Source: secalert@redhat.com
The isdn_ppp_ccp_reset_alloc_state function in drivers/isdn/isdn_ppp.c in the Linux 2.4 kernel before 2.4.34-rc4 does not call the init_timer function for the ISDN PPP CCP reset state timer, which has unknown attack vectors and results in a system crash.
Linux kernel versions 2.4.x are vulnerable to a denial-of-service (DoS) condition due to a missing timer initialization in the ISDN PPP CCP reset state function. This flaw allows an attacker to trigger a kernel panic, resulting in a system crash and potential data loss.
Step 1: Trigger ISDN PPP Connection: An attacker establishes or manipulates an ISDN PPP connection to the vulnerable system.
Step 2: CCP Reset Request: The attacker sends a crafted CCP (Compression Control Protocol) reset request or initiates a sequence of events that leads to the execution of the vulnerable isdn_ppp_ccp_reset_alloc_state function.
Step 3: Timer Activation: The isdn_ppp_ccp_reset_alloc_state function is called, and the uninitialized timer is used.
Step 4: Timer Expiration: The timer expires, and the kernel attempts to execute the timer's handler function.
Step 5: Kernel Panic: Due to the uninitialized timer data, the handler attempts to access invalid memory locations, triggering a kernel panic and system crash.
The vulnerability lies within the isdn_ppp_ccp_reset_alloc_state function in drivers/isdn/isdn_ppp.c of the Linux 2.4 kernel. The function fails to initialize the ISDN PPP CCP reset state timer using init_timer. This omission leads to a situation where the timer is used without proper initialization. When the timer fires, it attempts to access uninitialized memory, leading to a kernel panic. The root cause is a simple coding error: a missing call to init_timer before the timer is used. This can be triggered by specific ISDN PPP communication patterns.
Due to the age of the vulnerability and the limited scope (ISDN PPP), it's unlikely to be actively targeted by sophisticated APTs. However, it could be exploited in specific environments where older Linux kernels are still in use, particularly in embedded systems or legacy network infrastructure. Not listed on CISA KEV.
Monitor system logs for kernel panics (e.g., kernel: BUG: unable to handle kernel NULL pointer dereference at ... or similar error messages).
Analyze network traffic for unusual ISDN PPP communication patterns, especially CCP reset requests or malformed packets.
Examine system logs for events related to ISDN PPP connections and disconnections.
Use intrusion detection systems (IDS) with rules that detect suspicious ISDN PPP traffic.
Upgrade to a patched Linux kernel version (2.4.34-rc4 or later).
Disable ISDN PPP support if not required. This eliminates the attack surface.
Implement network segmentation to isolate systems running vulnerable kernels.
Regularly scan systems for vulnerabilities using vulnerability scanners.
Review and harden ISDN PPP configurations to minimize attack surface.