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
EPSS (Exploit Prediction Scoring System) is a daily probability model maintained by FIRST.org. It estimates the likelihood a CVE will be exploited in production environments within the next 30 days, derived from real-world threat intelligence signals.
How urgent is this, really
CVE-2026-34829 plotted by exploitation likelihood (EPSS) against impact (CVSS). The shaded corner — EPSS 50%+ and CVSS 7.0+ — is where this CVE doesn't sit, though severity or exploitability alone can still warrant action.
Where this sits among everything scored
Of 378,156 CVEs with a current EPSS score, this one falls in the < 10% band (highlighted). Real counts from FIRST.org, not a sample — log-scaled since the landscape is heavily right-skewed.
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 |
Detection & 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
If you can't upgrade right away: gate or disable the affected feature, validate untrusted input at the boundary, and avoid passing attacker-controlled data into the vulnerable path. O3's runtime protection blocks exploitation in production as an interim safeguard until the upgrade lands.
How O3 protects you
O3 Security's impact-aware SCA analyses which vulnerable code paths your application actually calls, so a match like CVE-2026-34829 can be triaged on real exposure rather than presence alone.
Tailored to CVE-2026-34829. Runtime protection reduces exposure until a permanent patch is applied and verified — it complements patching, it doesn't replace it.
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.
Frequently Asked Questions
Is CVE-2026-34829 in your dependencies?
O3 Security finds CVE-2026-34829 across RubyGems dependencies, including transitive ones, and its impact-aware SCA ranks findings by whether your code actually calls the vulnerable path.