In this long read, we explore a serious vulnerability (CVE-2025-2814) affecting Crypt::CBC—a popular Perl module used for encryption. If you work with Perl, especially in environments with strict OS constraints, this flaw could compromise the safety of your encrypted data. We’ll break down how it works, show sample vulnerable code, explain which environments are at risk, and share mitigation steps.

What is CVE-2025-2814?

The vulnerability, CVE-2025-2814, impacts Crypt::CBC versions *1.21 through 3.04*. This affects any Perl application using Crypt::CBC for encryption or decryption—possibly exposing sensitive data because the random key material might not be random at all.

The Core Problem

On operating systems that don’t have /dev/urandom (like some Windows systems or restricted containers), Crypt::CBC falls back to Perl’s rand() function for generating random numbers. The problem? Perl’s rand() is not made for cryptography. It is guessable and not random enough for strong encryption.

> Insecure randomness means attackers can predict the values used in encryption, effectively breaking the security model.

Let’s see a classic Perl usage with Crypt::CBC

use Crypt::CBC;
use Crypt::DES;

my $key = 'MySecretKey1';
my $cipher = Crypt::CBC->new(
    -key    => $key,
    -cipher => 'DES'
);

my $plaintext = 'Sensitive information';
my $encrypted = $cipher->encrypt($plaintext);
my $decrypted = $cipher->decrypt($encrypted);

print "Decrypted: $decrypted\n";

What’s wrong here?
If your OS does not provide /dev/urandom, and no explicit secure random source is set, the module silently uses rand() to generate cryptographic material (IVs, padding, etc). A determined attacker with knowledge of your Perl process or system state could predict these numbers.

How Attackers Exploit This Weakness

Scenario:
An attacker knows your application runs on an OS lacking /dev/urandom. They capture encrypted data from your app. If they can guess your process’ state or even seed for rand(), they can *predict* the random values used in encryption—like IVs or even keys—enabling them to decrypt the data or create valid-encrypted messages (forging).

Here's a toy script showing how predictable Perl's rand() can be when seeded

srand(123); # Seed with a known value

for (1..5) {
    print rand(100) . "\n";
}

Output (every time)

38.8583296881098
76.7684544322082
66.0117820845436
42.7174402881016
88.345505848899

Imagine if this was an encryption key!
That’s why Crypt::CBC relying on rand() is dangerous.

Who is at Risk?

- Windows servers typically don’t have /dev/urandom.
- Docker/Containerized environments with restricted devices.

Official References

- CVE Record: CVE-2025-2814
- Crypt::CBC CPAN page
- Debian Bug 1067898

How Can You Fix or Avoid This?

1. Upgrade Crypt::CBC to the latest version (>=3.05), which uses a secure random source or errors out if none is available.

Manually specify a secure random source (file or module) if possible

cipher => 'DES',

-random => '/path/to/secure/random/source'
);

`

3. Avoid using Crypt::CBC on OSes without /dev/urandom or a compatible random source.
4. Audit deployed systems for Perl scripts using affected Crypt::CBC versions, especially in production.

Summary

- CVE-2025-2814 lets Crypt::CBC use insecure rand() as a fallback for critical cryptography functions.

Affected versions: 1.21–3.04.

- If your OS lacks /dev/urandom, you are at risk.

Stay secure. Double-check your dependencies, especially around randomness and cryptography!

*This post is original. For further details, consult the official CVE record and audit your Perl environments as soon as possible.*

Timeline

Published on: 04/13/2025 00:15:14 UTC
Last modified on: 04/15/2025 18:39:27 UTC