Security readout for executives and security teams
Plain-English summary
This flaw lets a malicious web page learn whether specific local files exist on a victim’s computer through Adobe Acrobat Reader’s Internet Explorer add-on. It does not grant file contents or system control, but it can expose sensitive clues about installed software, documents, or environment details.
Executive priority
Treat as a moderate endpoint hygiene issue, not an emergency breach indicator. Prioritize where legacy Internet Explorer and Acrobat Reader ActiveX remain in use, especially for users exposed to untrusted web content.
Technical view
CVE-2021-39857 is a CWE-200 information disclosure issue in the Adobe Acrobat Reader DC ActiveX add-on for Internet Explorer. A remote unauthenticated attacker can check for local file existence if a user visits an attacker-controlled page. CVSS is 4.3: network reachable, low complexity, user interaction required, confidentiality impact low.
Likely exposure
Exposure is mainly Windows endpoints using Internet Explorer with the Adobe Acrobat Reader DC add-on at affected versions: 2021.005.20060 and earlier, 2020.004.30006 and earlier, or 2017.011.30199 and earlier.
Exploitation context
The bundle does not show CISA KEV listing or other evidence of active exploitation. Exploitation requires social engineering or web traffic redirection to make a victim visit an attacker-controlled page.
Researcher notes
The source bundle identifies file-existence disclosure only; it does not support claims of arbitrary file read, code execution, privilege escalation, or active exploitation. Fixed-version details were not included in the bundle, so rely on Adobe APSB21-55 for remediation specifics.
Mitigation direction
- Review Adobe APSB21-55 and apply Adobe’s recommended update or remediation guidance.
- Inventory Acrobat Reader DC versions against the affected version thresholds.
- Reduce reliance on Internet Explorer and legacy ActiveX add-ons where business permits.
- Prioritize remediation for users who browse external websites from affected endpoints.
Validation and detection
- Confirm whether Internet Explorer has the Acrobat Reader DC add-on installed or enabled.
- Check endpoint software inventory for affected Acrobat Reader DC version branches.
- Verify remediation status against Adobe APSB21-55 guidance.
- Document any remaining legacy Internet Explorer dependency and compensating controls.
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
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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-2021-39857 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
- 4.3 (3.0)
- Known Exploited
- No
- Published
Vector: CVSS:3.0/AV:N/AC:L/PR:N/UI:R/S:U/C:L/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:3.0/AV:N/AC:L/PR:N/UI:R/S:U/C:L/I:N/A:N2.81.4Primary CVE scoreVulnerability scoring details
Base CVSS 3.0 score
4.3MediumVector: CVSS:3.0/AV:N/AC:L/PR:N/UI:R/S:U/C:L/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.