CVE-2008-2136 in Linux
Summary
by MITRE
Memory leak in the ipip6_rcv function in net/ipv6/sit.c in the Linux kernel 2.4 before 2.4.36.5 and 2.6 before 2.6.25.3 allows remote attackers to cause a denial of service (memory consumption) via network traffic to a Simple Internet Transition (SIT) tunnel interface, related to the pskb_may_pull and kfree_skb functions, and management of an skb reference count.
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Analysis
by VulDB Data Team • 08/10/2019
The vulnerability described in CVE-2008-2136 represents a critical memory management flaw within the Linux kernel's IPv6 tunneling implementation that specifically affects the ipip6_rcv function in the sit.c file. This issue exists in kernel versions prior to 2.4.36.5 and 2.6.25.3, creating a persistent security risk for systems utilizing Simple Internet Transition (SIT) tunnel interfaces. The flaw manifests as a memory leak that occurs during the processing of network traffic destined for these tunnel interfaces, allowing remote attackers to systematically consume system memory resources through carefully crafted network packets.
The technical root cause of this vulnerability lies in improper handling of socket buffer (skb) reference counting within the kernel's network stack processing logic. Specifically, the ipip6_rcv function fails to correctly manage the relationship between pskb_may_pull and kfree_skb operations, which are fundamental kernel functions for manipulating and freeing network packet data structures. When network traffic arrives at an SIT tunnel interface, the function processes the incoming packets but does not properly decrement the reference count on the socket buffer, leading to a situation where memory allocated for packet data remains allocated even after the packet has been processed. This reference count management failure creates a memory leak that accumulates over time as attackers send multiple packets through the vulnerable tunnel interface.
The operational impact of this vulnerability is significant and directly translates to a denial of service condition where the targeted system gradually consumes all available memory resources. Remote attackers can exploit this weakness by sending specially crafted packets to the SIT tunnel interface, causing the kernel to continuously allocate memory without properly releasing it. As the memory leak compounds over time, system performance degrades substantially, eventually leading to system instability, application crashes, and complete system unresponsiveness. This makes the vulnerability particularly dangerous in network environments where continuous traffic flow is expected, as the memory exhaustion can occur gradually but inevitably leads to system failure.
This vulnerability maps to CWE-401: Improper Release of Memory Before Removing Last Reference, which specifically addresses memory management issues where resources are not properly deallocated despite the removal of the last reference. From an ATT&CK perspective, this represents a resource exhaustion attack pattern under T1499.004: Endpoint Denial of Service, where adversaries leverage system vulnerabilities to consume computational resources. The exploitation requires minimal privileges and can be performed remotely, making it an attractive vector for attackers seeking to disrupt network services. The vulnerability also aligns with T1595.001: Network Device Discovery, as attackers must first identify systems running vulnerable kernel versions that support SIT tunneling. The impact extends beyond simple service disruption to potentially affecting system availability for legitimate users and can be combined with other attacks to amplify their effectiveness.
Mitigation strategies for this vulnerability include immediate kernel updates to versions 2.4.36.5 or 2.6.25.3, which contain the necessary fixes for proper reference count management. Organizations should also implement network segmentation to isolate systems with SIT tunnel interfaces from critical network segments, and establish monitoring systems to detect unusual memory consumption patterns that might indicate exploitation attempts. Additionally, disabling unused tunnel interfaces and implementing proper network access controls can reduce the attack surface. For systems where immediate updates are not feasible, network-level filtering can be implemented to restrict traffic to SIT interfaces, though this approach only provides partial protection and does not address the underlying kernel flaw.