CVE-2026-125111: Rack: Multipart parsing allows DoS via unbounded scan buffer growth
Rack is a modular Ruby web server interface. Prior to 2.2.25, 3.1.23, and 3.2.8, Rack::Multipart::Parser can retain parsed data in its internal StringScanner buffer when reads end inside or immediately after multipart headers. An unauthenticated attacker can align many small parts with rack.multipart.buffersize boundaries so the buffer grows with the request despite per-field, header, and preamble limits, causing a worker to exhaust memory. The attack requires only a well-formed multipart body and affects applications that parse multipart requests. This issue is fixed in versions 2.2.25, 3.1.23, and 3.2.8.
Affected Software
Remediation
Recommended actions to resolve this vulnerability, in priority order.
- Upgrade
Upgrade
Rackto a version that resolves this vulnerability.Fixed in 2.2.25
Event History
Frequently Asked Questions
Which deployments are exposed?
Applications using Rack versions earlier than 2.2.25, 3.1.23, or 3.2.8 are affected if they parse multipart requests. The issue applies to multipart request handling rather than requiring a particular authentication state.
What does an attacker need to exploit this?
An attacker needs only to send a well-formed multipart request body. They can arrange many small parts so reads end inside or immediately after multipart headers at rack.multipart.buffer_size boundaries.
Are existing multipart limits sufficient protection?
No. Per-field, header, and preamble limits can still be bypassed because parsed data may remain in the internal StringScanner buffer, allowing memory consumption to grow with the request.
How can this be remediated?
Upgrade Rack to 2.2.25, 3.1.23, or 3.2.8, depending on the release line in use. These versions contain the fix.