CVE-2025-15638: Net::Dropbear Bundles Vulnerable libtomcrypt — CVSS 10.0

Executive Summary

CVE-2025-15638 is a critical vulnerability affecting Net::Dropbear, a Perl interface to the Dropbear SSH library. Versions before 0.14 bundle an outdated version of Dropbear (2019.78 or earlier), which in turn ships with libtomcrypt v1.18.1 or earlier. This version of libtomcrypt is affected by two known critical vulnerabilities:

• CVE-2016-6129 — ECC key validation flaw in libtomcrypt
• CVE-2018-12437 — RSA/DSA key generation weakness in libtomcrypt

CVSS Score: 10.0 (Critical)

The transitive nature of this vulnerability means that any Perl application using Net::Dropbear for SSH functionality inherits these cryptographic weaknesses, potentially exposing SSH sessions to decryption or man-in-the-middle attacks.

Vulnerability Overview

Affected Products

Technical Analysis

Vulnerability Chain

CVE-2025-15638 is a supply chain / bundled dependency vulnerability. The flaw is not in Net::Dropbear's own code — it arises from bundling outdated upstream components:

Net::Dropbear < 0.14
  └── Bundles Dropbear SSH <= 2019.78
        └── Includes libtomcrypt <= 1.18.1
              ├── CVE-2016-6129 (ECC key validation flaw)
              └── CVE-2018-12437 (RSA/DSA key generation weakness)

CVE-2016-6129 — ECC Key Validation Flaw

This vulnerability in libtomcrypt allows an attacker to supply an invalid elliptic curve public key point. Without proper validation, the library processes the malformed point and can expose private key material or enable invalid curve attacks, allowing an adversary to:

• Extract private keys from the affected SSH server or client
• Perform unauthorized decryption of captured session traffic
• Forge cryptographic signatures

CVE-2018-12437 — RSA/DSA Key Generation Weakness

This flaw affects the random number generation used during RSA and DSA key creation. Predictable or biased key material generated through the vulnerable libtomcrypt code weakens the resulting keys, making them susceptible to:

• Factorization attacks against RSA keys
• Lattice attacks against DSA keys derived with biased nonces
• Retrospective decryption of SSH sessions using affected keys

Attack Surface

Any Perl application using Net::Dropbear for SSH functionality is exposed:

• SSH servers built on Net::Dropbear handling inbound client connections
• SSH clients using Net::Dropbear to connect to remote systems
• Embedded or IoT SSH implementations written in Perl using this module
• Automation scripts performing SSH operations via Net::Dropbear

Impact Assessment

Remediation

Step 1: Upgrade Net::Dropbear

# Check current Net::Dropbear version
perl -MNet::Dropbear -e 'print "$Net::Dropbear::VERSION\n"'
 
# Upgrade via CPAN
cpan Net::Dropbear
 
# Or force install the latest version
cpan -f Net::Dropbear

Step 2: Regenerate Any SSH Keys Created with Affected Versions

If Net::Dropbear was used to generate SSH host keys or user keys, those keys should be considered compromised and regenerated:

# Remove and regenerate SSH host keys if using Dropbear standalone
rm /etc/dropbear/dropbear_rsa_host_key
rm /etc/dropbear/dropbear_ecdsa_host_key
rm /etc/dropbear/dropbear_ed25519_host_key
 
dropbearkey -t rsa -f /etc/dropbear/dropbear_rsa_host_key -s 4096
dropbearkey -t ecdsa -f /etc/dropbear/dropbear_ecdsa_host_key
dropbearkey -t ed25519 -f /etc/dropbear/dropbear_ed25519_host_key

Step 3: Verify the Bundled libtomcrypt Version

After upgrading Net::Dropbear, verify the bundled libtomcrypt has been updated:

# Check that Net::Dropbear 0.14+ is installed
use Net::Dropbear;
print "Net::Dropbear version: $Net::Dropbear::VERSION\n";
# Should be >= 0.14

Step 4: Audit CPAN Dependencies

# Scan for other modules bundling outdated cryptographic libraries
cpan-outdated | grep -i crypt
perl -MCPAN -e 'CPAN::Shell->upgrade()'

Detection Indicators

Post-Remediation Checklist

1. Upgrade Net::Dropbear to 0.14 or later on all affected systems
2. Regenerate all SSH host keys and user keys created with affected versions
3. Rotate any credentials or secrets accessed over SSH sessions that used vulnerable Net::Dropbear instances
4. Audit all CPAN modules for similar transitive dependency vulnerabilities
5. Monitor SSH authentication logs for anomalous patterns following remediation
6. Update dependency scanning tools to flag Net::Dropbear below 0.14

References

• NVD — CVE-2025-15638
• NVD — CVE-2016-6129
• NVD — CVE-2018-12437
• MetaCPAN — Net::Dropbear
• Dropbear SSH Project
• libtomcrypt Project