CriticalCVE-2026-34621

    Adobe Acrobat and Reader Prototype Pollution Vulnerability (CVE-2026-34621)

    Published Last modified In CISA KEV since Sources: CVE.org, Adobe, pwn.watch

    What is affected, what to do

    Acrobat Reader versions 24.001.30356, 26.001.21367 and earlier are affected by an Improperly Controlled Modification of Object Prototype Attributes ('Prototype Pollution') vulnerability that could result in arbitrary code execution in the context of the current user. Exploitation of this issue requires user interaction in that a victim must open a malicious file.

    The vulnerability is being actively exploited; treat it as a priority. Update acrobat to version 24.001.30360, acrobat_dc to version 26.001.21411, acrobat_reader_dc to version 26.001.21411. Immediate actions, mitigation and detection rules are in the runbook below.

    Affected
    • Adobe Acrobat DC lte *..26.001.21367
    • Adobe Acrobat Reader DC lte *..26.001.21367
    • Adobe Acrobat 2024 lte *..24.001.30356

    Not sure whether and how you are affected? Ask a question about this vulnerability

    Metrics

    8.6

    Priority

    XIEM Effective CVSS: highest available CVSS rating from NVD and vendor data.

    Source: NVD, CVSS v3

    8.6

    CVSS v3

    Severity High on the scale from 0 to 10.

    Source: CVE.org, Adobe, pwn.watch

    2.2 %

    EPSS score

    Moderate — model estimates 1-10% exploitation likelihood.

    Source: FIRST EPSS, percentile 82.3

    Show all metrics
    Severity
    Critical
    Priority (0–325)

    The priority score is a consolidated metric that condenses several individual signals into a single, action-guiding ranking: technical severity (CVSS), exploitation likelihood (EPSS), known-exploited status (KEV) and the context of the affected asset. The aim is to answer the only truly important question in vulnerability management: what first?

    Read in glossary
    213.39 Methodology
    Exploit maturity

    An exploit is code or a technique that specifically uses a concrete vulnerability to force unintended behaviour — such as executing code or escalating privileges. It is the tool that turns a theoretical flaw into a practical attack.

    Read in glossary
    actively exploitedSource: VulnCheck KEV (opens in a new tab) How we classify
    XIEM Effective CVSS

    XIEM® is the tiered managed-security operating model from NEOSEC and a registered trademark of NEOSEC GmbH. It extends classic SIEM — the central collection, normalisation and correlation of security-relevant events — with endpoint forensics, network visibility and deception technology, and consolidates them into three building tiers: Sentry, Orchestrate and Command.

    Read in glossary
    8.6 Methodology
    CVSS v3

    CVSS is the globally established, open standard for assessing the technical severity of a vulnerability on a scale from 0 to 10. It describes how easily a flaw can be exploited and what damage it can cause — but not how likely its actual exploitation is. CVSS and the complementary EPSS deliberately answer different questions.

    Read in glossary
    8.6
    EPSS score

    The EPSS score is the absolute model output for an individual vulnerability: the estimated probability of real-world exploitation within the next 30 days, expressed as a value between 0 and 1 (i.e. 0 % to 100 %). A score of 0.42 means „the model estimates a 42 % chance of real-world exploitation within 30 days“.

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    2.2 %
    Moderate — model estimates 1-10% exploitation likelihood.
    EPSS percentile

    The EPSS percentile is the relative position of a vulnerability within all EPSS-scored CVEs, expressed as a value between 0 and 1. A percentile of 0.90 means „this CVE has a higher EPSS score than 90 % of all other scored CVEs“. Unlike the EPSS score, it is not a probability — but a positioning within a ranking.

    Read in glossary
    82.3
    Critical — this CVE ranks in the top fifth of all CVEs scored today (rank ≥ 80%).
    KEV status

    The KEV catalogue is a list maintained by the US authority CISA of vulnerabilities for which active exploitation in the wild has been confirmed. It answers not the question „how severe?“ but „is it already being attacked?“ — thereby sharply prioritising patching.

    Read in glossary
    Actively exploited
    Published
    In CISA KEV since

    The KEV catalogue is a list maintained by the US authority CISA of vulnerabilities for which active exploitation in the wild has been confirmed. It answers not the question „how severe?“ but „is it already being attacked?“ — thereby sharply prioritising patching.

    Read in glossary
    CWE

    CWE is a catalogue of typical weakness types in software and hardware — such as flawed input validation, unsafe memory handling or poor authentication. Where a CVE names a concrete vulnerability in a concrete product, a CWE describes the underlying class of defect.

    Read in glossary
    CWE-1321

    Response and mitigation

    Why act now?

    Prioritisation rationale

    Exploitation requires local access and user interaction but no authentication. Because the scope is changed, it can also break out of the Reader sandbox. That makes it a natural fit for weaponised-PDF phishing aimed at document-heavy staff, where a single opened file means full workstation compromise. Exploitation likelihood is clearly elevated but not top-tier. CISA has not flagged ransomware use, which points more towards targeted espionage or initial-access operations. Given that Acrobat sits on virtually every office endpoint, treat remediation as high priority within days, and immediate for exposed roles and shared terminal servers.

    1. Runbook step 1

      Immediate response (0-24 h)

      • Deploy the vendor patch: Update Acrobat DC and Acrobat Reader DC (Continuous track) to 26.001.21411 or later, and Acrobat 2024 (Classic track) to 24.001.30360 or later, preferably 24.001.30362. Look up the APSB bulletin ID in the vendor advisory. Push via Intune, SCCM/MECM or the Adobe Admin Console as a top-priority deployment.
      • Inventory vulnerable installs: Query EDR or software inventory for versions at or below 24.001.30356 (Classic) and 26.001.21367 (Continuous). Don't forget terminal servers, VDI golden images and Citrix hosts.
      • Disable Acrobat JavaScript until patching is complete: Prototype pollution is reached through the embedded JS engine. Set HKLM\SOFTWARE\Policies\Adobe\Adobe Acrobat\DC\FeatureLockDown\bDisableJavaScript = 1 via GPO, with the equivalent under ...\Acrobat Reader\DC\... for Reader. Grant exceptions only to teams with a proven dependency on scripted forms.
      • Tighten mail filtering: Quarantine or sandbox-detonate external PDF attachments containing /JavaScript, /JS, /OpenAction or /AA.
      • Warn users: Send a short alert to high-document-volume roles (procurement, finance, HR, sales): don't open unexpected PDFs, and report anything suspicious to the SOC.

    2. Runbook step 2

      Mitigation layers

      • Endpoint hardening (Microsoft Defender ASR): Switch "Block Adobe Reader from creating child processes" (GUID 7674ba52-37eb-4a4f-a9a1-f0f9a1619a2c) to Block mode. This breaks the usual post-exploitation chain.

      • Enforce sandboxing: Enable the following via GPO:

        • Protected Mode: bProtectedMode = 1
        • Protected View for all files: iProtectedView = 2
        • Enhanced Security: bEnhancedSecurityStandalone = 1, bEnhancedSecurityInBrowser = 1

        Lock these settings through FeatureLockDown so users cannot turn them off.

      • Network layer: Restrict egress from Acrobat.exe, AcroRd32.exe and AcroCEF.exe to Adobe update and licensing domains via host firewall or proxy. Block direct connections to uncategorised domains or raw IPs to stop second-stage downloads.

      • Least privilege: Code runs as the logged-on user. Remove local admin rights (LAPS, endpoint privilege management) and keep privileged accounts strictly separated from email use.

      • Alternative viewers: Where patching lags, route externally sourced PDFs to the browser's built-in viewer or to isolated environments (Application Guard, remote browser isolation).

    3. Runbook step 3

      Detection rules

      • Sysmon EID 1 / EDR process ancestry: Acrobat.exe or AcroRd32.exe spawning cmd.exe, powershell.exe, wscript.exe, mshta.exe, rundll32.exe or regsvr32.exe. KQL: DeviceProcessEvents | where InitiatingProcessFileName in~ ("acrobat.exe","acrord32.exe") and FileName in~ ("cmd.exe","powershell.exe","mshta.exe","rundll32.exe","wscript.exe","regsvr32.exe")
      • Sysmon EID 11: Acrobat processes writing executables or scripts (.exe, .dll, .js, .hta, .lnk) to %TEMP%, %APPDATA% or autostart locations.
      • Sysmon EID 3 / Zeek conn.log: Outbound connections from Acrobat processes to non-Adobe destinations, especially right after a file is opened.
      • Mail and proxy logs: PDFs with embedded JavaScript containing __proto__, constructor.prototype or Object.prototype in decompressed streams. For retro-hunting, use a YARA rule that pairs /JavaScript with these strings.
      • Windows Defender Event ID 1121/1122: Raise ASR blocks for the Adobe child-process rule as high-severity alerts in the SIEM.

    Detection rules

    No NEOSEC detection rule is available for this vulnerability yet.

    Classification under NIS2 and CRA

    Rule-based classification from exploitation status and severity. It does not replace a case-by-case assessment.

    NIS2

    NIS2 is EU Directive (EU) 2022/2555 to raise the level of cybersecurity across the Union. It substantially broadens obligations compared with the first NIS directive: more sectors, mandatory risk management, staged reporting duties and personal accountability of senior management.

    Read in glossary
    Actively exploited: treat the vulnerability as a priority in vulnerability handling under Art. 21(2)(e) NIS2. If it is exploited in your environment, check the reporting obligation under Art. 23 (early warning within 24 hours).
    CRA

    The CRA is EU Regulation (EU) 2024/2847 setting horizontal cybersecurity requirements for „products with digital elements“ — from industrial controllers and software to connected consumer devices. For the first time manufacturers must ensure security across the entire product lifecycle, handle vulnerabilities and provide security updates.

    Read in glossary
    Since 11.09.2026, manufacturers of affected products with digital elements must report actively exploited vulnerabilities under Art. 14 CRA, with an early warning within 24 hours via ENISA's single reporting platform.
    ISO/IEC 27001

    ISO/IEC 27001 is the internationally leading standard for an information security management system (ISMS). It describes a risk-based, process-oriented framework to systematically plan, implement, review and improve information security.

    Read in glossary
    Control A.8.8, management of technical vulnerabilities: assess relevance to your systems, plan remediation and keep evidence.

    Description

    Acrobat Reader versions 24.001.30356, 26.001.21367 and earlier are affected by an Improperly Controlled Modification of Object Prototype Attributes ('Prototype Pollution') vulnerability that could result in arbitrary code execution in the context of the current user. Exploitation of this issue requires user interaction in that a victim must open a malicious file.

    Source: CISA KEV Catalog, adobe_bulletin, cvelistv5, NVD (NIST), CVE.org (MITRE), pwn.watch

    Affected products

    The following versions are affected. Older releases of the same product line are also vulnerable unless stated otherwise.

    Affected products
    Affected versionsFixed in
    AdobeAcrobat 2024
    • ≤ 24.001.30356
    • —
    AdobeAcrobat DC
    • ≤ 26.001.21367
    • —
    AdobeAcrobat Reader DC
    • ≤ 26.001.21367
    • —
    adobeacrobat
    • 24.0.0 – < 24.001.30360
    • 24.0.0 – < 24.001.30362
    • 24.001.30360
    • 24.001.30362
    adobeacrobat_dc
    • < 26.001.21411
    • 26.001.21411
    adobeacrobat_reader_dc
    • < 26.001.21411
    • 26.001.21411

    Vendors and products

    Weakness classes (CWE)

    • CWE-1321Improperly Controlled Modification of Object Prototype Attributes ('Prototype Pollution')

      Variant

      The product receives input from an upstream component that specifies attributes that are to be initialized or updated in an object, but it does not properly control modifications of attributes of the object prototype.

      Entry at MITRE CWE-1321

    Public exploit references

    Publicly reported proof-of-concepts, exploit modules and detection templates for this vulnerability. NEOSEC links to the original sources only and does not provide any exploit code.

    Public exploit references
    CISA KEV CatalogCISA Known Exploited Vulnerability (opens in a new tab)cisa.govactively exploited

    References & sources

    • helpx.adobe.comAdobe Acrobat and Reader — CVE-2026-34621Vendor advisory, pwn-watch
    • cisa.govknown-exploited-vulnerabilities-catalog?field_cve=CVE-2026-34621government-resource
    • pwn.watchpwn.watch: Adobe Acrobat and Reader: Adobe Acrobat and Reader Prototype Pollution Vulnerabilitypwn-watch
    • NVD (NIST)NVD: CVE-2026-34621pwn-watch
    • cisa.govCISA Known Exploited Vulnerabilities Catalogpwn-watch

    Reanalysis & status changes

    Chronological NVD audit events for this CVE — reanalyses, CVSS updates, CPE diffs.

    1. CVE Modifiedpsirt@adobe.com

      • Affected: after Affected value modified. <a href="https://github.com/CVEProject/cvelistV5/blob/main/cves/2026/34xxx/CVE-2026-34621.json">CVE-2026-34621</a>

    2. CVE Modified134c704f-9b21-4f2e-91b3-4a467353bcc0

      • Reference: after https://www.cisa.gov/known-exploited-vulnerabilities-catalog?field_cve=CVE-2026-34621
      • Reference: before https://www.cisa.gov/known-exploited-vulnerabilities-catalog?field_cve=CVE-2026-34621
      • Reference Type: before https://www.cisa.gov/known-exploited-vulnerabilities-catalog?field_cve=CVE-2026-34621 Types: US Government Resource

    3. CVE Modifiedpsirt@adobe.com

      • Affected: after Affected value modified. <a href="https://github.com/CVEProject/cvelistV5/blob/main/cves/2026/34xxx/CVE-2026-34621.json">CVE-2026-34621</a>
      • Reference: after https://helpx.adobe.com/security/products/acrobat/apsb26-43.html
      • Reference: before https://helpx.adobe.com/security/products/acrobat/apsb26-43.html
      • Reference Type: before https://helpx.adobe.com/security/products/acrobat/apsb26-43.html Types: Vendor Advisory
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