CVE-2026-123205
Description
A flaw was found in accel-ppp. This vulnerability allows an unauthenticated remote attacker to cause a Denial of Service (DoS) by crashing the daemon through a heap-based buffer overflow. When verbose logging is enabled, an attacker can send a Layer 2 Tunneling Protocol (L2TP) connection request containing numerous attributes, generating oversized log entries that exceed fixed buffer limits in the logging targets. Successful exploitation corrupts memory and terminates the service.
Statement
This vulnerability is rated as Moderate severity rather than Important because successful exploitation requires non-default high-verbosity logging configured across specific sinks (such as syslog, database, or network targets) alongside reachability from client IP spaces, and the impact is limited to service crash. In default deployments, these verbose logging options are typically disabled during production use, substantially reducing the likelihood of heap corruption. Furthermore, accel-ppp is not shipped or supported in Red Hat Enterprise Linux, meaning Red Hat products are not affected by default.
Mitigation
Disable verbose logging and avoid using tcp, syslog, or pgsql logging targets in the daemon configuration, or restrict access to the L2TP service port via firewall controls.
1. In the accel-ppp configuration file (typically /etc/accel-ppp.conf), ensure verbose logging is turned off:
[log]Avoid configuring log-tcp, log-syslog, or log-pgsql targets.
log-file=/var/log/accel-ppp/accel-ppp.log
verbose=0
2. Restrict access to the L2TP port (UDP 1701) to trusted IP addresses only using firewalld:
firewall-cmd --permanent --zone=public --remove-port=1701/udp
firewall-cmd --permanent --zone=trusted --add-source=<trusted_ip_range>
firewall-cmd --reload
Caveats: Disabling verbose logging limits detailed debugging information for connection troubleshooting. Restricting UDP port 1701 blocks incoming tunnel connection requests from unlisted networks.
Warning: Modifying daemon configurations or firewall settings requires reloading or restarting the daemon (systemctl restart accel-ppp), which will terminate active VPN sessions and disconnect active tunnels.
Understanding the Weakness (CWE)
Integrity
Technical Impact: Modify Memory; Execute Unauthorized Code or Commands
Write operations could cause memory corruption. In some cases, an adversary can modify control data such as return addresses in order to execute unexpected code.
Availability
Technical Impact: DoS: Crash, Exit, or Restart
Attempting to access out-of-range, invalid, or unauthorized memory could cause the product to crash.
Other
Technical Impact: Unexpected State
Subsequent write operations can produce undefined or unexpected results.
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