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CVE-2026-34835· NVD / CVE Program· CNA GitHub_M

Rack: `Rack::Request` accepts invalid Host characters, enabling host allowlist bypass.

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::Request parses the Host header using an AUTHORITY regular expression that accepts characters not permitted in RFC-compliant hostnames, including /, ?, #, and @. Because req.host returns the full parsed value, applications that validate hosts using naive prefix or suffix checks can be bypassed. This can lead to host header poisoning in applications that use req.host, req.url, or req.base_url for link generation, redirects, or origin validation. This issue has been patched in versions 3.1.21 and 3.2.6.

MediumCVSS 6.5 · v3.1—Proof of concept Fix available
Published
Apr 2, 2026
Updated
Jul 24, 2026
EPSS
0.3% · 18th percentile
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Report tools

JSON

Action score

26

Monitor

Low priority for now.

CVSS
26 / 40 · 6.5 / 10
CISA KEV
0 / 30 · Not listed
EPSS
0 / 30 · 0.3%

CISA SSVC decision

Exploitation
none
Automatable
no
Technical impact
partial

Vulnrichment: CISA's decision-tree inputs.

CNA vs NVD score

NVD
6.5
CNA · GitHub_M
4.8
1.7 point gap

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

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

Affected systems

VendorProduct
rackrack

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.

EcosystemPackageAffected rangeFix
Debian:13ruby-rackbefore 3.1.20-0+deb13u23.1.20-0+deb13u2
Debian:14ruby-rackbefore 3.2.6-23.2.6-2
RubyGemsrackfrom 3.0.0.beta1 · before 3.1.213.1.21
RubyGemsrackfrom 3.2.0 · before 3.2.63.2.6

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
  • CVE-2026-34827Rack: Algorithmic-Complexity DoS in Rack::Multipart::Parser
    30Monitor
  • CVE-2026-34830Rack: Rack::Sendfile regex injection via HTTP_X_ACCEL_MAPPING header allows arbitrary file reads through nginx
    30Monitor
  • CVE-2026-34829Rack: Denial of Service via Unbounded Multipart File Upload Without Content-Length
    30Monitor

Remediation

Which version to upgrade to

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

Product / packageFixed versionSource
debian:ruby-rack3.1.20-0+deb13u2 · Debian:13Package registry (OSV)
RubyGems:rack3.1.21Package registry (OSV)
RubyGems:rack3.2.6Package registry (OSV)

Exploit status

Proof of concept available

A public proof of concept or detection signature exists. Exploit knowledge is out there; don't delay patching.

Evidence

  • Proof-of-concept repository on GitHub ×1

Noroxi does not host or link to exploit code. Metasploit module names and Nuclei template ids are shown (names, not code); Exploit-DB entries and PoC repositories are given as counts only.

Public exploit evidence
SourceKindDate
Proof-of-concept repository on GitHubstars:62026-07-03

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.

  1. Published
  2. First PoC+91 d
  3. TodaySep 30, 2026+181 d
EPSS, last 120 days

FIRST EPSS daily score; only changes of 0.01 or more are recorded (step chart).

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.

No nightly-computed relations.

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
low · data can be read
Integrity
low · data or configuration can be modified
Availability
none
Attack vector
Network
Attack complexity
Low
Privileges required
None
User interaction
None
Scope
Unchanged
Confidentiality impact
Low
Integrity impact
Low
Availability impact
None

Weakness class (CWE)

CWE-1286 · Improper Validation of Syntactic Correctness of Input

CVSS vector

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

nvd-primary

Attack context

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

ATT&CK techniques

—

Noroxi analysis

No Noroxi analysis for this record yet

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Change log

  1. Fix✗ → ✓

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