Heap Buffer Overflow in AX.25 Packet Parsing: How a Missing Bounds Check Could Let Attackers Hijack Your System

Severity: 🔴 Critical | CVE Class: Heap-Based Buffer Overflow (CWE-122) | Attack Vector: Remote (RF or Network)

Introduction

In the world of amateur radio and packet networking, the AX.25 protocol is a cornerstone — a link-layer protocol used by ham radio operators, satellite ground stations, and embedded communication systems worldwide. It's the kind of infrastructure that quietly hums in the background, often assumed to be safe because of its niche audience.

That assumption just got a reality check.

A critical heap buffer overflow vulnerability was identified and patched in src/ax25.c — the kind of bug that security researchers describe as a "write primitive," a foundational building block for full remote code execution. The root cause? A single memcpy call that trusted a number it should never have trusted: a length value supplied directly by the remote sender of an AX.25 packet.

If you write C or C++ code that parses network packets, reads binary data from external sources, or works with embedded protocol stacks, this post is required reading. The pattern is devastatingly common, and the consequences are severe.

The Vulnerability Explained

What Is AX.25?

AX.25 is a data link layer protocol derived from the X.25 standard, designed for use over amateur radio. An AX.25 packet contains several fields, including a variable-length information field — the payload of the packet. When software receives an AX.25 packet, it typically:

1. Allocates a buffer to hold the information field.
2. Reads the info_len field from the packet header to know how many bytes to copy.
3. Uses memcpy (or equivalent) to copy the payload into the allocated buffer.

Step 3 is where things went wrong.

The Vulnerable Code Pattern

The vulnerability exists at src/ax25.c:208. Here's the conceptual pattern of what the vulnerable code looked like:

// Vulnerable pattern — DO NOT USE
int parse_ax25_packet(const uint8_t *raw_buf, size_t raw_len, ax25_frame_t *out) {

    // info_len is read DIRECTLY from the incoming packet
    out->info_len = read_uint16(raw_buf, OFFSET_INFO_LEN);

    // A buffer is allocated — but based on what size?
    // If the allocation uses a fixed size or a different calculation,
    // this is already dangerous.
    out->information = malloc(MAX_INFO_SIZE);
    if (!out->information) return -1;

    // ⚠️ CRITICAL: info_len comes from the attacker.
    // There is NO check that info_len <= MAX_INFO_SIZE.
    memcpy(out->information, raw_buf + OFFSET_INFO_DATA, out->info_len);
    //     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    //     If info_len > MAX_INFO_SIZE, this writes PAST the buffer!

    return 0;
}

The problem is elegant in its simplicity: the code allocates a buffer of a certain size, but copies a number of bytes controlled by the attacker. If the attacker sets info_len to a value larger than the allocated buffer, memcpy will happily write bytes beyond the end of the heap allocation.

Why Is This So Dangerous?

On the heap, memory allocations sit next to each other. When you overflow one allocation, you start overwriting the contents of adjacent allocations. In a modern heap layout, those adjacent regions might contain:

• Heap metadata (chunk headers used by malloc/free) — corrupting these can cause free() to execute attacker-controlled code
• Function pointers — overwriting a stored callback means the next time it's called, the attacker's code runs instead
• Other data structures — credentials, session tokens, or configuration data
• vtable pointers (in C++ code) — leading directly to virtual dispatch hijacking

This transforms a "simple" memory corruption into a potential remote code execution (RCE) primitive.

The Attack Scenario

Let's walk through a realistic attack:

[Attacker's Radio/Network Node]
        |
        |  Crafted AX.25 packet:
        |    info_len = 0xFFFF  (65535 bytes)
        |    actual payload = 512 bytes of shellcode + padding
        |
        v
[Vulnerable AX.25 Receiver]
        |
        |  malloc(512)  → allocates 512-byte heap buffer
        |  memcpy(..., 0xFFFF)  → writes 65535 bytes!
        |                          ^^^^^^^^^^^^^^^^^^^^^^
        |                          Overwrites adjacent heap chunks
        v
[Heap Corruption → Crash or Code Execution]

What makes this especially alarming:

1. No authentication required. AX.25 operates at the link layer. Anyone within RF range — or connected to a network feed — can send packets. There's no login, no handshake, no credential check before the vulnerable code runs.

2. The attack is remote. An attacker doesn't need physical access to the machine running this software. A malicious packet transmitted over radio or injected into a network APRS feed is sufficient.

3. The attack is reliable. Heap overflows, especially with full control over the overflow size and content, are among the most exploitable memory corruption vulnerabilities. Modern exploit techniques like heap grooming can make these highly reliable even with ASLR enabled.

The Fix

What Changed

The fix introduces bounds validation before the memcpy call. The principle is straightforward: before copying N bytes into a buffer of size M, verify that N ≤ M. If the check fails, the packet is rejected as malformed.

Here's the corrected pattern:

// Fixed version — proper bounds checking
int parse_ax25_packet(const uint8_t *raw_buf, size_t raw_len, ax25_frame_t *out) {

    // Read info_len from the packet (still attacker-controlled)
    out->info_len = read_uint16(raw_buf, OFFSET_INFO_LEN);

    // ✅ FIX #1: Validate info_len against the maximum allowed size
    if (out->info_len > MAX_INFO_SIZE) {
        log_error("AX.25: info_len %u exceeds maximum %u, dropping packet",
                  out->info_len, MAX_INFO_SIZE);
        return -EINVAL;  // Reject the malformed packet
    }

    // ✅ FIX #2: Also validate against the actual raw buffer length
    // to prevent reading past the input buffer (a separate vulnerability)
    if (OFFSET_INFO_DATA + out->info_len > raw_len) {
        log_error("AX.25: packet truncated, dropping");
        return -EINVAL;
    }

    // Allocate exactly what we need (or use a pre-allocated buffer)
    out->information = malloc(out->info_len);
    if (!out->information) return -ENOMEM;

    // ✅ Now safe: we've verified info_len <= MAX_INFO_SIZE
    //              and the source data is within raw_buf bounds
    memcpy(out->information, raw_buf + OFFSET_INFO_DATA, out->info_len);

    return 0;
}

Key Security Improvements

The Two-Check Pattern

Notice that the fix includes two separate bounds checks, not one. This is intentional and important:

1. Check against the destination buffer size — prevents heap overflow on write
2. Check against the source buffer size — prevents heap over-read on read (a separate vulnerability that could leak memory contents)

Both checks are necessary. A fix that only adds one of them is incomplete.

Conclusion

A single missing bounds check in a memcpy call. That's all it takes to turn a packet parser into a remote code execution vulnerability. This isn't a theoretical concern — heap buffer overflows are regularly weaponized in real-world attacks, and AX.25 implementations are particularly exposed because they accept input from anyone within radio range, with no authentication barrier.

The fix is simple: validate before you copy. Check that the attacker-supplied length fits within your buffer. Check that it fits within the source data. Reject anything that doesn't comply. This three-line fix closes an attack surface that could have allowed a malicious actor to execute arbitrary code on any system running this software.

Key Takeaways

• ✅ Never trust length fields from external input — always validate against known bounds
• ✅ Apply two-sided bounds checks — validate against both source and destination sizes
• ✅ Add ASan to your test suite — it catches these bugs immediately during development
• ✅ Use static analysis in CI/CD — tools like CodeQL can find taint flows from network input to memcpy
• ✅ Consider memory-safe languages for new protocol parser implementations
• ✅ Treat all network-facing code as adversarial — assume the worst about every field in every packet

Security is not about perfection — it's about making exploitation as difficult as possible. Proper bounds checking is one of the most fundamental and effective defenses in the C programmer's toolkit. Use it, every time, without exception.

This vulnerability was identified and patched as part of an automated security review. If you maintain software that processes AX.25 packets or other binary protocols, audit your memcpy call sites today.

Fixed by OrbisAI Security — automated security analysis for modern codebases.