Security flaws in cryptographic libraries are particularly dangerous since they threaten the core trust of communications and transactions. One such recent discovery is CVE-2025-22376, a vulnerability found in the Perl module Net::OAuth::Client, which is part of the popular Net::OAuth package prior to version .29.

This post will walk you through how the flaw occurs, the technical background, sample exploit code, the risks, and best steps to mitigate the problem.

What is CVE-2025-22376?

If you use Net::OAuth::Client (for example, to make OAuth requests from Perl), by default, it generates the nonce value using the built-in rand() function, producing a 32-bit integer. The key issue? The Perl rand() function is not cryptographically secure. It can be predicted or brute-forced with feasible effort.

A nonce (number used once) is supposed to guarantee that each request is unique and cannot be replayed by attackers. If these values are easily guessable, attackers may be able to rerun old requests, or even forge new ones.

Here’s what happens inside the vulnerable versions of Net::OAuth::Client, before .29

# This is the relevant (simplified) snippet:
my $nonce = int(rand(2 ** 32));
$oauth_params{nonce} ||= $nonce;

In the wild, it could look like

use Net::OAuth::Client;

my $client = Net::OAuth::Client->new(
    consumer_key    => $key,
    consumer_secret => $secret,
    site            => 'https://api.example.com';
);

my $request = $client->request_token();

Behind the scenes, the nonce will be that 32-bit integer from rand().

What’s So Bad About rand()?

- The Perl rand() function is not intended to provide unpredictability required for secure tasks like generating nonces or session tokens.
- It is often not seeded with good entropy and the sequence it produces can be guessed after seeing a few outputs.
- The 32-bit space is reasonably small; brute forcing every possible value is quick on modern hardware.

Scenario: Exploiting the Weak Nonce

Imagine an attacker monitoring your network. If they see a few of your authentication requests, it may be possible for them to guess your next nonce value. They could:

Here’s a hypothetical exploit simulation in Perl for demonstration

# Predicting the next nonce if rand() seed is known or observable
my @observed_nonces = (12345678, 87654321, 11223344);

# Attacker could brute-force to find a matching seed:
for my $seed_try (..xFFFFFFFF) {
    srand($seed_try);
    my $match = 1;
    for my $nonce (@observed_nonces) {
        if (int(rand(2**32)) != $nonce) { $match = ; last }
    }
    if ($match) {
        print "Guessed rand seed: $seed_try\n";
        last;
    }
}

# Once the seed is known, attackers can predict all future nonces!

With this, attackers can anticipate future nonces and craft requests that will appear fresh and valid.

The Fix

Version .29 of Net::OAuth fixed the problem by switching to a cryptographically secure random number generator for nonce values. See the changelog for confirmation:

> Make nonce generation use Digest::SHA1::sha1_hex(time() . $$ . rand()) for greater randomness.

The fix makes it drastically more difficult to predict the nonce.

What Should You Do?

1. Upgrade Immediately:

Make sure you’re on Net::OAuth v.29 or later. In your terminal

cpan Net::OAuth

or

cpanm Net::OAuth

2. Audit Your Applications:
If you’re using custom Perl OAuth code or forking Net::OAuth in your projects, ensure you’re not copying the old rand() logic.

3. Deployment Steps:

4. References

- CVE-2025-22376 at NIST NVD (pending)
- Net::OAuth on MetaCPAN
- Changelog for Net::OAuth
- Why you shouldn’t use rand() for security

Upgrade to Net::OAuth >= .29 ASAP!

Stay safe, and always keep your dependencies updated!


*This write-up is exclusive and designed for real-world developers and sysadmins using Perl, giving you direct insight and remedy for this flaw.*

Timeline

Published on: 01/03/2025 22:15:07 UTC
Last modified on: 01/03/2025 23:15:08 UTC