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Multer vulnerable to Denial of Service via deeply nested field names

High severity GitHub Reviewed Published Jun 15, 2026 in expressjs/multer • Updated Jun 17, 2026

Package

npm multer (npm)

Affected versions

>= 1.0.0, < 2.2.0
>= 3.0.0-alpha.1, < 3.0.0-alpha.2

Patched versions

2.2.0
3.0.0-alpha.2

Description

Impact

Multer is vulnerable to a Denial of Service (DoS) via deeply nested field names in multipart form data. The append-field dependency parses bracket notation in field names (e.g., a[b][c]) with no limit on nesting depth, allowing an attacker to force allocation of deeply nested object structures that consume CPU and memory. A single HTTP request with a crafted multipart body is sufficient to exploit this.

Patches

Users should upgrade to 2.2.0 and configure limits.fieldNestingDepth to the minimum depth their application requires.

Workarounds

Set limits.fields to a reasonable value to reduce the number of fields an attacker can send per request. This does not fully mitigate the issue but limits the impact.

References

@UlisesGascon UlisesGascon published to expressjs/multer Jun 15, 2026
Published by the National Vulnerability Database Jun 15, 2026
Published to the GitHub Advisory Database Jun 17, 2026
Reviewed Jun 17, 2026
Last updated Jun 17, 2026

Severity

High

CVSS overall score

This score calculates overall vulnerability severity from 0 to 10 and is based on the Common Vulnerability Scoring System (CVSS).
/ 10

CVSS v3 base metrics

Attack vector
Network
Attack complexity
Low
Privileges required
None
User interaction
None
Scope
Unchanged
Confidentiality
None
Integrity
None
Availability
High

CVSS v3 base metrics

Attack vector: More severe the more the remote (logically and physically) an attacker can be in order to exploit the vulnerability.
Attack complexity: More severe for the least complex attacks.
Privileges required: More severe if no privileges are required.
User interaction: More severe when no user interaction is required.
Scope: More severe when a scope change occurs, e.g. one vulnerable component impacts resources in components beyond its security scope.
Confidentiality: More severe when loss of data confidentiality is highest, measuring the level of data access available to an unauthorized user.
Integrity: More severe when loss of data integrity is the highest, measuring the consequence of data modification possible by an unauthorized user.
Availability: More severe when the loss of impacted component availability is highest.
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H

EPSS score

Weaknesses

Uncontrolled Resource Consumption

The product does not properly control the allocation and maintenance of a limited resource. Learn more on MITRE.

CVE ID

CVE-2026-5079

GHSA ID

GHSA-72gw-mp4g-v24j

Source code

Credits

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