A serious vulnerability known as CVE-2022-48468 was discovered in protobuf-c (before version 1.4.1). It’s an unsigned integer overflow in the function parse_required_member, which is used for decoding Protocol Buffers data in C/C++ programs. If exploited, this bug can lead to memory corruption, crashes, denial of service, or even potential remote code execution depending on how the vulnerable code is used.

This exclusive post will explain the bug in simple terms, show you the dangerous code, walk through how an attacker could exploit it, and offer the steps you should take to stay safe.

What is protobuf-c?

protobuf-c is an implementation of Google’s Protocol Buffers (protobuf) for C. It lets you serialize and deserialize structured data interchangeably, useful for distributed computing, RPC, or saving configuration data.

- GitHub: protobuf-c repository
- Issue tracker: protobuf-c Issues

CVSS Score: 9.8 (Critical)

- More info: NVD Entry

The Vulnerable Code

In versions before 1.4.1, the function parse_required_member does not correctly handle certain malformed protobuf data. If a crafted message contains very large field lengths, the code performs an addition that can overflow the buffer boundary:

// Vulnerable excerpt from protobuf-c (before 1.4.1)
static void parse_required_member(SomeType *message, const uint8_t *data, size_t len) {
    // 'offset' is a size_t.
    size_t offset = ;
    while (offset < len) {
        uint32_t field_length = decode_varint(&data[offset], &len);
        offset += field_length;
        // ... do stuff with data[offset] ...
    }
}

If an attacker can make field_length large enough, offset + field_length will overflow, wrapping around to a small number. That means all the code relying on offset is now using a value that breaks out of the boundaries of data, causing reads/writes outside the intended buffer.

Exploit Scenario

1. Send Malformed Protobuf Data: The attacker crafts a protocol buffer message with intentionally corrupted length fields (e.g., field_length = UINT32_MAX or similar).
2. Trigger the Overflow: When the vulnerable code processes this data, offset wraps around due to the overflow.
3. Crash or Corrupt Memory: This can lead to out-of-bounds memory reading or writing. In some cases, it may cause a denial-of-service crash. In more severe deployments, with some luck and skill, it could be used for arbitrary code execution—especially if the code is running with privileges.

Here is a very simplified demonstration of what might go wrong

#include <stdio.h>
#include <stdint.h>
#include <string.h>

#define BUFFER_SIZE 16

// Dummy "decode_varint" function, always gives huge length:
uint32_t decode_varint(const uint8_t *data, size_t *len) {
    return xFFFFFFFF; // Simulate attacker input: unsigned int max
}

void parse_required_member(uint8_t *data, size_t len) {
    size_t offset = ;
    while (offset < len) {
        uint32_t field_length = decode_varint(&data[offset], &len);
        // Vulnerable addition:
        offset += field_length;
        if (offset > len) {
            printf("Buffer overflow detected!\n");
            return;
        }
        // Code using data[offset]...
    }
}

int main() {
    uint8_t data[BUFFER_SIZE];
    memset(data, , sizeof(data));
    // Pretend this is attacker-controlled input
    parse_required_member(data, sizeof(data));
    return ;
}

What happens:
The offset += field_length; causes offset to wrap around to a small value, which may lead to using the buffer incorrectly and possibly reading/writing beyond data[].

Real-World Impact

- Denial of Service (DoS): Most common, your software will crash upon parsing a malicious protobuf message.
- Arbitrary Code Execution: If other bugs or predictable data are nearby, and the attacker is persistent, this bug could be chained for RCE.
- Supply Chain Threat: Any application or IoT device using vulnerable versions of protobuf-c is at risk.

Upgrade!

The protobuf-c team released version 1.4.1 which includes a fix for this and related bugs. Always use the latest version.

Input Validation:

Never trust user-controlled data. Validate the expected size and structure of incoming protobuf data before parsing.

Fuzzing:

Protobuf parsers are a common target for fuzzers. Regularly fuzz your parsing code to catch such errors early.

Here’s a simplified fix

// Fixed: Check for overflow before the addition
if (field_length > len - offset) {
    // Error: field_length too big, abort parsing!
    return;
}
offset += field_length;

By checking if field_length > len - offset, you ensure that offset + field_length won’t overflow.

References and Further Reading

- CVE-2022-48468 on MITRE
- protobuf-c issue #480
- Patch and Release Notes
- Report on oss-fuzz

Conclusion

CVE-2022-48468 is a classic example of how unchecked arithmetic can become a highway for attackers. If your product, embedded device, or open-source project uses protobuf-c, make sure you’ve updated to at least version 1.4.1. Buffer overflows are dangerous and treat all serialization input as untrusted—because your attacker surely will.

Stay safe and patch your libraries!

*For any questions or help with remediating this vulnerability in your infrastructure, feel free to reach out or comment below.*

Timeline

Published on: 04/13/2023 21:15:00 UTC
Last modified on: 04/21/2023 18:25:00 UTC