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CVE Record

CVE-2021-39861: Adobe Acrobat Reader DC Catalog Plugin Out-of-Bounds Read Bug

Acrobat Reader DC versions 2021.005.20060 (and earlier), 2020.004.30006 (and earlier) and 2017.011.30199 (and earlier) are affected by an out-of-bounds read vulnerability that could lead to disclosure of arbitrary memory information in the context of the current user. Exploitation of this issue requires user interaction in that a victim must open a malicious file.

MediumCVSS 5.5Not KEV-listedUpdated
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Glexia's TakeAutomated analysismoderate

Security readout for executives and security teams

Plain-English summary

This is an information disclosure flaw in Adobe Acrobat Reader DC. A user would need to open a malicious file, after which the bug could expose memory contents from that user’s session. It is not listed as known exploited in the provided KEV signal.

Executive priority

Treat as a routine but necessary endpoint patching issue. It can expose sensitive memory data, but requires user interaction and has no provided evidence of active exploitation.

Technical view

CVE-2021-39861 is a CWE-125 out-of-bounds read in Acrobat Reader DC affecting 2021.005.20060 and earlier, 2020.004.30006 and earlier, and 2017.011.30199 and earlier. CVSS 3.1 is 5.5: local attack vector, low complexity, no privileges, user interaction required, high confidentiality impact.

Likely exposure

Exposure is mainly endpoint workstations running affected Acrobat Reader DC versions, especially users who open PDFs or other files from email, web downloads, or external parties.

Exploitation context

The provided sources say exploitation requires a victim to open a malicious file. The source bundle does not show CISA KEV listing or other evidence of active exploitation.

Researcher notes

The bundle identifies only the product family, affected version thresholds, CWE-125 class, CVSS vector, and Adobe advisory reference. It does not provide root-cause detail, proof-of-concept status, or named fixed builds.

Mitigation direction

  • Inventory Acrobat Reader DC versions across managed endpoints.
  • Review Adobe APSB21-55 for vendor update guidance.
  • Update affected installations according to Adobe guidance.
  • Limit opening untrusted files until affected clients are updated.
  • Use endpoint and email controls to reduce malicious file delivery.

Validation and detection

  • Confirm whether installed versions are at or below affected version thresholds.
  • Check software inventory for Acrobat Reader DC 2021, 2020, and 2017 tracks.
  • Verify remediation status against Adobe APSB21-55 guidance.
  • Review endpoint detections involving suspicious document handling.
  • Prioritize users who routinely receive external PDF attachments.
Prepared
Confidence
high
Sources
3

Public sources used

Generated from the cited source records. This long-tail analysis has not been individually reviewed by a named human.

Potential ATT&CK relevance

Conservative CVE-to-ATT&CK context

These mappings and lookup hints may be relevant to the vulnerability behavior, CWE, affected product, or exposure path. Glexia-inferred context is not an official MITRE, ATT&CK, CWE, or CVE Program mapping.

ATT&CK lookup starting points

Use these exact CWE pages and searches to review the Glexia ATT&CK library from this CVE's weakness and description context.

cwe · low confidence lookup

CWE-125: Exact CWE lookup

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cve · low confidence lookup

CVE-2021-39861 mapping review

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Vulnerability profileCVE Program record
Severity
Medium
CVSS
5.5 (3.1)
Known Exploited
No
Published

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

Official CVE source material

CNA and ADP enrichment extracted from CVE v5

These fields come from the CVE record and ADP containers, not from Glexia's Take. They preserve time-varying source decisions such as CISA SSVC, KEV status, CVSS metrics, and provider references.

1CVSS vectors
0Timeline events
0ADP providers
2Source links

CVSS vector scores

1 official score

We collect every scored CVSS vector available in the official CNA and ADP containers. When more than one version is present, the table keeps the source vectors side by side instead of collapsing them into the highest score.

ScoreVersionSeverityVectorExploitImpactSource
5.5CVSS 3.1MediumCVSS:3.1/AV:L/AC:L/PR:N/UI:R/S:U/C:H/I:N/A:N1.83.6Primary CVE score

Vulnerability scoring details

Base CVSS 3.1 score

5.5Medium
CVSS 3.1 vector shape for CVE-2021-39861Attack VectorAttack ComplexityPrivileges RequiredUser InteractionScopeConfidentiality ImpactIntegrity ImpactAvailability Impact

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

Attack Vector
NetworkAdjacentLocalPhysical
Attack Complexity
LowHigh
Privileges Required
NoneLowHigh
User Interaction
NoneRequired
Scope
ChangedUnchanged
Confidentiality Impact
HighLowNone
Integrity Impact
HighLowNone
Availability Impact
HighLowNone

Source materials

Affected products

Products and packages named in the record

VendorProductVersion / packageStatus
AdobeAcrobat Readerunspecified, unspecified, unspecified, unspecifiedListed
Weakness

CWE details

CWE-125

CWE links open Glexia weakness intelligence pages with official CWE context, developer remediation guidance, and related CVE mappings.

CWE-125 · source CWE mapping

Out-of-bounds Read

Out-of-bounds Read represents a recurring weakness pattern that can create exploitable paths when design, validation, or implementation controls are missing.

Open CWE intelligenceSame-CWE CVEs