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CVE Record

CVE-2026-78207: exceljs through 4.4.0 Prototype Pollution via deepMerge Reached From Note Serialization

exceljs through 4.4.0 contains a prototype pollution vulnerability in the deepMerge helper that fails to reject __proto__, constructor, or prototype keys when merging note objects. Attackers can assign parsed JSON with a malicious __proto__ property to cell notes, modifying Object.prototype and affecting all plain objects created in the process.

CriticalCVSS 9.4Not KEV-listedUpdated
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Glexia's TakeAutomated analysiscritical

Security readout for executives and security teams

Plain-English summary

Applications using exceljs through 4.4.0 may let attacker-controlled spreadsheet note data alter shared JavaScript object behavior. Because the corruption can influence unrelated objects in the same process, a successful attack could compromise data, application decisions, or service availability.

Executive priority

Treat as urgent where internet-facing or partner-facing inputs can reach spreadsheet note serialization. Prioritize confirming reachability immediately, then contain exposed workflows. Systems using exceljs without untrusted note data require prompt inventory and review, but the source bundle does not prove direct exploitability.

Technical view

The deepMerge helper does not reject __proto__, constructor, or prototype keys. When parsed attacker-controlled JSON is assigned to cell notes and serialized, these keys can modify Object.prototype, affecting subsequently created plain objects throughout the process. The supplied CVSS 3.1 score is 9.4.

Likely exposure

Highest exposure is in server applications using affected exceljs versions that accept untrusted parsed JSON, assign it to cell notes, and serialize workbooks. Exposure is lower where note content is trusted or strictly validated. The bundle does not establish risk from merely opening arbitrary workbooks.

Exploitation context

The CVSS vector describes network-accessible, low-complexity exploitation requiring neither privileges nor user interaction. However, CISA KEV status is false, and the supplied sources provide no evidence of active exploitation. Practical reachability depends on application data flow.

Researcher notes

The bundle’s affected metadata lists version 0 with defaultStatus unaffected, conflicting with the narrative claim that versions through 4.4.0 are vulnerable. No fixed upstream version is identified. Researchers should verify package lineage, exact deployed code, note-serialization reachability, and authoritative vendor guidance before declaring remediation complete.

Mitigation direction

  • Inventory applications and services using exceljs, including transitive dependencies and deployed lockfile versions.
  • Prevent untrusted input from reaching cell-note objects until a confirmed remediation is available.
  • Recursively reject __proto__, constructor, and prototype keys before merging or serializing note data.
  • Upgrade only to a vendor-confirmed fixed release; this bundle does not identify one.
  • Isolate workbook generation from sensitive, long-lived application processes where practical.

Validation and detection

  • Inspect manifests, lockfiles, and deployed artifacts for exceljs versions through 4.4.0.
  • Trace whether attacker-controlled parsed JSON can populate cell notes before workbook serialization.
  • Verify input validation recursively blocks dangerous property names at every object depth.
  • Test remediation in an isolated environment and confirm Object.prototype remains unchanged after note serialization.
  • Review downstream authorization and configuration logic that depends on ordinary object properties.
Prepared
Confidence
medium
Sources
6

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

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ATT&CK lookup starting points

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cwe · low confidence lookup

CWE-1321: Exact CWE lookup

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cve · low confidence lookup

CVE-2026-78207 mapping review

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Vulnerability profileCVE Program record
Severity
Critical
CVSS
9.4 (3.1)
Known Exploited
No
Published

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

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.

2CVSS vectors
3Timeline events
1ADP providers
5Source links

SSVC decision data

CISA-ADPCISA Coordinator
Timestamp
Version
2.0.3
Exploitation: noneAutomatable: yesTechnical Impact: total

CVSS vector scores

2 official scores

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
9.4CVSS 3.1CriticalCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:L3.95.5VulnCheck
9.3CVSS 4.0CriticalCVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:H/VI:H/VA:L/SC:N/SI:N/SA:N——VulnCheck

Vulnerability scoring details

Base CVSS 4.0 score

9.3Critical
CVSS 4.0 vector shape for CVE-2026-78207Attack VectorAttack ComplexityAttack RequirementsPrivileges RequiredUser InteractionVS ConfidentialityVS IntegrityVS AvailabilitySS ConfidentialitySS IntegritySS Availability

Vector: CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:H/VI:H/VA:L/SC:N/SI:N/SA:N

Attack Vector
NetworkAdjacentLocalPhysical
Attack Complexity
LowHigh
Attack Requirements
NonePresent
Privileges Required
NoneLowHigh
User Interaction
NonePassiveActive
VS Confidentiality
HighLowNone
VS Integrity
HighLowNone
VS Availability
HighLowNone
SS Confidentiality
HighLowNone
SS Integrity
HighLowNone
SS Availability
HighLowNone

Vulnerability timeline

Timeline events are normalized from CVE metadata, CNA source timelines, ADP timelines, and KEV metadata when present.

  1. CVE reservedCVE Program

    The CVE ID was reserved by the assigning CNA.

  2. CVE publishedCVE Program

    The CVE record was published.

  3. CVE updatedCVE Program

    The CVE record metadata indicates this as the latest update time.

ADP provider summaries

CISA-ADPCISA ADP Vulnrichment
other:ssvc

Source materials

Affected products

Products and packages named in the record

VendorProductVersion / packageStatus
exceljsexceljsexceljs, 0unaffected
Weakness

CWE details

CWE-1321

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

CWE-1321 · source CWE mapping

Improperly Controlled Modification of Object Prototype Attributes ('Prototype Pollution')

Improperly Controlled Modification of Object Prototype Attributes ('Prototype Pollution') represents a recurring weakness pattern that can create exploitable paths when design, validation, or implementation controls are missing.

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