What’s Vulnerable › Vulnerabilities

CVE-2026-77063: multer vulnerable to file size limit bypass via async fileFilter race condition

• Severity: low (CVSS 3.7 v3.1)
• Exploited in the wild: not listed in CISA KEV
• EPSS: 0.2% probability of exploitation in the next 30 days
• Public exploit or detection template: none known
• Published: 2026-08-28 · updated 2026-09-02
• Weakness: CWE-362

Description

multer is a middleware for handling multipart/form-data in Node.js. When an application uses an asynchronous fileFilter together with the fileSize limit, a race condition in multer's file stream handling can allow a file that exceeds the configured size limit to bypass the size-limit rejection. All versions before 2.3.0 are affected. The impact is limited because the underlying multipart parser still truncates the stream at the size limit, so this is a bypass of the limit rejection rather than uncontrolled resource consumption. The issue is fixed in multer 2.3.0. Upgrade to multer 2.3.0 to remediate.

Affected products

• multer: before 2.3.0 — fixed in 2.3.0

How to fix

Upgrade to a fixed version: multer 2.3.0. Check the vendor advisory for mitigations.

References

• https://cna.openjsf.org/security-advisories.html
• https://github.com/expressjs/multer/security/advisories/GHSA-qvfw-j98x-7q72

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