CVE-2026-34829 | Rack: Denial of Service via Unbounded Multipart File Upload Without Content-Length

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.

Published: 2026-04-02 Last update: 2026-04-16 Assigner: [email protected] Source: [email protected]
NVD Status：Analyzed，CVE State: published
View at NVD, CVE.org, EUVD

CVE-2026-34829 is rated Low Risk (36/100): CVSS High severity, with low exploitation likelihood (EPSS 0.05%). Monitor for updates and reassess as exploit intelligence or EPSS changes.

Risk is dynamic; we continuously reassess and refresh what is shown on this page as upstream context changes.

Exploit prediction scoring system (EPSS) score for CVE-2026-34829

EPSS lead: Daily EPSS estimates relative likelihood of exploitation; percentile ranks this CVE among scored vulnerabilities (higher = more severe relative rank).

# Date Old EPSS score New EPSS score Delta (New - Old)
1 2026-04-08 0.04% 0.05% +0.01%
2 2026-04-03 — 0.04% —

Full EPSS history (2 records total)

Common vulnerability scoring system (CVSS) metrics for CVE-2026-34829

CVSS metrics for this CVE.

Base score Version Severity Vector Exploitability Impact Score source
7.5 3.1 HIGH
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H Click to expand
Attack vector (AV:N)
Could be attacked over the internet or any normal routed network—not just someone sitting at the machine.
Attack complexity (AC:L)
Once they can reach the bug, pulling it off is straightforward—no weird race conditions or rare setup.
Privileges required (PR:N)
No account or special rights needed—anonymous or random user is enough.
User interaction (UI:N)
Nobody has to click “OK” or open a trap file; it can work without a victim helping.
Scope (S:U)
Damage stays in the same “trust bubble” as the broken component—no big spill into unrelated systems.
Confidentiality (C:N)
Doesn’t really leak secrets in a meaningful way.
Integrity (I:N)
Data isn’t meaningfully altered or forged.
Availability (A:H)
Could take the service down hard or make it unusable for people who depend on it.
 3.9 3.6 [email protected]

Weakness enumeration for CVE-2026-34829

GitHub Security Advisory for CVE-2026-34829

GHSA-8vqr-qjwx-82mw · Severity: high · Ecosystem: rubygems — Rack's multipart parsing without Content-Length header allows unbounded chunked file uploads

OS Trackers for CVE-2026-34829

vendor priority summary link
debian not yet assigned CVE-2026-34829 not yet assigned priority: Debian including 1 source packages (ruby-rack), 5 status rows across 5 suites (bookworm, bullseye, forky, sid, trixie): open 3, resolved 2. https://security-tracker.debian.org/tracker/CVE-2026-34829
redhat high — https://access.redhat.com/security/cve/CVE-2026-34829
suse high CVE-2026-34829 severity important: SUSE including 2 source package names (hawk2, ruby4.0-rubygem-rack-2.2-2.2.23-1.1), 7 product×package rows across 7 product lines (SUSE Linux Enterprise High Availability Extension 12 SP5, SUSE Linux Enterprise High Availability Extension 15 SP4, … (7 product lines)): Known Not Affected 6, Fixed 1. https://www.suse.com/security/cve/CVE-2026-34829/
ubuntu medium CVE-2026-34829 medium priority: Ubuntu including 1 source packages (ruby-rack), 9 status rows across 9 suites (bionic, focal, jammy, noble, questing, resolute, trusty, upstream, xenial): released 4, ignored 3, needed 1, not-affected 1. https://ubuntu.com/security/CVE-2026-34829

Affected software / configurations for CVE-2026-34829

Vendor Product Version Raw CPE
rack rack < 2.2.23 cpe:2.3:a:rack:rack:*:*:*:*:*:ruby:*:*
rack rack >= 3.0.0, < 3.1.21 cpe:2.3:a:rack:rack:*:*:*:*:*:ruby:*:*
rack rack >= 3.2.0, < 3.2.6 cpe:2.3:a:rack:rack:*:*:*:*:*:ruby:*:*

References for CVE-2026-34829

cvelogic Threat Intelligence