CVE-2018-20250: Understanding the Vulnerability, Exploitation Risks, Affected Software, Detection Methods, and Security Mitigations

Patch or replace old WinRAR installations now, and remove unacev2.dll wherever you find it. CVE-2018-20250 is not a flashy memory-corruption bug. It is worse in a practical sense: a poisoned archive can drop a file into a Windows Startup folder, then wait for the next login to run malware.

TLDR: CVE-2018-20250 is a path traversal flaw in the old ACE extraction component used by WinRAR before version 5.70 beta 1. An attacker can disguise a malicious ACE archive as a RAR file and place malware in sensitive folders, such as the Windows Startup directory. For example, in a small office with 40 PCs, if 25% still use outdated WinRAR builds, 10 machines may be exposed through one malicious email attachment. The fastest fix is to upgrade WinRAR and delete the old ACE library.

What CVE-2018-20250 Is

CVE-2018-20250 is a path traversal vulnerability tied to unacev2.dll, a decades-old library used by WinRAR to unpack ACE archives. The issue was publicly reported in 2019, but the vulnerable component itself had been around for about 19 years. That long shelf life is part of what made the bug so painful.

The flaw allows a crafted archive to write extracted files outside the folder chosen by the user. In plain terms, the archive says, “Extract this innocent file here,” but the vulnerable library follows a hidden path and writes it somewhere else.

A common target was:

• %APPDATA%\Microsoft\Windows\Start Menu\Programs\Startup
• C:\ProgramData\Microsoft\Windows\Start Menu\Programs\Startup

Files placed in these locations can run automatically when the user logs in. That turns a simple extraction action into a persistence method. No complex exploit chain is needed. No kernel trick is required. Just a malicious archive and a user who opens it.

Why This Bug Was So Risky

The risk came from a nasty mix of trust, old code, and file confusion. Many users treat compressed files as harmless containers. Attackers know that. They also know that file extensions can be misleading.

The catch is that the malicious file did not need to end in .ace. An ACE archive could be renamed to look like a .rar file. Older WinRAR versions would still process it using the ACE component after identifying the real format internally.

That means a user might receive something like:

• Invoice.rar
• Photos.rar
• Resume.rar
• ShippingDocuments.rar

Behind the scenes, the archive could be an ACE file with a crafted path. When extracted, it could plant a script, executable, or shortcut into a startup folder. The payload might not run right away, which makes the attack feel less obvious. It may trigger on the next reboot or login, when the user has forgotten about the archive.

Honestly, it feels like the kind of bug that punishes normal behavior. People extract files every day. They do not expect an archive tool to write into startup paths behind their back.

Affected Software

The main affected product was WinRAR before version 5.70 beta 1. Instead of trying to patch the obsolete ACE library, RARLAB removed ACE extraction support from WinRAR. That was the right call. The source code for the ACE component was not available, and the format itself was old and rarely needed.

Systems may be exposed if they have:

• WinRAR 5.61 or older, especially versions released before ACE support was removed.
• A copy of unacev2.dll in the WinRAR installation directory.
• Third-party tools that bundled the same vulnerable ACE extraction library.
• Portable WinRAR copies stored outside standard software inventory paths.

One annoying detail: portable tools often escape normal patch checks. Expect to waste time on shared folders, old admin toolkits, and “temporary” utilities that somehow survived for six years. A clean software inventory helps, but only if it checks actual files, not just installed program lists.

How Exploitation Usually Works

A typical attack path is simple:

1. The attacker creates a malicious ACE archive with crafted file paths.
2. The archive is renamed with a more trusted extension, such as .rar.
3. The file is sent through email, chat, file sharing, or a compromised website.
4. The victim extracts it using a vulnerable WinRAR version.
5. A payload lands in a startup folder or another sensitive location.
6. The payload runs later, often at the next user login.

This pattern was attractive to threat actors because it was cheap and reliable. It did not require a browser zero-day. It did not need administrator rights in many cases. If the user had write access to their own Startup folder, that was enough for persistence under that account.

Possible payloads included:

• Remote access trojans
• Password stealers
• Downloaders for later malware
• Cryptocurrency miners
• Scripts that weaken local security settings

Detection Methods

Detection should focus on both vulnerable software and suspicious file activity. Start with the easy win: find old WinRAR versions and the ACE DLL.

1. Search for vulnerable components

Look for:

• unacev2.dll
• WinRAR versions before 5.70
• Portable copies of WinRAR.exe and UnRAR.exe

Common locations include:

• C:\Program Files\WinRAR\
• C:\Program Files (x86)\WinRAR\
• User download folders
• Admin utility shares
• Software deployment caches

2. Monitor suspicious writes

Watch for archive tools writing files into startup paths. With Sysmon, Event ID 11 for file creation can help. Useful signals include WinRAR.exe creating files in:

• \Start Menu\Programs\Startup\
• \AppData\Roaming\Microsoft\Windows\
• \ProgramData\Microsoft\Windows\

3. Inspect archive attachments

Email gateways should not trust extensions alone. They should inspect file headers and archive structure. A file named .rar but identified as ACE deserves extra scrutiny or outright blocking.

4. Review endpoint alerts

EDR tools may flag:

• Archive extraction followed by startup folder modification
• New shortcut files in startup locations
• Script files created by compression utilities
• Delayed execution after reboot

Security Mitigations

The best fix is direct: upgrade WinRAR to a modern version. Version 5.70 beta 1 and later removed ACE support, which removes the vulnerable attack surface from WinRAR.

Recommended actions:

• Update WinRAR on all endpoints and servers.
• Delete unacev2.dll if it remains after upgrades or in portable folders.
• Block ACE archives at email and web gateways unless there is a real business need.
• Block renamed ACE files by checking file content, not only extensions.
• Restrict startup folder writes where possible.
• Use application control to stop unknown scripts and executables from user-writable paths.
• Train users to treat unexpected compressed attachments as risky.
• Audit old utilities in admin shares and software repositories.

For higher-risk environments, add rules that alert when compression tools create executable content. This includes .exe, .scr, .bat, .cmd, .vbs, .js, and shortcut files. That control catches more than this one CVE.

Why CVE-2018-20250 Still Matters

CVE-2018-20250 is old, but old bugs do not vanish. They sit quietly on forgotten laptops, lab machines, vendor workstations, and unmanaged desktops. Attackers like that. A vulnerability with public details, broad past exposure, and easy delivery can stay useful for years.

The lesson is simple: archive tools need the same patch discipline as browsers, document readers, and email clients. They parse untrusted files. They handle strange formats. They often run with the user’s full file permissions.

If you manage Windows systems, treat this CVE as a quick hygiene test. If you still find unacev2.dll, your software inventory has gaps. If users can freely run old portable tools, your endpoint controls need work. Fixing CVE-2018-20250 is not just about WinRAR. It is about closing a quiet, boring, very real path from “open this archive” to “malware runs at login.”