multer vulnerable to file size limit bypass via async fileFilter race condition

低 3.7CVE-2026-77063 · GHSA-qvfw-j98x-7q72 · 28/08/2026 公開 · 02/09/2026 更新

要点

CVE-2026-77063 は multer (expressjs), multer に影響する深刻度「低」 (CVSS 3.7) の脆弱性です。
今後30日間に悪用される確率（EPSS）：0.23%（全脆弱性の 14.3% より高い）。
対処：2.3.0 以降に更新してください。

パッケージ	影響を受けるバージョン	修正済みバージョン
multer expressjs	`< 2.3.0`	`2.3.0`
multer npm	`< 2.3.0`	`2.3.0`

概要

Impact

When multer is configured with an asynchronous fileFilter, the limits.fileSize limit can be bypassed. The 'limit' event is registered inside the async fileFilter callback, so if a file exceeds limits.fileSize before that callback runs, the event is missed and the oversized upload is accepted instead of being rejected with a LIMIT_FILE_SIZE error. Applications that rely on limits.fileSize to reject oversized uploads are affected on all upload methods (.single(), .array(), .fields(), .any()). Uploads using a synchronous fileFilter are not affected.

Patches

Users should upgrade to 2.3.0.

Workarounds

Use a synchronous fileFilter, or validate the uploaded file size after the upload completes.

NVD の説明

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.