Overview
CVE-2018-1196 describes a symlink-based attack in the embedded launch script shipped with Spring Boot, affecting Spring Boot 1.5.9 and earlier and 2.0.0.M1 through 2.0.0.M7. When the application is installed as a system service and the run_user has shell access, an attacker could leverage a symbolic link to cause the script to write to arbitrary files on the host, potentially taking ownership or corrupting sensitive resources. This creates a broken object level authorization scenario where deployment artifacts and service configuration (objects) can be manipulated by an attacker with limited access, bypassing intended authorization boundaries at the OS level. The underlying issue is not in application logic itself but in the packaging/launch wrapper that interacts with privileged filesystem locations, enabling unauthorized object modification through the wrapper script.
Exploitation typically requires the attacker to have some level of access to the host (shell access) and to position a symlink in a directory or path that the embedded launch script touches during startup or service management. When the script resolves or follows that symlink, the run_user may end up writing to, or taking ownership of, the target file, thereby escalating privileges or disrupting service. Because the vulnerability hinges on how deployment artifacts and startup automation interact with the file system, it is classed as a broken object level authorization scenario where the objects (startup scripts, service ownership, configuration files) can be manipulated by an attacker with access to the host.
Fixing this class of vulnerability in Spring Boot involves upgrading to patched releases that remove the symlink vulnerability in the embedded launcher, and adopting safer deployment practices. Specifically, upgrade to releases where the embedded launch script has been corrected (2.0.0.M8+ and later as applicable, and patch levels for the 1.5 line as released). If upgrading is not feasible, decouple from the embedded launch script for service management and rely on OS-managed service wrappers (e.g., systemd units) or containerized deployment, ensuring the application does not operate on or modify privileged system files via user-controlled paths. In code, enforce strict path validation and avoid allowing user-controlled paths to target privileged files; treat startup/launch artifacts as untrusted, and audit all deployment scripts that interact with the filesystem.
In practice for application code, implement path validation and least privilege controls to minimize the impact of such issues. Avoid relying on startup-time wrappers that write to privileged locations; if wrappers are necessary, ensure they operate only within a restricted, well-defined directory structure and cannot be influenced by untrusted input. Regularly scan deployment tooling for symlink and path-traversal vulnerabilities and validate that service startup procedures do not expose object-level resources to manipulation by unprivileged users.
Affected Versions
Spring Boot 1.5.9 and earlier; 2.0.0.M1 through 2.0.0.M7
Code Fix Example
Spring Boot API Security Remediation
import java.io.IOException;
import java.io.OutputStream;
import java.nio.charset.StandardCharsets;
import java.nio.file.Files;
import java.nio.file.Path;
import java.nio.file.Paths;
import java.nio.file.StandardOpenOption;
public class SpringBootSymlinkVulnerabilityDemo {
public static void main(String[] args) throws IOException {
String userProvidedPath = args.length > 0 ? args[0] : "/var/myapp/data/output.txt";
// Vulnerable pattern: write to a user-supplied path without validation
vulnerableWrite(userProvidedPath);
// Fixed pattern: validate path is within allowed directory
fixedWrite(userProvidedPath);
}
// Vulnerable pattern: writes to any path provided by user input
public static void vulnerableWrite(String pathStr) throws IOException {
Path p = Paths.get(pathStr).toAbsolutePath().normalize();
try (OutputStream os = Files.newOutputStream(p, StandardOpenOption.CREATE, StandardOpenOption.TRUNCATE_EXISTING)) {
os.write("vulnerable".getBytes(StandardCharsets.UTF_8));
}
System.out.println("WROTE (VULNERABLE) TO: " + p);
}
// Fixed pattern: restrict writes to a known, safe directory
public static void fixedWrite(String pathStr) throws IOException {
Path allowedDir = Paths.get("/var/myapp/data").toAbsolutePath().normalize();
Path p = Paths.get(pathStr).toAbsolutePath().normalize();
if (!p.startsWith(allowedDir)) {
throw new IOException("Attempt to write outside allowed directory: " + p);
}
try (OutputStream os = Files.newOutputStream(p, StandardOpenOption.CREATE, StandardOpenOption.TRUNCATE_EXISTING)) {
os.write("fixed".getBytes(StandardCharsets.UTF_8));
}
System.out.println("WROTE (FIXED) TO: " + p);
}
}