{"document":{"category":"csaf_vex","csaf_version":"2.0","title":"CVE-2026-43974: gun HTTP/1.1 client accepts unsolicited 101 Switching Protocols response allowing server-driven protocol hijack and OOM","publisher":{"category":"vendor","name":"HarborGuard Database","namespace":"https://database.harborguard.co"},"tracking":{"id":"CVE-2026-43974","status":"final","version":"1","initial_release_date":"2026-06-08T14:12:36.957Z","current_release_date":"2026-06-08T16:34:38.989Z","revision_history":[{"date":"2026-06-08T14:12:36.957Z","number":"1","summary":"Initial machine-readable export from HarborGuard."}]},"distribution":{"tlp":{"label":"WHITE"},"text":"Public CVE data; freely redistributable."},"notes":[{"category":"description","text":"Unexpected Status Code or Return Value vulnerability in ninenines gun (gun_http module) allows a malicious HTTP server to force the client into raw protocol mode via an unsolicited 101 Switching Protocols response.\n\nIn gun_http:handle_inform/8, when a 101 Switching Protocols response is received over HTTP/1.1, the function verifies only that the Upgrade header is syntactically valid and that the stream reference is a plain reference(). It does not check whether the client ever sent an Upgrade or Connection: upgrade header on the corresponding request. Because this check is absent, any 101 response (solicited or not) causes gun to dispatch a gun_upgrade message to the caller and transition the entire connection to raw protocol mode.\n\nA malicious or compromised HTTP server can send an unsolicited 101 response to any HTTP/1.1 request, causing the gun client to abandon HTTP framing for that connection. Once in raw mode, gun_raw applies no flow control (flow=infinity) and re-arms socket active mode after every received packet, so the server can flood the client with arbitrary bytes. These are forwarded as unbounded gun_data messages to the owner process, exhausting its mailbox and BEAM memory, ultimately crashing the VM.\n\nThis issue affects gun: from 2.0.0 before 2.4.0.","title":"CVE description"}],"references":[{"category":"self","summary":"CVE-2026-43974 on HarborGuard Database","url":"https://database.harborguard.co/cve/CVE-2026-43974"},{"category":"external","summary":"CVE Record","url":"https://www.cve.org/CVERecord?id=CVE-2026-43974"},{"category":"external","summary":"cna.erlef.org","url":"https://cna.erlef.org/cves/CVE-2026-43974.html"},{"category":"external","summary":"osv.dev","url":"https://osv.dev/vulnerability/EEF-CVE-2026-43974"},{"category":"external","summary":"github.com","url":"https://github.com/ninenines/gun/commit/5b48068c29ce5e112cb149b5857c7d4dc319a81b"}]},"product_tree":{"branches":[{"category":"vendor","name":"ninenines","branches":[{"category":"product_name","name":"gun","branches":[{"category":"product_version_range","name":">=2.0.0 <2.4.0","product":{"name":"ninenines gun >=2.0.0 <2.4.0","product_id":"CSAFPID-1","product_identification_helper":{"cpe":"cpe:2.3:a:ninenines:gun:*:*:*:*:*:*:*:*"}}}]}]},{"category":"vendor","name":"ninenines","branches":[{"category":"product_name","name":"gun","branches":[{"category":"product_version_range","name":">=a3c2edbb8c807717e2f10520c6cf1e77a62eab2e <5b48068c29ce5e112cb149b5857c7d4dc319a81b","product":{"name":"ninenines gun >=a3c2edbb8c807717e2f10520c6cf1e77a62eab2e <5b48068c29ce5e112cb149b5857c7d4dc319a81b","product_id":"CSAFPID-2","product_identification_helper":{"cpe":"cpe:2.3:a:ninenines:gun:*:*:*:*:*:*:*:*"}}}]}]}]},"vulnerabilities":[{"cve":"CVE-2026-43974","title":"gun HTTP/1.1 client accepts unsolicited 101 Switching Protocols response allowing server-driven protocol hijack and OOM","notes":[{"category":"description","text":"Unexpected Status Code or Return Value vulnerability in ninenines gun (gun_http module) allows a malicious HTTP server to force the client into raw protocol mode via an unsolicited 101 Switching Protocols response.\n\nIn gun_http:handle_inform/8, when a 101 Switching Protocols response is received over HTTP/1.1, the function verifies only that the Upgrade header is syntactically valid and that the stream reference is a plain reference(). It does not check whether the client ever sent an Upgrade or Connection: upgrade header on the corresponding request. Because this check is absent, any 101 response (solicited or not) causes gun to dispatch a gun_upgrade message to the caller and transition the entire connection to raw protocol mode.\n\nA malicious or compromised HTTP server can send an unsolicited 101 response to any HTTP/1.1 request, causing the gun client to abandon HTTP framing for that connection. Once in raw mode, gun_raw applies no flow control (flow=infinity) and re-arms socket active mode after every received packet, so the server can flood the client with arbitrary bytes. These are forwarded as unbounded gun_data messages to the owner process, exhausting its mailbox and BEAM memory, ultimately crashing the VM.\n\nThis issue affects gun: from 2.0.0 before 2.4.0.","title":"CVE description"}],"product_status":{"known_affected":["CSAFPID-1","CSAFPID-2"]},"scores":[{"cvss_v4":{"version":"4.0","vectorString":"CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N","baseScore":8.7,"baseSeverity":"HIGH"},"products":["CSAFPID-1","CSAFPID-2"]}],"remediations":[{"category":"vendor_fix","details":"Update to a fixed version: 2.4.0, 5b48068c29ce5e112cb149b5857c7d4dc319a81b.","product_ids":["CSAFPID-1","CSAFPID-2"],"url":"https://github.com/ninenines/gun/commit/5b48068c29ce5e112cb149b5857c7d4dc319a81b"}]}]}