CVE-2026-34835 — 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.

Published Apr 2, 2026Updated Apr 2, 2026Sources: cvelistV5, GitHub_M

Triage

Is it exploited, how likely is exploitation, what does it touch, and how severe do the scoring sources call it.

Exploitation

Exploit code

public exploit, none observed

EPSS

chance of exploitation in 30 days

Affects

rack

CVSS base

4.8

MEDIUM

CISA SSVC assessment

Three decision points CISA publishes for the CVEs it assesses · SSVC 2.0.3. A stakeholder decision, not a severity score.

CISA

Exploitation

None

none · proof-of-concept · active

Automatable

can an attacker script all four kill-chain steps

Technical impact

Partial

partial · total control of the vulnerable component

Affected scope

The catalog records vendors and products as separate lists, not pairs, so which product belongs to which vendor is not something this page can say.

Vendors (1)

rack

Products (1)

rack

Every base score collected

Sources score independently and disagree; each row says who scored it and under which version.

Score	Version	Severity	Expl.	Impact	Source
4.8	CVSS 3.1	MEDIUM	—	—	cvelistV5

Weakness & attack patterns

CWE-1286

Public exploit

Capability, not use: code existing is a different claim from anyone running it.

Indexed by

poc in github

Detection

Read off the CVSS vector and the weakness class. Starting points, not rules we have tested.

Search application, proxy, and WAF logs for requests touching /.

References

1 on the record

github.com/rack/rack/security/advisories/GHSA-g2pf-xv49-m2h5
x_refsource_CONFIRM

Elsewhere on this site

rackevery CVE for this vendor
CWE-1286other pages naming this weakness

Not in any source we poll

Listed rather than left blank: an empty field and an unmeasured one look identical on screen, and only one is a reason to look elsewhere.

No confirmed IOCs, IP addresses, domains, file hashes, or malware artifacts supplied.
No organization-specific asset inventory, compensating-control status, or patch deployment evidence supplied.
No exploit packet captures, log samples, or incident case IDs supplied.