CVE-2026-34230 Rack: Quadratic complexity in Rack::Utils.select_best_encoding via wildcard Accept-Encoding header
An unauthenticated attacker can cause denial of service in Rack applications using Deflater by sending a crafted Accept-Encoding header with many wildcards.
- Exploited in the wild
- Not known
- Login needed
- Not needed
- Attackable
- Over the internet
- Victim action needed
- Not needed
- Fix
- Available
What is CVE-2026-34230?
CVE-2026-34230 is a medium-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::Utils.select_best_encoding processes Accept-Encoding values with quadratic time complexity when the header contains many wildcard (*) entries. Because this method is used by Rack::Deflater to choose a response encoding, an unauthenticated attacker can send a single request with a crafted Accept-Encoding header and cause disproportionate CPU consumption on the compression middleware path. This results in a denial of service condition for applications using Rack::Deflater. This issue has been patched in versions 2.2.23, 3.1.21, and 3.2.6.
Which versions are affected by CVE-2026-34230?
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-2026-34230?
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-2026-34230 being exploited?
There is no known exploitation of CVE-2026-34230 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-2026-34829 Rack: Denial of Service via Unbounded Multipart File Upload Without Content-Length
- 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
Sources
Also known as DEBIAN-CVE-2026-34230, GHSA-v569-hp3g-36wr, MINI-24h4-c8xf-m83x, MINI-2533-8cmf-xvhf, MINI-3gxq-x68c-6cq4, MINI-4r8r-xhc6-9q4f and 21 more.