expressjs multer up to 2.2.x File Stream race condition

Summaryinfo

A vulnerability was found in expressjs multer up to 2.2.x. It has been classified as problematic. Affected by this issue is some unknown functionality of the component File Stream Handler. The manipulation leads to race condition. This vulnerability is uniquely identified as CVE-2026-77063. The attack is possible to be carried out remotely. No exploit exists.

Detailsinfo

A vulnerability, which was classified as problematic, has been found in expressjs multer up to 2.2.x. This issue affects an unknown code block of the component File Stream Handler. The manipulation with an unknown input leads to a race condition vulnerability. Using CWE to declare the problem leads to CWE-362. The product contains a code sequence that can run concurrently with other code, and the code sequence requires temporary, exclusive access to a shared resource, but a timing window exists in which the shared resource can be modified by another code sequence that is operating concurrently. Impacted is integrity. The summary by CVE is:

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.

It is possible to read the advisory at github.com. The identification of this vulnerability is CVE-2026-77063 since 08/20/2026. The exploitation is known to be easy. The attack may be initiated remotely. No form of authentication is needed for a successful exploitation. The technical details are unknown and an exploit is not publicly available.

Upgrading to version 2.3.0 eliminates this vulnerability.

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Productinfo

Vendor

• expressjs

Name

• multer

Version

• 2.0
• 2.1
• 2.2

Website

• Product: https://github.com/expressjs/multer/

CPE 2.3info

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CPE 2.2info

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