{"id":"CVE-2026-42091","aliases":["GHSA-rhf7-wvw3-vjvm","GO-2026-5625"],"url":"https://o3.security/vulnerability/CVE-2026-42091","summary":"goshs has Cross-Origin Arbitrary File Write via Missing CSRF on PUT and Wildcard CORS","details":"### Summary\nThe PUT upload handler (`httpserver/updown.go`) lacks the CSRF token validation that was added to the POST upload handler during the GHSA-jrq5-hg6x-j6g3 fix. Combined with the unconditional `Access-Control-Allow-Origin: *` on the OPTIONS preflight handler (`httpserver/server.go`), any website can write arbitrary files to a goshs instance through the victim's browser — bypassing network isolation (e.g. localhost, internal network).\n\n### Details\n**Root Cause 1 — Missing CSRF on PUT** (`httpserver/updown.go:19`)\n\nWhen GHSA-jrq5-hg6x-j6g3 was fixed (commit `e3c3d37`), `checkCSRF()` was added to the POST `upload()` function (line 78) but not to the PUT `put()` function directly above it in the same file. This means PUT requests are accepted without any CSRF token.\n\n```go\n// POST — protected \nfunc (fs *FileServer) upload(w http.ResponseWriter, req *http.Request) {\n    if !fs.checkCSRF(w, req) { return }\n    // ...\n}\n\n// PUT — unprotected \nfunc (fs *FileServer) put(w http.ResponseWriter, req *http.Request) {\n    // No checkCSRF call\n    // ...\n}\n```\n\n**Root Cause 2 — Wildcard CORS** (`httpserver/server.go:126`)\n\nThe OPTIONS handler unconditionally returns permissive CORS headers:\n\n```go\nw.Header().Set(\"Access-Control-Allow-Origin\", \"*\")\nw.Header().Set(\"Access-Control-Allow-Methods\", \"POST, PUT, OPTIONS\")\nw.Header().Set(\"Access-Control-Allow-Headers\", \"Content-Type, Authorization\")\n```\n\nThis allows any website's JavaScript to pass the browser's CORS preflight check and send PUT requests to the goshs server.\n\n### PoC\n[poc.zip](https://github.com/user-attachments/files/26828829/poc.zip)\n\nPlease extract the uploaded compressed file before proceeding\n\n1. bash poc.sh\n<img width=\"543\" height=\"376\" alt=\"스크린샷 2026-04-17 오후 11 08 13\" src=\"https://github.com/user-attachments/assets/a695cbc8-133e-4e80-a2f5-9fe9fd36b569\" />\n\n\n\n\n### Impact\n- Arbitrary file write to the goshs webroot from any website the victim visits\n- File overwrite — existing files can be silently replaced","published":"2026-05-04T17:24:47.890Z","modified":"2026-08-27T03:57:09.204127533Z","cvss":{"score":6.5,"severity":"MEDIUM","vector":"CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:N/I:H/A:N"},"epss":{"score":0.00165,"percentile":0.06156,"asOf":"2026-08-14"},"cisaKev":null,"exploitsKnown":0,"affectedPackages":[{"ecosystem":"Go","name":"github.com/patrickhener/goshs/v2","fixedVersion":"2.0.2"},{"ecosystem":"Go","name":"github.com/patrickhener/goshs","fixedVersion":null}],"fix":{"url":"https://github.com/patrickhener/goshs/commit/0e715b94e10c3d1aa552276000f15f104dee2f32","label":"patrickhener/goshs@0e715b9"},"references":[{"type":"WEB","url":"https://github.com/patrickhener/goshs/releases/tag/v2.0.2"},{"type":"ADVISORY","url":"https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/42xxx/CVE-2026-42091.json"},{"type":"ADVISORY","url":"https://github.com/patrickhener/goshs/security/advisories/GHSA-rhf7-wvw3-vjvm"},{"type":"ADVISORY","url":"https://nvd.nist.gov/vuln/detail/CVE-2026-42091"},{"type":"FIX","url":"https://github.com/patrickhener/goshs/commit/0e715b94e10c3d1aa552276000f15f104dee2f32"},{"type":"PACKAGE","url":"https://github.com/patrickhener/goshs"}],"provenance":{"sources":["OSV.dev","FIRST.org (EPSS)"],"lastVerified":"2026-08-27T03:57:09.204127533Z"}}