WinRAR 0-day that uses poisoned JPG and TXT files under exploit since April

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.

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.

RARLAB WinRAR before 6.23 mishandles ZIP archives that contain a benign file (such as a JPG) alongside a folder with the same name, causing the folder's contents - which can include malicious executable files - to be processed when the user merely attempts to view the benign file. By sending a crafted ZIP archive, an attacker gains arbitrary code execution on the victim's machine with the user's privileges. Because the flaw is local (AV:L) and requires user interaction, risk is limited to Windows systems running an unpatched copy of WinRAR, while machines without the tool are unaffected. The bug was actively exploited in the wild from April through October 2023, including by government-backed actors (APT28), SideCopy attacks on Indian government entities, ransomware operations, and trading-account theft campaigns, and it was added to CISA's Known Exploited Vulnerabilities catalog on 2023-08-24.

Do: Upgrade all Windows systems running WinRAR to version 6.23 or later, which fixes this flaw; if patching is not immediately possible, treat ZIP files from untrusted sources with caution and check archives for duplicate file/folder names before opening. Given KEV listing with known ransomware use and public proof-of-concept exploits, hunt for compromise by reviewing whether unexpected executables or scripts ran when ZIP archives were opened, and apply vendor mitigations per CISA's required action or discontinue use if mitigations are unavailable.