CVE-2026-34829 Rack: Denial of Service via Unbounded Multipart File Upload Without Content-Length
An unauthenticated attacker can cause denial of service by streaming unbounded multipart file uploads without Content-Length headers, exhausting disk space.
- Exploited in the wild
- Not known
- Login needed
- Not needed
- Attackable
- Over the internet
- Victim action needed
- Not needed
- Fix
- Available
What is CVE-2026-34829?
CVE-2026-34829 is a high-severity denial of service vulnerability in rack, published on 2 Apr 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
Official description
Rack is a modular Ruby web server interface. Prior to versions 2.2.23, 3.1.21, and 3.2.6, Rack::Multipart::Parser only wraps the request body in a BoundedIO when CONTENT_LENGTH is present. When a multipart/form-data request is sent without a Content-Length header, such as with HTTP chunked transfer encoding, multipart parsing continues until end-of-stream with no total size limit. For file parts, the uploaded body is written directly to a temporary file on disk rather than being constrained by the buffered in-memory upload limit. An unauthenticated attacker can therefore stream an arbitrarily large multipart file upload and consume unbounded disk space. This results in a denial of service condition for Rack applications that accept multipart form data. This issue has been patched in versions 2.2.23, 3.1.21, and 3.2.6.
Which versions are affected by CVE-2026-34829?
6 open source packages ship an affected version, including rack. It affects 1 product.
| Package | Affected | Fixed in |
|---|---|---|
rack RubyGems | All versions | 2.2.23 |
rack RubyGems | All versions | 2.2.23 |
rack RubyGems | From 3.0.0.beta1 | 3.1.21 |
rack RubyGems | From 3.0.0.beta1 | 3.1.21 |
rack RubyGems | From 3.2.0 | 3.2.6 |
rack RubyGems | From 3.2.0 | 3.2.6 |
Products include Rack Rack.
How do I fix CVE-2026-34829?
Upgrade rack to a fixed version for your release line: 2.2.23, 3.1.21 or 3.2.6.
Upgrade, then put limits on request size, rate and nesting depth so one client cannot exhaust the service. More about Denial of service.
Is CVE-2026-34829 being exploited?
There is no known exploitation of CVE-2026-34829 in the wild and no public exploit code.
Other vulnerabilities in rack
- CVE-2026-34785 Rack: Local file inclusion in `Rack::Static` via URL Prefix Matching
- CVE-2026-34827 Rack: Algorithmic-Complexity DoS in Rack::Multipart::Parser
- CVE-2026-22860 Rack has a Directory Traversal via Rack:Directory
- CVE-2025-61919 Rack is vulnerable to a memory-exhaustion DoS through unbounded URL-encoded body parsing
- CVE-2025-61770 Rack's unbounded multipart preamble buffering enables DoS (memory exhaustion)
Sources
Also known as DEBIAN-CVE-2026-34829, GHSA-8vqr-qjwx-82mw, MINI-39rc-69xv-8x4r, MINI-3mf8-x6mr-4pcr, MINI-3r7p-6gcw-gww5, MINI-5882-7pjf-25jc and 21 more.