CVE-2025-43970 - Input Length Validation Flaw in GoBGP (Pre-3.35.) Exposes Packet Parsing Vulnerability

In the world of network routing, GoBGP stands out as a popular, open-source BGP implementation written in Go. Unfortunately, a security flaw tagged as CVE-2025-43970 was discovered in GoBGP versions before 3.35., specifically involving the way the package handles MRT (Multi-threaded Routing Toolkit) packet parsing. The root of the issue lies in improper input length validation in pkg/packet/mrt/mrt.go, which can open the door to serious security problems.

This post breaks down the flaw in simple terms, provides example code, details how the vulnerability can be exploited, and wraps up with links to further reading.

Vulnerability Overview

Affected Component:
pkg/packet/mrt/mrt.go in GoBGP versions before 3.35..

What's the problem?

When the MRT parser processes network packets, it expects certain structures to have a fixed minimum length of 12 or 36 bytes, depending on the address family (IPv4 or IPv6). Older versions failed to validate if the input actually met the required length, opening the possibility for attackers to send malformed or truncated packets that could crash the service or trigger other unexpected behaviors.

Here’s a simplified version of the vulnerable code

// pkg/packet/mrt/mrt.go

func parseMRTMessage(data []byte, afi uint16) error {
   // Expected size based on address family
   var expectedLen int
   switch afi {
   case AFI_IP:    // IPv4
      expectedLen = 12
   case AFI_IP6:   // IPv6
      expectedLen = 36
   }
   
   // MISSING: Input validation!
   // Vulnerable: No check if len(data) >= expectedLen

   // Processes data assuming enough bytes are present
   addr := data[:expectedLen]
   // ... continue processing ...
   return nil
}

The Issue

Notice above: There's no validation that data is at least expectedLen bytes. If an attacker sends a short packet, something like data=[]byte{x00, x10}, the code won't panic right away (because slicing in Go is checked), but any subsequent logic that assumes the full data is there may malfunction, or parts of the data may be missing, resulting in incorrect parsing or possible panics elsewhere.

How Could Attackers Exploit This?

1. Denial of Service (DoS): By sending malformed or truncated MRT packets, an attacker could cause GoBGP to panic and crash, disrupting network routing.
2. Potential Out-of-bounds Reads: While Go’s slice bounds checks avoid direct buffer overflows, incorrect handling could lead to skipped error checks, missed routing events, or logging sensitive information.
3. Chained Attacks: If GoBGP is running with high privileges, a crash can lead to service interruption or even provide hints for more complex exploits.

Example Exploit

Here’s a minimal example of how such an attack could be simulated.

package main

import (
    "fmt"
    "github.com/osrg/gobgp/pkg/packet/mrt"
)

func main() {
    // Simulate a too-short MRT packet for IPv6 (should be 36 bytes, here only 10)
    shortPacket := make([]byte, 10)
    err := mrt.ParseMRTMessage(shortPacket, mrt.AFI_IP6)
    if err != nil {
        fmt.Println("Error:", err)
    } else {
        fmt.Println("Possibly exploited: Function did not fail on short input!")
    }
}

Expected Output on Vulnerable Versions

Without proper length checking, the function may not immediately error, or may crash deeper inside.

The Fix

In GoBGP 3.35., validation was added to ensure input data meets the minimum required length:

if len(data) < expectedLen {
    return fmt.Errorf("not enough input data: expected %d bytes, got %d", expectedLen, len(data))
}

See the commit diff here.

Mitigation

- Update GoBGP: Upgrade to 3.35. or later.

References

- Official CVE Report: CVE-2025-43970
- GoBGP GitHub Repository
- GoBGP 3.35. Release Notes
- GoBGP mrt.go File

Conclusion

CVE-2025-43970 might seem minor, but input validation is often the thin wall that protects critical network services from annoying to catastrophic failures. Always keep your routers updated; a tiny missing check could bring down your network.

If you use GoBGP in production, upgrade now.

Feel free to share, fork, or link back — and keep your packets the right size!

Timeline

Published on: 04/21/2025 01:15:45 UTC
Last modified on: 05/08/2025 15:45:51 UTC