The Common Vulnerabilities and Exposures (CVE®) project has published a vulnerability, identifier CVE-2025-2814, which affects Crypt::CBC (Cipher Block Chaining) mode encryption in Perl for versions between 1.21 and 3.04. The issue occurs when Crypt::CBC uses the insecure rand() function as the default source of entropy when /dev/urandom is unavailable, leading to potentially weak encryption.

Detailed Analysis

Crypt::CBC is a popular encryption method in Perl, implementing the well-known cipher block chaining mode. In versions 1.21 to 3.04, there's a risk that the rand() function is used as the default source of entropy for cryptographic functions when /dev/urandom is not available on the operating system. Unfortunately, the rand() function is not cryptographically secure, making the resulting ciphertext generated by Crypt::CBC easier to crack or tamper with than if a secure entropy source was used.

The vulnerability was introduced in Crypt::CBC version 1.21 when the default entropy source behavior was changed and persisted until version 3.04.

Here's a code snippet demonstrating the potentially insecure default behavior

use Crypt::CBC;
my $cipher = Crypt::CBC->new( -key => 'a_key', -cipher => 'Blowfish' );
my $ciphertext = $cipher->encrypt_hex("plaintext");

In this example, since no secure entropy source is specified, Crypt::CBC may use the insecure rand() function when the /dev/urandom device is not present.

Operating Systems Affected

This vulnerability affects any Unix, Linux, or embedded system that does not have a /dev/urandom device. Some examples include:

- Operating systems where /dev/urandom has been intentionally disabled
- Older Unix systems that do not have /dev/urandom
- Low-resource embedded systems without a /dev/urandom device

Exploit Details and References

- CVE Identifier: CVE-2025-2814
- perl-Crypt-CBC Issue on GitHub: Issue #52

This vulnerability can be exploited by an attacker with enough computing resources to perform a brute-force or statistical attack on encrypted data, especially when the encryption key is derived using the insecure rand() function. The reduced keyspace due to the poor entropy caused by relying on rand() makes it significantly easier for an attacker to crack the encrypted data.

As always, it is crucial to keep up-to-date with the latest security patches and ensure that only secure configurations are used.

Users of Crypt::CBC on affected versions should upgrade to version 3.05 or later, which includes a fix for the insecure entropy source. Upgrading can be accomplished using:

cpan Crypt::CBC

Alternatively, you can update your Crypt::CBC installation by downloading the latest version from its CPAN page: Crypt::CBC on CPAN.

If you are unable to upgrade, you may also explicitly specify a secure source of entropy, such as the the Crypt::URandom module, when initializing Crypt::CBC:

use Crypt::CBC;
use Crypt::URandom qw(urandom);

my $key = urandom(32);

my $cipher = Crypt::CBC->new( -key => $key, -cipher => 'Blowfish', -iv => urandom(8), -header => 'randomiv' );
my $ciphertext = $cipher->encrypt_hex("plaintext");

In this example, Crypt::URandom::urandom() is utilized, providing cryptographically secure random numbers for key generation and the IV (initialization vector).

Conclusion

The CVE-2025-2814 vulnerability in Crypt::CBC for Perl can lead to weak encryption, especially in scenarios where a secure source of entropy is not available. It is essential for developers and system administrators to ensure that their systems are using the latest version of Crypt::CBC and apply the appropriate mitigations for a secure encryption implementation. The use of a properly secured Crypt::CBC configuration will help protect sensitive data and provide a safer digital environment.

Timeline

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