UBUNTU-CVE-2026-125111: Medium severity ubuntu/ruby-rack vulnerability
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 - Upgrade
Upgrade
rackto a version that resolves this vulnerability.Fixed in 3.1.23 - Upgrade
Upgrade
rackto a version that resolves this vulnerability.Fixed in 3.2.8
Event History
Frequently Asked Questions
Which applications are exposed to this issue?
Applications using affected Rack versions that parse multipart requests are exposed. The issue affects Rack versions before 2.2.25, 3.1.23, and 3.2.8.
What does an attacker need to exploit it?
An attacker needs only to send a well-formed multipart request body. No authentication is required; the request is constructed with many small parts aligned to rack.multipart.buffer_size boundaries.
Are Rack multipart limits sufficient to prevent exploitation?
No. The buffer can grow despite per-field, header, and preamble limits when reads end inside or immediately after multipart headers.
What is the impact of a successful attack?
A worker can exhaust available memory while parsing the request, resulting in denial of service.
How can this be remediated?
Upgrade Rack to version 2.2.25, 3.1.23, or 3.2.8, as appropriate for the deployed release line.