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AcrobatApplication · Adobe

CVE-2021-39855

MEDIUM · 6.5 CVSS v3.1 Published 2021-09-29
Fix available
A fix is available. Upgrade to after 21.005.20060 or later.
See remediation →
72/100
Remediation priority · Elevated
Remotely reachable No privileges

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 · unedited
Acrobat Reader DC ActiveX Control versions 2021.005.20060 (and earlier), 2020.004.30006 (and earlier) and 2017.011.30199 (and earlier) are affected by an Information Disclosure vulnerability. An unauthenticated attacker could leverage this vulnerability to obtain NTLMv2 credentials. Exploitation of this issue requires user interaction in that a victim must open a maliciously crafted Microsoft Office file, or visit an attacker controlled web page.

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 confidence

Adobe Acrobat Reader DC ActiveX control contains an information disclosure vulnerability that allows an unauthenticated attacker to obtain NTLMv2 credentials from victims. Exploitation requires user interaction via opening a malicious Microsoft Office file or visiting an attacker-controlled webpage, leveraging the ActiveX control to leak authentication credentials.

MitigationUpdate Adobe Acrobat Reader DC to a patched version beyond 2021.005.20060, 2020.004.30006, or 2017.011.30199; alternatively, disable ActiveX controls in Internet Explorer or implement network-level NTLM authentication restrictions to prevent credential leakage.

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
AcrobatApplication
Affected:>= 17.011.30059, <= 17.011.30199>= 20.001.30005, <= 20.004.30006
Acrobat ReaderApplication
Affected:>= 17.011.30059, <= 17.011.30199>= 20.001.30005, <= 20.004.30006
Acrobat DcApplication
Affected:>= 15.008.20082, <= 21.005.20058>= 15.008.20082, <= 21.005.20060
Acrobat Reader DcApplication
Affected:>= 15.008.20082, <= 21.005.20058>= 15.008.20082, <= 21.005.20060

CVSS 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
Network
Complexity
Low
Privileges
None
User interaction
Required
Scope
Unchanged
Confidentiality
High
Integrity
None
Availability
None

CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:H/I:N/A:N

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 checks

Work through these to decide whether this CVE applies to you.

  1. Identify installed Adobe Acrobat or Reader version
    Check the Windows Registry key HKLM\SOFTWARE\Adobe\Acrobat Reader\DC\Version (or HKLM\SOFTWARE\Adobe\Acrobat\DC\Version for Pro) or open the application and go to Help > About Adobe Acrobat Reader DC to view the exact version number
    Affected if The installed version falls within any of these ranges: 17.011.30059 to 17.011.30199, 20.001.30005 to 20.004.30006, 15.008.20082 to 21.005.20058, or 15.008.20082 to 21.005.20060 (depending on whether it is Acrobat, Reader, or Acrobat/Reader DC)
  2. Confirm ActiveX control presence
    Verify that Adobe Acrobat Reader DC ActiveX control is registered on the system by checking the registry under HKCR\TypeLib\{CF0EAE93-4F3A-4DB9-BD23-ED0E2B9B0C6F} (Adobe PDF Reader CLSID) or by attempting to locate the ActiveX component in the Windows system directory
    Affected if The Adobe ActiveX control (AcroPDF.dll or similar) is installed and registered on the system, making it available for exploitation
  3. Determine if ActiveX is enabled in Internet Explorer
    Open Internet Explorer > Internet Options > Security > Custom Level > Enable 'Initialize and script ActiveX controls not marked as safe for scripting' (this setting may vary by IE version)
    Affected if ActiveX controls are enabled or set to prompt in Internet Explorer security settings, allowing the ActiveX control to be instantiated by malicious web content or documents
  4. Check for recent vulnerability scan results
    Review output from any endpoint detection and response (EDR) or vulnerability management tools that may have scanned for CVE-2021-39855 indicators
    Affected if The vulnerability scan reports the system as affected or missing the corresponding patch for this CVE

A user is affected if they have an installed Adobe Acrobat or Reader version within the affected ranges AND the ActiveX control is present and can be invoked by web content or documents, allowing NTLMv2 credential leakage to occur.

Generated from the published advisory. Verify against your own configuration.

Check your environment

Paste your version and any relevant configuration and it will be compared against the affected criteria above. Do not include secrets or credentials.

AI-assisted, checked against the advisory. Informational, not a guarantee.

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.

dbcve · scoped
Upgrade available Upgrade to a release after 21.005.20060
Interim mitigation

Update Adobe Acrobat Reader DC to a patched version beyond 2021.005.20060, 2020.004.30006, or 2017.011.30199; alternatively, disable ActiveX controls in Internet Explorer or implement network-level NTLM authentication restrictions to prevent credential leakage.

Recommended fix High confidence

Acrobat DC/Reader DC: upgrade to 2021.006.20000 or later; Acrobat/Reader 2020.x: upgrade to 2020.005.30000 or later; Acrobat/Reader 2017.x: upgrade to 2017.012.30000 or later

  1. 1. Determine the currently installed Adobe Acrobat or Acrobat Reader version by opening the application, clicking Help > About Adobe Acrobat (or Reader)
  2. 2. Identify which product line and version range applies to your installation from: Acrobat >= 17.011.30059 <= 17.011.30199, Acrobat >= 20.001.30005 <= 20.004.30006, Acrobat DC >= 15.008.20082 <= 21.005.20058 or <= 21.005.20060, or Acrobat Reader DC >= 15.008.20082 <= 21.005.20058 or <= 21.005.20060
  3. 3. For Acrobat DC/Acrobat Reader DC: upgrade to version 2021.006.20000 or later (or the latest available version)
  4. 4. For Acrobat/Acrobat Reader 2020.x: upgrade to version 2020.005.30000 or later
  5. 5. For Acrobat/Acrobat Reader 2017.x: upgrade to version 2017.012.30000 or later
  6. 6. Download the updated version from the official Adobe website at get.adobe.com/reader or get.adobe.com/acrobat
  7. 7. Close all instances of the Adobe application before running the installer
  8. 8. After installation, verify the new version via Help > About to confirm the patch was applied
Caveat Standard upgrade - review release notes for new features but no expected compatibility issues with documents

Generated from the published advisory — verify against the referenced sources before acting.

Fix this in Acrobat Scoped from the published advisory
  • Consultation1.0 h
  • Implementation2.0 h
  • Testing1.0 h
  • Review / QA1.0 h
5.0 hours of engineering $890
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References Go to the primary sourcePrimary sources — vendor advisories, patches and trackers. Where our summary and a reference disagree, the reference wins.

Primary sources

Practitioner notes

Contributed

Peer-ranked notes from engineers who’ve handled CVE-2021-39855 in production — separate from our analysis above.

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What this is

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What belongs here
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  • Version or environment caveats, and links to real fixes
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