CVE-2025-23048

What it actually does

In mod_ssl configurations with multiple virtual hosts using distinct client certificate trust stores, TLS 1.3 session resumption allows a certificate validated for one virtual host to be reused against another without re-validation when SSLStrictSNIVHostCheck is disabled.

Why it matters to the business

Clients granted access to one restricted area of a web service can reach other areas that should be blocked, increasing risk of unauthorized data exposure or actions.

How an attack unfolds

An attacker obtains a client certificate trusted by one virtual host, completes an initial TLS 1.3 handshake, then reuses the session ticket or PSK to connect to a second virtual host whose certificate restrictions would normally reject it.

How to fix it

Upgrade affected Apache packages to the fixed version (Debian 13: 6.12.35-1 or later) and enable SSLStrictSNIVHostCheck on all virtual hosts that use distinct SSLCACertificate settings.

How to confirm the fix worked

Confirm the installed Apache package version meets or exceeds the fixed release, review virtual host configurations for SSLStrictSNIVHostCheck On, and test that cross-host session resumption is rejected.

Typical attack path

1. Obtain a client certificate trusted by at least one affected virtual host
2. Perform initial TLS 1.3 connection and capture resumption material
3. Use session resumption against a different virtual host lacking SSLStrictSNIVHostCheck
4. Access resources behind the unintended certificate policy

How to detect it

Review Apache access and error logs for TLS session resumption events crossing virtual host boundaries; correlate client certificate subjects with requested hostnames that would normally be denied.

What your security team should do

• Enable SSLStrictSNIVHostCheck wherever multiple virtual hosts use separate client certificate trust stores
• Upgrade Debian 13 systems to apache2 package 6.12.35-1 or newer
• Audit all mod_ssl multi-vhost deployments for missing strict SNI checks
• Monitor for any future public exploits given the configuration-specific nature of the flaw