Security readout for executives and security teams
Plain-English summary
This vulnerability can expose a user’s NTLMv2 credentials if they use an affected Adobe Acrobat Reader DC ActiveX Control and are tricked into visiting an attacker-controlled web page. It is mainly a credential-theft risk, not a system takeover flaw by itself.
Executive priority
Treat this as a moderate-priority credential exposure issue. It needs timely remediation on legacy endpoints, especially where stolen NTLMv2 material could support lateral movement or unauthorized access.
Technical view
CVE-2021-39856 is a CWE-200 information disclosure issue in Acrobat Reader DC ActiveX Control through listed 2021, 2020, and 2017 builds. It is network-reachable, requires no attacker authentication, requires user interaction, and affects confidentiality only, with CVSS 3.1 score 6.5.
Likely exposure
Exposure is most likely on Windows environments still running affected Acrobat Reader DC ActiveX Control versions, especially where browser-based ActiveX use or legacy Reader integrations remain available.
Exploitation context
The sources state exploitation requires a victim to visit an attacker-controlled web page. The bundle does not identify CISA KEV listing or active exploitation evidence.
Researcher notes
The source bundle names LoadFile and NTLMv2 credential disclosure but does not provide exploit mechanics. Affected version details are specific in the description, while CPE and detailed product enumeration are incomplete.
Mitigation direction
- Review Adobe APSB21-55 and apply the vendor security update guidance.
- Inventory Acrobat Reader DC ActiveX Control versions across managed endpoints.
- Reduce or disable legacy ActiveX exposure where business use does not require it.
- Monitor for unexpected NTLM authentication attempts from user workstations.
- Prioritize users handling sensitive documents or high-value credentials.
Validation and detection
- Confirm installed Reader builds are newer than affected versions listed in the CVE.
- Check endpoint software inventory for Acrobat Reader DC ActiveX Control presence.
- Review browser and endpoint policies for ActiveX availability.
- Look for NTLM authentication attempts to unusual or external destinations.
- Document any exceptions requiring legacy Reader or ActiveX support.
Public sources used
Generated from the cited source records. This long-tail analysis has not been individually reviewed by a named human.
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-200: Information exposure and cloud metadata lookup
Information exposure and SSRF weaknesses can make discovery, cloud metadata, and credential material review relevant. Open the exact CWE lookup page first, then review the ATT&CK searches from that MITRE weakness context. This is a Glexia lookup hint, not an official ATT&CK mapping.
Open ATT&CK lookupCVE-2021-39856 mapping review
Open the CVE-to-ATT&CK bridge for reviewed, inferred, or future official mappings tied to this CVE.
Open ATT&CK lookup- Severity
- Medium
- CVSS
- 6.5 (3.1)
- Known Exploited
- No
- Published
Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:H/I:N/A:N
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.
CVSS vector scores
1 official scoreWe 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.
CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:H/I:N/A:N2.83.6Primary CVE scoreVulnerability scoring details
Base CVSS 3.1 score
6.5MediumVector: CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:H/I:N/A:N
Source materials
- CVE List V5 sourceCVE List V5
- https://helpx.adobe.com/security/products/acrobat/apsb21-55.htmlCVE reference · x_refsource_MISC
Products and packages named in the record
CWE details
CWE links open Glexia weakness intelligence pages with official CWE context, developer remediation guidance, and related CVE mappings.
Exposure of Sensitive Information to an Unauthorized Actor
Exposure of Sensitive Information to an Unauthorized Actor represents a recurring weakness pattern that can create exploitable paths when design, validation, or implementation controls are missing.