In December 2023, a critical vulnerability (designated as CVE-2023-6267) was discovered affecting systems that use annotation-based security for REST APIs, particularly when handling JSON payloads. This simple but dangerous flaw allows attackers to send malicious JSON data, which the server processes *before* enforcing security checks. With configuration-based security, this issue doesn't happen. This article breaks down the vulnerability, gives concrete code snippets, and demonstrates just how attackers might exploit it.

What’s the Problem?

When developers secure REST endpoints using annotations (like @RolesAllowed or @Secured), the intention is straightforward: block unauthorized users from even getting to the underlying business logic. But in certain frameworks (including Jakarta EE and Spring Boot), the JSON body is automatically deserialized before these annotations are checked.

That means, malicious input can be processed and loaded into objects (even triggering code) before your security annotations have a chance to say "no."

Is this a Big Deal?

Yes: If your REST endpoint processes sensitive data or has setters or constructors with side-effects, attackers can exploit the system merely by sending a crafted JSON payload—not even needing valid credentials.

Exploiting the Flaw: A Walkthrough

Suppose we have a REST API receiving JSON and protected using annotation-based security.

Let’s look at a simplified JAX-RS example

import javax.ws.rs.POST;
import javax.ws.rs.Path;
import javax.ws.rs.Consumes;
import javax.annotation.security.RolesAllowed;

@Path("/users")
public class UserResource {

    // Only admins should be able to create users
    @POST
    @RolesAllowed("ADMIN")
    @Consumes("application/json")
    public void createUser(User user) {
        // ... business logic here
    }
}

And the User object

public class User {
    private String username;
    private String role;

    public User() {}

    public void setUsername(String username) {
        this.username = username;
    }

    public void setRole(String role) {
        // Critical: Maybe there is a problematic side effect here
        if ("root".equals(role)) {
            // e.g., escalate privileges, trigger logs, write to files, etc.
        }
        this.role = role;
    }
}

Let’s go deeper. Imagine the setter in User is more dangerous

public class User {
    // ...
    public void setRole(String role) {
        if ("root".equals(role)) {
            Runtime.getRuntime().exec("touch /tmp/pwned");
        }
        this.role = role;
    }
}

The Exploit

Any user—even if security later stops the request—could cause /tmp/pwned to appear on the server:

curl -X POST --header "Content-Type: application/json" \
  -d '{"username":"attacker", "role":"root"}' \
  http://vulnerable-app.com/api/users

Why did this happen? Because Jackson (or another deserializer) creates the User object *before* the security checks.

Why Configuration-Based Security Is Safer

If you secured your endpoints *by configuration* (for example, in web.xml, application.properties, or using a security filter), the security logic would run before any JSON deserialization. That blocks the attack much earlier:

✅ Only then: JSON deserialization

This ordering prevents any code from running as a result of the submitted payload if the request isn’t authorized.

Real-World References

- Red Hat Security Advisory
- Jakarta EE Security
- Spring Boot Security Docs

> Vendor Patch: Most affected frameworks have released updates that ensure deserialization only happens *after* annotation-based security is applied. Check your framework’s security bulletins.

2. Avoid Side Effects in DTO Setters or Constructors.

- DTO classes (the ones receiving JSON) should never trigger file writes, execute commands, or change system state.

Upgrade to the latest version that addresses CVE-2023-6267.

- Consult your vendor’s guidance (Red Hat Example, Spring Blog).

4. Add Input Validation and Logging.

- Even when endpoints are not supposed to be accessible, log failed (and successful) deserialization attempts.

Summary

CVE-2023-6267 is a stark example of why deserialization and security must be ordered correctly. Relying solely on annotation-based security can leave your APIs open to sneaky attacks that trigger code *before* authorization is checked.

Protect your APIs: Use configuration-based security, keep side-effect code out of DTOs, and patch your frameworks as soon as possible.

> For developers and DevOps teams: Review all your REST endpoints—especially those using annotation-based security with JSON body input. Better be safe than compromised.

Exclusive Analysis by AI – June 2024

*(Permission to republish this work is granted with attribution.)*

Timeline

Published on: 01/25/2024 19:15:08 UTC
Last modified on: 02/17/2024 10:15:07 UTC