CVE-2022-4087 is a security vulnerability that was discovered in iPXE, a popular open-source network boot firmware. It affects the TLS (Transport Layer Security) implementation, specifically in how it handles message padding in its code. If left unpatched, attackers could exploit this flaw to gain access to sensitive information—the type of bug that can have ripple effects in secure networks, especially when iPXE is used to load operating systems over the network.

In this article, we break down the vulnerability, look at the vulnerable code, show how it could be exploited, and provide mitigation steps. We’ll use simple, clear explanations—no cryptic security lingo.

What is iPXE and Why Does This Matter?

iPXE lets computers boot using network protocols such as HTTP or iSCSI. You might find it running in data centers or cloud environments where bare-metal systems are provisioned on the fly. Because it often operates before the operating system even starts, any bug in iPXE can be a real headache—especially if it affects security.

Where’s the Bug? The tls_new_ciphertext Function

The vulnerability is in tls_new_ciphertext, a function found in the src/net/tls.c file, inside the TLS component of iPXE. It’s all about how the code handles padding in encrypted TLS messages.

Here’s a simplified version of the vulnerable code, focusing on the handling of pad_len

/* Vulnerable snippet from src/net/tls.c */
static int tls_new_ciphertext( ... , size_t pad_len, ... ) {
    ...
    if ( pad_len >= TLS_BLOCK_SIZE ) {
        return -EINVAL;
    }
    /* Calculate actual data length */
    data_len = length - pad_len - 1;
    ...
}

The crucial part is that pad_len comes from user-controlled input and isn’t properly validated before the function acts on it.

What’s the Risk? Information Disclosure Through Discrepancy

When handling encrypted data, code often needs to be very careful not to behave differently based on the values of encrypted (or attacker-controlled) inputs. Otherwise, attackers might be able to piece together information (think “padding oracle” attacks).

In this bug

- If an attacker sends a TLS message with funny padding (an unusual pad_len), iPXE can leak clues about how it’s processing the message.
- Different errors, or different processing times, might let an attacker figure out the underlying plaintext, or otherwise gather information they shouldn’t have.

While this isn't a full-on “remote code execution” vulnerability, it IS a classic *information disclosure* flaw.

The vulnerability is officially tracked as VDB-214054.

The attacker crafts a TLS message with an invalid or edge-case padding value for pad_len.

- The attacker sends this message to an iPXE client/server.
- By observing how iPXE responds (maybe a specific error code, maybe a different delay), the attacker collects clues.
- After many messages, the attacker can reconstruct secrets or session data—depending on how much info leaks and the surrounding context.

Here’s a super-simplified Python-style pseudo-code attack to illustrate

# Pseudocode: Attack TLS padding in iPXE
for pad_len_guess in range(, 256):
    crafted_message = build_tls_message(pad_len=pad_len_guess)
    send_to_ipxe(crafted_message)
    response = receive_from_ipxe()
    if "Error" in response:
        print(f"Pad len {pad_len_guess} causes an error")
    else:
        print(f"Pad len {pad_len_guess} processed without error")
# Repeat and analyze for behavior differences

Over enough tries, timing or error discrepancies can reveal information about the decrypted data.

How Do You Fix It?

A patch was issued to tightly validate padding and avoid any processing differences attackers could use. The patch has the Git identifier: 186306d6199096b7a7c4b4574d4be8cdb8426729.

Here’s what the maintainers changed (simplified)

/* Patched code: stricter padding check */
if ( pad_len >= TLS_BLOCK_SIZE || pad_len != expected_pad_len ) {
    return -EINVAL;
}

To fix the issue:

Upgrade to the latest iPXE code or apply the patch above.

- If you build iPXE from source, pull the latest commit from their official repository.

References and More Reading

- CVE-2022-4087 Vulnerability Details - NIST NVD
- VulDB Entry: VDB-214054
- iPXE GitHub Issue / Commit

Conclusion

The CVE-2022-4087 vulnerability in iPXE’s TLS handling is a classic example of why *every single byte* matters in security code. If you manage iPXE servers or boot environments—check your versions, patch up ASAP, and stay secure.

If you want to dig even deeper, the public commit and CVE resources above provide full details.

Don’t leave your network’s boot process exposed!

*This article was written exclusively for educational and defensive purposes. Never attempt to exploit vulnerabilities on systems you do not own or have explicit permission to test.*

Timeline

Published on: 11/21/2022 07:15:00 UTC
Last modified on: 02/03/2023 14:38:00 UTC