NVD / CVE Program· CNA GitHub_M

CVE-2026-34827: Rack: Algorithmic-Complexity DoS in Rack::Multipart::Parser

Noroxi assessment

Priority: Monitor (action score 30/100)

The sources we track (CISA KEV, Exploit-DB, Metasploit, Nuclei, GitHub) show no public exploit code or exploitation in the wild.
Probability of exploitation in the next 30 days: 0.49% (FIRST EPSS).
Anyone who can reach it over the internet can trigger it. No account or password is required. No user action is required. No special conditions are required; it is repeatable.
According to CISA the attack is automatable: targets can be found by mass scanning.
Fixed version: ruby-rack (Debian:13) 3.1.20-0+deb13u2; ruby-rack (Debian:14) 3.2.6-2; rack (RubyGems) 3.1.21, 3.2.6.

Low priority for now. Upgrade to ruby-rack (Debian:13) 3.1.20-0+deb13u2; ruby-rack (Debian:14) 3.2.6-2; rack (RubyGems) 3.1.21, 3.2.6 or later.

Generated automatically from the record's data: CVSS (NVD and CNA), CISA KEV and SSVC, FIRST EPSS, exploit sources, fixed versions. How we calculate it

Primary sources: NVD · CVE.org · FIRST EPSS

Rack is a modular Ruby web server interface. From versions 3.0.0.beta1 to before 3.1.21, and 3.2.0 to before 3.2.6, Rack::Multipart::Parser#handle_mime_head parses quoted multipart parameters such as Content-Disposition: form-data; name="..." using repeated String#index searches combined with String#slice! prefix deletion. For escape-heavy quoted values, this causes super-linear processing. An unauthenticated attacker can send a crafted multipart/form-data request containing many parts with long backslash-escaped parameter values to trigger excessive CPU usage during multipart parsing. This results in a denial of service condition in Rack applications that accept multipart form data. This issue has been patched in versions 3.1.21 and 3.2.6.

HighCVSS 7.5 · v3.1—No exploit Fix available

Published: Apr 2, 2026
Updated: Jul 24, 2026
EPSS: 0.4% · 42th percentile
CWE: CWE-400

Report tools

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Action score

Monitor

Low priority for now.

CVSS: 30 / 40 · 7.5 / 10
CISA KEV: 0 / 30 · Not listed
EPSS: 0 / 30 · 0.4%

CISA SSVC decision

Exploitation: none
Automatable: yes
Technical impact: partial

Vulnrichment: CISA's decision-tree inputs.

CNA vs NVD score

NVD: —
CNA · GitHub_M: 7.5; NVD has not scored this yet; the score shown is the CNA’s.

The score given by the assigning authority versus NVD’s independent score. A gap means the severity is contested.

Get told when this record changes

Not in KEV: no record of exploitation in the wild
No public exploit code on record
A fix has been released

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Noroxi analysis

What does it mean in your environment?

The assessment above is generated automatically from the record's data; it doesn't know your environment. Our team verifies on your systems whether this vulnerability is actually exploitable and which mitigation is enough.

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Affected systems

Is your version affected?

Pick the product and type the version you run. It's compared with this record's version ranges in your browser; the version isn't sent anywhere.

Vendor	Product	CPE
rack	rack	cpe:2.3:a:rack:rack

Affected versions

NVD version ranges (for catalog products). Add the product to your stack with a version and matching uses these.

rack rack3.0.0 and later · before 3.1.21
rack rack3.2.0 and later · before 3.2.6

Versions reported by the vendor

Affected version ranges reported by the assigning authority (GitHub_M). Independent of NVD's CPE analysis and usually ahead of it.

rack rack
- >= 3.0.0.beta1, < 3.1.21, >= 3.2.0, < 3.2.6affected

Package-level exposure

OSV and GitHub Advisory data: ecosystem, package and range. SBOM matching uses this table.

Ecosystem	Package	Affected range	Fix
Debian:13	ruby-rack	before 3.1.20-0+deb13u2	3.1.20-0+deb13u2
Debian:14	ruby-rack	before 3.2.6-2	3.2.6-2
RubyGems	rack	from 3.0.0.beta1 · before 3.1.21	3.1.21
RubyGems	rack	from 3.2.0 · before 3.2.6	3.2.6

Same product

rack: all records

Other highest-scoring records for the same primary product.

CVE-2024-25126Rack ReDos in content type parsing (2nd degree polynomial)
41Plan
CVE-2024-26146Possible Denial of Service Vulnerability in Rack Header Parsing
31Monitor
CVE-2023-27530A DoS vulnerability exists in Rack <v3.0.4.2, <v2.2.6.3, <v2.1.4.3 and <v2.0.9.3 within in the Multipart MIME parsing code in which could al
31Monitor

Published before and after this one in the same product

← EarlierCVE-2026-26962Apr 2, 2026 · Rack: Header injection in multipart requests
26Monitor
← EarlierCVE-2026-32762Apr 2, 2026 · Rack: Forwarded Header semicolon injection enables Host and Scheme spoofing
26Monitor
Later →CVE-2026-34835Apr 2, 2026 · Rack: `Rack::Request` accepts invalid Host characters, enabling host allowlist bypass.
26Monitor

Remediation

Which version to upgrade to

Fix versions compiled from the vendor, package registries and Microsoft. Verify the vendor's note before upgrading.

Product / package	Fixed version	Source
debian:ruby-rack	3.1.20-0+deb13u2 · Debian:13	Package registry (OSV)
RubyGems:rack	3.1.21	Package registry (OSV)
RubyGems:rack	3.2.6	Package registry (OSV)

Exploit status

No known public exploit

No public exploit has been observed yet. That doesn't mean you're safe, only that the bar is a little higher.

Research context

For pentesters and researchers: attack profile, score disagreement, timeline, patch commits, credits, variant and chain candidates, bug bounty scope. All derived from existing data; no exploit code.

Timeline

From publication to today: proof of concept, Metasploit module, CISA KEV and fix record. Dates are as reported by the sources.

No dated events beyond publication.

No history yet (it accumulates with the daily EPSS job).

Patch and commit links

Commit, PR and diff links among the references. A starting point for patch-diffing and variant hunting; fixes, not exploits.

No commit or PR link among the references.

Credits

All researchers

Finders, reporters and analysts named in the CNA record. Click a name for that researcher’s other records.

No credits in the CNA record.

Variant candidates

Same product, same weakness class, within 18 months. If the patch missed the root cause, the sibling bug is here.

CVE-2026-342300 days apartRack: Quadratic complexity in Rack::Utils.select_best_encoding via wildcard Accept-Encoding header
30Monitor
CVE-2026-348260 days apartRack: Unbounded Range Count in get_byte_ranges Enables DoS
30Monitor
CVE-2026-348290 days apartRack: Denial of Service via Unbounded Multipart File Upload Without Content-Length
30Monitor
CVE-2025-61919174 days apartRack is vulnerable to a memory-exhaustion DoS through unbounded URL-encoded body parsing
30Monitor
CVE-2025-61770177 days apartRack's unbounded multipart preamble buffering enables DoS (memory exhaustion)
30Monitor
CVE-2025-61771177 days apartRack's multipart parser buffers large non‑file fields entirely in memory, enabling DoS (memory exhaustion)
30Monitor

Chain candidates

An authentication bypass and a privilege-requiring bug in the same product, published close together: combined they may become an unauthenticated path.

—

Bug bounty scope

No known public program.

Source: bounty-targets-data (public HackerOne, Bugcrowd, Intigriti, YesWeHack listings).

Technical details

Attack conditions

Anyone who can reach it over the internet can trigger it.
No account or password is required.
No user action is required.
No special conditions are required; it is repeatable.

If successful

Confidentiality: none
Integrity: none
Availability: high · the service can be disrupted

Attack vector: Network
Attack complexity: Low
Privileges required: None
User interaction: None
Scope: Unchanged
Confidentiality impact: None
Integrity impact: None
Availability impact: High

Weakness class (CWE)

CWE-400 · Uncontrolled Resource Consumption

CVSS vector

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H

nvd-secondary

Root cause

Code that parses nested structures sets no depth or size limit. A small input can turn into a disproportionate processing load.

The bug in code

The whole CWE-400 class

A representative example of this vulnerability class. Not the vendor's source code; it shows the faulty pattern and its fix.

Vulnerable

1function expand(node) {
2  return node.children.map(expand);
3}

Fixed

1const MAX_DEPTH = 32;
2function expand(node, depth = 0) {
3  if (depth > MAX_DEPTH) throw new Error("too deep");
4  return node.children.map((c) => expand(c, depth + 1));
5}

Attack context

MITRE CAPEC attack patterns and ATT&CK techniques for this weakness class (CWE). A starting point for detection rules and threat hunting.

Attack patterns (CAPEC)

ATT&CK techniques

Endpoint Denial of ServiceT1499

Change log

Fix✗ → ✓Sep 28, 2026, 10:47 PM UTC

For records you follow, these changes also arrive as notifications. →

References

github.com/rack/rack/security/advisories/GHSA-v6x5-cg8r-vv6x

Show all references (3 more)

Vendor advisories and official records. Exploit/PoC links are deliberately left out.

All records