g., startup folders) and treat this as a KEV entry with the corresponding federal remediation deadline.