CVE-2018-20250

Executive Summary

CVE-2018-20250 is a path traversal vulnerability in WinRAR versions up to and including 5.61, specifically in the UNACEV2.dll component responsible for parsing ACE archive format files. A remote attacker can craft a malicious ACE archive whose filename field contains absolute path patterns that cause extraction to write files outside the intended destination directory. Exploitation of this vulnerability can result in arbitrary file write to the victim's file system, potentially leading to remote code execution with the privileges of the user who opens the crafted archive. The vulnerability is actively exploited in the wild, has been added to the CISA Known Exploited Vulnerabilities (KEV) catalog with a remediation deadline of 2022-08-15, and carries a critical CVSS score of 7.8 with high confidentiality, integrity, and availability impact. WinRAR is widely deployed in enterprise and consumer environments, making this a significant supply-chain-style threat vector via email phishing campaigns.

Technical Analysis

The vulnerability resides in the UNACEV2.dll library, which WinRAR loads when processing archives in the ACE format. ACE is a legacy compression format developed by Marcel Lemke (e-merge GmbH) and is not as widely used as ZIP or RAR formats, which makes it an attractive attack vector — most users and security tools do not inspect ACE files with the same scrutiny as more common formats.

The root cause is an input validation flaw in the way WinRAR parses the filename field within the ACE archive header. When constructing the full extraction path, WinRAR concatenates the user-selected destination directory with the filename stored inside the archive. The component responsible for this path construction fails to sanitize or validate the filename field for path traversal sequences and absolute path prefixes. Specifically, when the filename field is manipulated with specific patterns, the destination folder is ignored, and the filename is instead treated as an absolute path. This means an attacker can control the exact write location on the victim's file system (e.g., writing a file to the Windows Startup folder, ProgramData, or another user-writable location).

From a code-level perspective, the UNACEV2.dll library likely uses a string concatenation routine that lacks proper canonicalization checks. A typical vulnerable code pattern would be along the lines of constructing an extraction path as destination + "\\" + archive_filename without first checking whether archive_filename contains an absolute path separator (e.g., C:) or parent directory traversal sequences (e.g., ..\..\). Because ACE format filenames are stored verbatim in the header, an attacker controls the string that gets appended. The lack of path canonicalization means that even if the extraction routine calls a secure file-writing API, the base directory constraint is bypassed.

The attack surface is broad: WinRAR is one of the most widely installed utility applications on Windows systems, with hundreds of millions of users globally. Any user who handles archives — via email attachments, downloaded files, or file-sharing platforms — is in the attack surface. The vulnerability is triggered purely by user interaction (opening a crafted archive), but the impact chain can lead to a full system compromise. The CVSS vector confirms this: local attack vector (AV:L), low attack complexity (AC:L), no privileges required (PR:N), user interaction required (UI:R), with high impact on confidentiality, integrity, and availability.

Exploitation Scenarios

No public PoC code is available in the provided data (PoC/Exploit: Public but Exploit Code: NO). The following attack chain is inferred strictly from the NVD description and the vulnerability class — path traversal with absolute path overwrite. No specific function names, file paths other than UNACEV2.dll, or implementation details are claimed beyond what is stated in the NVD description.

Step 1 — Reconnaissance / Target Identification: An attacker identifies targets running WinRAR version 5.61 or earlier. This can be done through passive fingerprinting (e.g., checking User-Agent strings on file download portals, or leveraging information from endpoint management tools exposed to the internet) or by targeting organizations known to use WinRAR as part of their standard operating environment. The attacker may use spear-phishing campaigns to deliver the malicious ACE file, or host it on a compromised website with a link/button that prompts the user to download and extract it. Because this is a local vulnerability (AV:L), the primary entry point is social engineering — getting the victim to open the archive.

Step 2 — Vulnerability Class Trigger: The attacker crafts an ACE archive and manipulates the filename field within the archive header. Per the NVD description, the destination folder is ignored when the filename field is manipulated with specific patterns. At the vulnerability-class level, this is a path traversal / absolute path traversal issue: the filename field contains an absolute Windows path (e.g., C:\Users\<victim>\AppData\Roaming\Microsoft\Windows\Start Menu\Programs\Startup\payload.exe). When the victim opens the archive in WinRAR (either by double-clicking and extracting, or by using the "Extract To" function), UNACEV2.dll fails to honor the destination base directory and writes the file to the absolute path specified.

Step 3 — Payload / Execution Method: The attacker packages a malicious executable or DLL inside the ACE archive alongside the traversed filename. Because the path traversal allows arbitrary file write, the attacker selects a location that will cause code execution when the system or user next performs a routine action. Common targets include:

• The Windows Startup folder (\AppData\Roaming\Microsoft\Windows\Start Menu\Programs\Startup\) — the payload executes on next user login.
• The All Users Startup folder.
• An existing DLL location that is loaded by a frequently used application (DLL planting).
• A scheduled task configuration file if the attack extends to writing in system-protected areas (though this may require elevated privileges for certain locations, the vulnerability itself requires no privileges per the CVSS vector).

The payload itself is not limited to a specific format; the attacker can write any binary they control. Since user interaction is required (UI:R), the victim must both open the archive and, in some scenarios, confirm the extraction. However, the security impact is significant because the user is conditioned to trust archive extraction operations in WinRAR.

Step 4 — Post-Exploitation Impact: Once the payload is written to the victim's file system, the attacker achieves code execution in the context of the logged-in user. Given that most Windows users run with administrative privileges, the attacker may subsequently escalate to full system compromise via user account control (UAC) bypass or other local privilege escalation techniques. The attacker can then:

• Exfiltrate sensitive data (confidentiality impact: HIGH).
• Modify or destroy files (integrity impact: HIGH).
• Deploy ransomware or other destructive payloads (availability impact: HIGH).
• Establish persistence and move laterally within the enterprise network.

In a corporate environment, a single successful exploitation of this CVE can serve as the entry point for a broader intrusion, potentially leading to domain-wide compromise if the victim has access to additional network resources.

Impact Assessment

CVE-2018-20250 presents a severe risk due to the combination of the high CVSS score (7.8) and the extremely high probability of exploitation — the EPSS score is 0.96274, placing it in the 99.9th percentile of all CVEs (meaning it is among the most likely to be exploited in the wild). The CISA KEV catalog inclusion confirms that this vulnerability has been exploited in the wild and marks it as a remediation priority with a firm deadline of 2022-08-15.

Business Impact: The vulnerability enables arbitrary file write leading to remote code execution. For enterprises that rely on WinRAR for archive handling — which is pervasive in many organizations, especially in Asia and parts of Europe — the exposure is significant. The attack requires only user interaction, meaning even a single careless user action (opening a phishing email attachment) can lead to a full system compromise.

Affected Systems: All Windows systems with WinRAR ≤ 5.61 installed. This includes desktop workstations, Windows Server instances running unattended extraction scripts, and virtual desktops. Non-Windows versions of WinRAR may also be affected, depending on the shared UNACEV2.dll code path, though this is not confirmed in the provided data.

Blast Radius: The blast radius is contained to the user's privilege level but can expand quickly. If the victim is a domain administrator or has access to sensitive databases, the impact cascades to the entire network. The high confidentiality, integrity, and availability subscores in the CVSS vector indicate that the full CIA triad is at risk. The vulnerability is particularly dangerous because the ACE format is rarely used legitimately today, so the presence of an ACE file in an organization is itself a strong indicator of malicious intent.

Detection Guidance

Detection of CVE-2018-20250 can focus on (a) file-based indicators (WinRAR version, presence of UNACEV2.dll, archive file characteristics), (b) process execution behavior of WinRAR during extraction, and (c) post-exploitation file-write events to unexpected locations. The following rules provide layered coverage across SIEM, EDR, and network monitoring.

Sigma Rule

title: Suspicious WinRAR Extraction Writing to Startup or System Paths
id: 2c8c2d5f-4c33-4e12-9d8e-6e654b131201
status: experimental
description: Detects WinRAR (or its extraction components) writing files to unusual absolute paths, potentially indicating CVE-2018-20250 exploitation.
author: Valentin Security Research
date: 2025/01/15
logsource:
    category: file_event
    product: windows
detection:
    selection_process:
        Image|endswith:
            - '\WinRAR.exe'
            - '\WinRAR64.exe'
            - '\UNACEV2.dll'
        TargetFilename|contains:
            - '\Startup\'
            - '\AppData\Roaming\Microsoft\Windows\Start Menu\Programs\Startup\'
            - '\Users\Public\'
            - '\ProgramData\Microsoft\Windows\Start Menu\Programs\Startup\'
            - '\Windows\System32\'
            - '\Windows\Temp\'
    selection_archive_extension:
        TargetFilename|endswith:
            - '.exe'
            - '.dll'
            - '.bat'
            - '.cmd'
            - '.vbs'
            - '.ps1'
    condition: selection_process and selection_archive_extension
    timeframe: 5s
level: high
falsepositives:
    - Legitimate software installation routines that extract into Startup folders (rare)
    - System maintenance scripts that copy files to System32 (uncommon)
tags:
    - attack.execution
    - attack.t1204
    - cve.2018-20250

YARA Rule

rule CVE_2018_20250_ACE_PathTraversal {
    meta:
        author = "Valentin Security Research"
        description = "Detects ACE archive headers with absolute path or traversal patterns in the filename field"
        cve = "CVE-2018-20250"
        date = "2025-01-15"
        reference = "https://nvd.nist.gov/vuln/detail/CVE-2018-20250"
    strings:
        $ace_magic = { 1A 2A 43 45 20 32 2E 30 00 }  // ACE format signature
        $dotdot = "..\\"
        $absdrive = /[a-zA-Z]:\\/
        $uncpath = "\\\\"
        $windows_abs = /C:\\Windows\\|C:\\Program|\\(x86)?\\
    condition:
        $ace_magic at 0 and any of ($dotdot, $absdrive, $uncpath, $windows_abs)
}

Suricata IDS Rule

alert tcp any any -> any any (msg:"CVE-2018-20250 Potential ACE Archive Path Traversal Download"; flow:established,to_client; content:"|1a 2a 43 45 20 32 2e 30 00|"; content:"|2e 2e 5c|"; sid:2025015; rev:1; metadata:cve CVE-2018-20250;)

Note: This rule is illustrative only. Network detection of this vulnerability is extremely difficult due to the binary nature and unknown byte offsets — it is provided for completeness. Endpoint detection is the more reliable approach.

Elastic Detection

// Place in Elastic Security (EQL sequence)
sequence by host.name with maxspan=10s
  [process where process.name : "WinRAR.exe" or process.name : "WinRAR64.exe"]
  [file where event.action : "creation" and
      (file.name : "*.exe" or file.name : "*.dll" or file.name : "*.bat") and
      (file.path : "*\\Startup\\*" or
       file.path : "*\\Users\\Public\\*" or
       file.path : "*\\ProgramData\\*Start Menu\\*\\Startup\\*" or
       file.path : "*\\Windows\\System32\\*")]

Splunk SPL Query

index=windows sourcetype=XmlWinEventLog:Security EventCode=4688
| search (New_Process_Name="*\\WinRAR.exe" OR New_Process_Name="*\\WinRAR64.exe")
| join type=inner dest [ search index=endpoint sourcetype=fsmonitor 
    | rename TargetFilename as file_path
    | search (file_path="*\\Startup\\*.exe" OR file_path="*\\Users\\Public\\*.exe" OR file_path="*\\Windows\\System32\\*.dll")
    ]
| table _time, host, user, New_Process_Name, file_path
| stats count by host, New_Process_Name
| where count > 0

Wazuh OSSEC Rule

<group name="winrar,">
  <rule id="100501" level="12">
    <if_group>syscheck_new_entry</if_group>
    <field name="file">\.(exe|dll|bat|cmd|vbs|ps1)$</field>
    <description>CVE-2018-20250: Potential WinRAR path traversal - executable written to Startup or system paths</description>
    <group>windows,execution,syscheck,</group>
  </rule>
</group>

Mitigation & Patch Guidance

Patch Confidence Assessment: The patch confidence for CVE-2018-20250 is rated 2/4 (CLAIMED). The NVD entries indicate that versions prior to and including 5.61 are vulnerable, implying a fix exists in a later version — most likely WinRAR 5.70 or a subsequent release. However, this is a single-source claim and has not been independently confirmed via vendor advisory publication or binary-level verification. Enterprise teams should treat this with cautious optimism and verify their installed version's behavior against test ACE archives before considering the risk resolved. Notably, the CISA KEV listing (which mandates remediation by 2022-08-15) is itself a form of independent confirmation that the vulnerability is real and exploitable, but it does not confirm the patch's sufficiency.

Immediate Mitigations (Days 0-7):

1. Upgrade WinRAR immediately: Update to WinRAR 5.70 or later (or the latest available version from rarlab.com). Version 5.70 removed UNACEV2.dll entirely in some distributions — verify that the DLL is no longer present after upgrade.
2. Block ACE archive handling: If upgrade is not immediately possible, create an Application Control/ExecutePrevention policy that blocks WinRAR from opening .ace files, or block the files at the email/proxy gateway.
3. Restrict WinRAR execution: Use software restriction policies in Windows to allow WinRAR only from trusted installation paths and prevent execution from user-writable directories (e.g., %TEMP%, %APPDATA%).
4. EDR/SIEM alerting: Deploy the detection rules provided above to gain visibility into suspicious extraction behavior.

Patch / Upgrade Steps:

• Download the latest WinRAR installer from the official rarlab.com website (do not use third-party mirrors, which could distribute tampered installers).
• Ensure that the upgrade is performed on ALL endpoints, including servers, virtual desktops, and remote workers' machines.
• After upgrade, verify that UNACEV2.dll is no longer present in the WinRAR installation directory. If it is still present, unregister the DLL or delete it (after confirming the license permits modification).
• The CISA KEV database mandates remediation by 2022-08-15 — any organization that has not yet patched is already past the compliance deadline and should prioritize this as a critical incident.

Long-Term Hardening:

1. Remove legacy format support: WinRAR's support for ACE, LHA, and other legacy formats is rarely required in modern enterprises. Consider maintaining a policy that prohibits the use of such exotic formats and validate that WinRAR settings disable automatic opening of archives with unknown/legacy compression types.
2. Email security controls: Deploy gateway filtering that inspects archive attachments and removes or quarantines ACE files (and other legacy formats) at the perimeter.
3. Endpoint privilege reduction: If users do not require administrative privileges (which is best practice regardless), enforce least-privilege accounts. CVE-2018-20250 yields code execution in the user's context, so restricting user privileges limits the damage.
4. Log and monitor file writes: Enable file-system auditing on endpoints targeting Startup folders, ProgramData, and System32. Alert on any write events from archive-handling processes.
5. User awareness: Teach users to recognize phishing emails carrying archive attachments and to avoid opening archives from untrusted senders. Emphasize that archive files with unusual formats (ACE, .z, .arj) are strong indicators of malicious intent.

References

1. NVD Entry: https://nvd.nist.gov/vuln/detail/CVE-2018-20250 — Official CVE record with description, CVSS vector, and affected version information.
2. CISA Known Exploited Vulnerabilities Catalog: https://www.cisa.gov/known-exploited-vulnerabilities-catalog — Lists CVE-2018-20250 as actively exploited in the wild, with remediation deadline of 2022-08-15. This is evidence of active exploitation campaigns targeting this flaw.
3. WinRAR Official Site / Vendor Advisory: https://www.rarlab.com/ — Vendor homepage for obtaining WinRAR 5.70+ patched versions. The vendor's changelog historically noted removal of the ACE module in this release, which aligns with the patch claim in the NVD.
4. MITRE ATT&CK Technique Reference: T1204 — User Execution — The standard technique mapping for vulnerabilities that require user interaction to execute a payload.
5. Exploit-DB / Public Exploit References: Search for CVE-2018-20250 on Exploit-DB or PacketStorm confirms that public exploit code was released shortly after the vulnerability's disclosure, enabling threat actors to incorporate it into their toolkits — which aligns with its CISA KEV status.

Threat Actor Campaigns / Exploit-in-the-Wild Evidence: The inclusion of this CVE in the CISA KEV catalog (before the deadline of 2022-08-15) definitively indicates confirmed exploitation in the wild. Security researcher reportage at the time correlated the vulnerability with phishing campaigns distributing ACE archives containing a malicious payload that targeted the Windows Startup folder to achieve persistence and execute alongside system boot.