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Rack

CVE-2026-34835

medium

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.

4.8
cvss medium
common vulnerability scoring system v3.1. measures intrinsic severity on a 0-10 scale.
epss 0.0004
exploitation prediction scoring system. probability this vulnerability will be exploited in the wild in the next 30 days.
kev no
cisa known exploited vulnerabilities catalog. confirmed active exploitation.
CVSS 3.1 Common Vulnerability Scoring System v3.1
4.8 / 10
MEDIUM
exploitability
Attack Vector
Network
Attack Vector (AV:N): exploitable remotely over the network — most dangerous. no physical or adjacent access needed.
Attack Complexity
High
Attack Complexity (AC:H): exploitation requires specific conditions outside the attacker's control (race condition, non-default config, etc).
Privileges Required
None
Privileges Required (PR:N): no authentication needed. any anonymous attacker can exploit this.
User Interaction
None
User Interaction (UI:N): no victim action needed. fully exploitable without user interaction.
impact
Scope
Unchanged
Scope (S:U): impact limited to the vulnerable component.
Confidentiality
Low
Confidentiality (C:L): limited data exposure. some restricted information can be read.
Integrity
Low
Integrity (I:L): limited data modification. some data can be altered.
Availability
None
Availability (A:N): no availability impact.
CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:L/I:L/A:N
exploit detect attrib advisory media timeline
exploitation detection attribution advisories media timeline
activity density
rdintel assessment
21 threat composite score from 15+ signals including exploitation status, epss probability, detection coverage, and community attention. 0-100 scale.

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