File-size rejection bypass via late limit listener
Published Aug 28, 2026 · Updated Aug 28, 2026
Race condition in Multer before 2.3.0 allows remote attackers to bypass file-size rejection through asynchronous upload filtering. The file stream handler registers its limit-event listener only after the asynchronous fileFilter callback returns, so an earlier over-limit event is missed. Exploitation requires an application to combine an asynchronous fileFilter with limits.fileSize; the parser still truncates data at the limit, but the upload is accepted instead of returning LIMIT_FILE_SIZE.
Summary
What happened
Race condition in Multer before 2.3.0 allows remote attackers to bypass file-size rejection through asynchronous upload filtering. The file stream handler registers its limit-event listener only after the asynchronous fileFilter callback returns, so an earlier over-limit event is missed. Exploitation requires an application to combine an asynchronous fileFilter with limits.fileSize; the parser still truncates data at the limit, but the upload is accepted instead of returning LIMIT_FILE_SIZE.
The record
- CVE
- CVE-2026-77063
- Published
- Aug 28, 2026
- Updated
- Aug 28, 2026
- Vendor
- body-parser
- Product
- Multer
- Classifications
- CWE-362
- Attack vector
- network
- Privileges
- unauthenticated
Timeline
How it unfolded
- Aug 28, 2026CVE publishedPublication date reported by the CVE source.
- Aug 28, 2026Record updatedLatest update available in the CVE record.
Exploitability
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Is a vulnerable build present?
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- Affected versionversion=0 <2.3.0
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What is affected?
Published CVSS scores
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Attacks
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