CVE-2011-1767 in Linux
Summary
by MITRE
net/ipv4/ip_gre.c in the Linux kernel before 2.6.34, when ip_gre is configured as a module, allows remote attackers to cause a denial of service (OOPS) by sending a packet during module loading.
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Analysis
by VulDB Data Team • 12/03/2021
The vulnerability described in CVE-2011-1767 represents a critical denial of service flaw within the Linux kernel's Generic Routing Encapsulation implementation. This issue specifically affects systems running Linux kernel versions prior to 2.6.34 where the ip_gre module is configured as a loadable kernel module rather than being built into the kernel. The flaw arises from improper handling of network packets during the dynamic loading process of the GRE module, creating a race condition that can be exploited by remote attackers to trigger kernel oops conditions and subsequent system instability.
The technical root cause of this vulnerability stems from the kernel's handling of GRE packet processing when the ip_gre module is being loaded. During module initialization, the kernel sets up various data structures and function pointers necessary for packet processing. However, when remote attackers send specially crafted GRE packets while the module is in the process of loading, the kernel's packet handling code fails to properly validate incoming packets against the incomplete module state. This results in the kernel attempting to access uninitialized or improperly configured data structures, leading to kernel oops conditions that effectively crash the system. The vulnerability operates at the network layer and specifically targets the Generic Routing Encapsulation protocol implementation within the Linux kernel networking stack.
From an operational impact perspective, this vulnerability presents a significant risk to systems that rely on dynamic module loading for network functionality. Network services and routers that depend on the ip_gre module for tunneling operations become vulnerable to remote exploitation, potentially allowing attackers to perform denial of service attacks against critical infrastructure. The attack requires minimal privileges and can be executed from any remote location, making it particularly dangerous for publicly accessible network equipment. Systems running kernel versions before 2.6.34 are at risk regardless of whether the ip_gre module is actively in use, as the vulnerability exists during the module loading phase itself.
The security implications extend beyond simple denial of service, as this vulnerability can be leveraged to disrupt network services and potentially provide a foothold for more sophisticated attacks. According to CWE classification, this vulnerability maps to CWE-119: Improper Access to Memory, as it involves the kernel accessing memory locations that have not been properly initialized. The ATT&CK framework categorizes this under T1499.004: Endpoint Denial of Service, representing a method for attackers to disrupt services through kernel-level exploitation. Organizations should prioritize immediate patching of affected systems, as the vulnerability does not require authentication or special privileges to exploit, making it accessible to any remote attacker with network connectivity to the target system.
Mitigation strategies for this vulnerability include immediate deployment of kernel updates to version 2.6.34 or later, where the issue has been resolved through proper initialization checks and race condition handling. Administrators should also consider disabling the ip_gre module if it is not actively required, preventing the vulnerable loading sequence entirely. Network monitoring should be enhanced to detect unusual packet patterns that might indicate exploitation attempts, and systems should be configured to automatically apply security patches upon release. Additionally, implementing proper network segmentation and access controls can limit the potential impact of successful exploitation attempts.