CVE-2026-34829 — rack
HIGHCVE-2026-34829 is a high-severity (CVSS 7.5) Uncontrolled Resource Consumption vulnerability in rack. A fix is available for rack — see the affected versions and patch details below.
Rack: Denial of Service via Unbounded Multipart File Upload Without Content-Length
Exploitation Status
No confirmed exploitation observed yet
- CISA assesses this as automatable — exploitation doesn’t require manual, per-target effort, which raises the odds of mass scanning and opportunistic attacks.
- CISA’s own triage has not observed active exploitation or public proof-of-concept code for this CVE as of its last assessment.
Exploitation and automatability from CISA’s SSVC triage for CVE-2026-34829.
EPSS Exploitation Probability
Probability of exploitation in the next 30 days, from FIRST.org EPSS.
How urgent is this, really
Where this sits among everything scored
Real-World Exposure
rack💎rack💎rackReal-time download stats are indexed for npm and PyPI packages. This vulnerability affects RubyGems packages — download data is not available via public APIs for these ecosystems.
Description
Summary
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.
Details
Rack::Multipart::Parser.parse applies BoundedIO only when content_length is not nil:
io = BoundedIO.new(io, content_length) if content_length
When CONTENT_LENGTH is absent, the parser reads the multipart body until EOF without a global byte limit.
Although Rack enforces BUFFERED_UPLOAD_BYTESIZE_LIMIT for retained non-file parts, file uploads are handled differently. When a multipart part includes a filename, the body is streamed to a Tempfile, and the retained-size accounting is not applied to that file content. As a result, file parts are not subject to the same upload size bound.
An attacker can exploit this by sending a chunked multipart/form-data request containing a file part and continuously streaming data without declaring a Content-Length. Rack will continue writing the uploaded data to disk until the client stops or the server exhausts available storage.
Impact
Any Rack application that accepts multipart/form-data uploads may be affected if no upstream component enforces a request body size limit.
An unauthenticated attacker can send a large chunked file upload to consume disk space on the application host. This may cause request failures, application instability, or broader service disruption if the host runs out of available storage.
The practical impact depends on deployment architecture. Reverse proxies or application servers that enforce upload limits may reduce or eliminate exploitability, but Rack itself does not impose a total multipart upload limit in this code path when CONTENT_LENGTH is absent.
Mitigation
- Update to a patched version of Rack that enforces a total multipart upload size limit even when
CONTENT_LENGTHis absent. - Enforce request body size limits at the reverse proxy or application server.
- Isolate temporary upload storage and monitor disk consumption for multipart endpoints.
Affected Packages
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| 💎RubyGems | rack | all versions | 2.2.23bundle update rack --conservative |
| 💎RubyGems | rack | ≥ 3.0.0.beta1&&< 3.1.21 | 3.1.21bundle update rack --conservative |
| 💎RubyGems | rack | ≥ 3.2.0&&< 3.2.6 | 3.2.6bundle update rack --conservative |
Affected Products
rackrackDetection & mitigation playbook
Open-source dependencyDetect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for rack, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
Fix
Update rack to 2.2.23 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2026-34829 is resolved across your whole dependency graph.
Workarounds
Cap what an attacker can consume: apply request size, rate and timeout limits in front of the affected component, and run it with memory and CPU limits so exhaustion degrades one worker rather than the whole service.
Fixing This On Your OS
If you run this on a Linux distribution, patch through your package manager against the distro's own security advisory below — it tracks the exact backported fix for your release, which can ship on a different timeline (and sometimes a different severity) than the upstream project.
Mitigation for this issue is either not available or the currently available options do not meet the Red Hat Product Security criteria comprising ease of use and deployment, applicability to widespread installation base or stability.Source: Red Hat security advisory for CVE-2026-34829 (CC BY 4.0)
Frequently Asked Questions
Is CVE-2026-34829 in your dependencies?
Find it across RubyGems, including transitive dependencies.