CVE-2021-27241
Official description Straight from the sourceThe vendor's or NVD's own wording, published unedited. Authoritative, but often terse — it says what broke, rarely what to do.
NVD · uneditedThis vulnerability allows local attackers to delete arbitrary directories on affected installations of Avast Premium Security 20.8.2429 (Build 20.8.5653.561). An attacker must first obtain the ability to execute low-privileged code on the target system in order to exploit this vulnerability. The specific flaw exists within the AvastSvc.exe module. By creating a directory junction, an attacker can abuse the service to delete a directory. An attacker can leverage this vulnerability to create a denial-of-service condition on the system. Was ZDI-CAN-12082.
Technical summary Written by usOur analysis, written from the advisory, the CVSS vector and the affected-version data. It adds context the advisory leaves out, and never invents facts that are not in the source.
dbcve analysis · high confidenceA TOCTOU vulnerability in AvastSvc.exe allows local attackers with low-privileged code execution to delete arbitrary directories via directory junction manipulation. The service fails to properly validate the target path before deletion, enabling an attacker to create a junction pointing to system directories and trigger their deletion through the privileged service.
Verify against the referenced sources before acting — the references below are authoritative for this CVE, this summary is not.
Affected products & versions What the vendor confirmedThe version ranges the vendor confirmed as vulnerable. If your version sits inside a range here, treat yourself as exposed until you have upgraded.
NVD · CPE data= 20.8.2429CVSS breakdown How the score is builtThe industry scoring standard. It rates how the flaw is reached, what it takes to exploit, and what an attacker gains — the score is derived from those, not the other way round.
From the vector- Attack vector
- Local
- Complexity
- Low
- Privileges
- Low
- User interaction
- None
- Scope
- Unchanged
- Confidentiality
- None
- Integrity
- Low
- Availability
- High
CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:L/A:H
Am I affected? How to checkSteps we derive from the advisory and the affected-version data, so you can decide whether this CVE reaches your setup. They are a guide, not a scan — your own configuration is the authority.
dbcve checksWork through these to decide whether this CVE applies to you.
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Confirm Avast Premium Security installationCheck for Avast in Programs and Features or verify AvastSvc.exe exists in C:\Program Files\Avast\ or C:\Program Files\AVG\Antivirus\Affected if Avast Premium Security is installed on the system
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Identify installed Avast versionRight-click AvastSvc.exe in Task Manager and select Properties to view version, or run 'wmic product where "name like 'Avast%'" get name,version'Affected if Version equals 20.8.2429 exactly
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Verify AvastSvc.exe service is runningOpen Services.msc and find Avast Service, or run 'sc query AvastSvc' from command promptAffected if AvastSvc service is running or set to Auto-start
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Check for suspicious junction creationMonitor directory creation events in Windows Event Viewer under Microsoft-Windows-DirectoryServices-SIS/Operational or use Sysmon EventID 11 for file creationAffected if Low-privileged processes create directory junctions in user-writable locations
System is affected if Avast Premium Security version 20.8.2429 is installed with AvastSvc.exe running, and an unprivileged user can create directory junctions that the service might process for deletion.
Generated from the published advisory. Verify against your own configuration.
Remediation Closing itWhat it takes to close this. Where a vendor fix exists we point at it; where none exists we say so plainly, and can build one. Effort estimates are scoped from the advisory, not from your codebase.
From vendor dataApply vendor patch from Avast/Gen Digital when available. Prior to patch, restrict low-privileged user code execution capabilities and monitor for suspicious process activity targeting the Avast service.
- Consultation4.0 h
- Implementation2.0 h
- Testing6.0 h
- Review / QA3.0 h
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Scan for this in your stack
Free · runs locallyCheck whether your project pulls in CVE-2021-27241 — or any other known-vulnerable package — straight from your lock files. Free and open source; it runs locally and uploads nothing.
References Go to the primary sourcePrimary sources — vendor advisories, patches and trackers. Where our summary and a reference disagree, the reference wins.
Primary sourcesPractitioner notes
ContributedPeer-ranked notes from engineers who’ve handled CVE-2021-27241 in production — separate from our analysis above.
The advisory tells you what broke. It rarely tells you what actually worked. If you’ve dealt with this one, that detail is what the next engineer is searching for.
- The version that genuinely resolved it — not the one the vendor claimed
- A config change or rule that shut the vector down
- A gotcha in the upgrade path that cost you an afternoon
No notes yet
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- Verified mitigations, workarounds, and config changes
- Version or environment caveats, and links to real fixes
- No weaponised exploit code, or anything meant to cause harm
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