CVE-2011-1927 in Linux
Summary
by MITRE
The ip_expire function in net/ipv4/ip_fragment.c in the Linux kernel before 2.6.39 does not properly construct ICMP_TIME_EXCEEDED packets after a timeout, which allows remote attackers to cause a denial of service (invalid pointer dereference) via crafted fragmented packets.
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Analysis
by VulDB Data Team • 12/03/2021
The vulnerability identified as CVE-2011-1927 represents a critical flaw in the Linux kernel's handling of IP fragment expiration mechanisms. This issue resides within the net/ipv4/ip_fragment.c file and affects kernel versions prior to 2.6.39. The problem manifests when the ip_expire function attempts to construct ICMP_TIME_EXCEEDED packets after packet fragmentation timeouts occur. The improper construction of these ICMP packets creates a scenario where invalid pointer dereferences can occur, leading to system instability and potential denial of service conditions.
The technical implementation of this vulnerability stems from the kernel's failure to properly validate and construct ICMP_TIME_EXCEEDED packets when processing expired IP fragments. When fragmented packets exceed their time-to-live values, the kernel's ip_expire function is responsible for generating appropriate ICMP responses to notify the sender of the timeout. However, the flawed implementation does not adequately handle the memory references required for constructing these responses, resulting in situations where the kernel attempts to dereference invalid memory pointers. This type of error falls under the CWE-476 category of NULL Pointer Dereference, which represents a fundamental memory management flaw that can lead to system crashes and denial of service conditions.
From an operational perspective, this vulnerability presents a significant risk to network infrastructure and systems running affected Linux kernel versions. Remote attackers can exploit this weakness by crafting specifically formatted fragmented packets that trigger the vulnerable code path when the kernel processes packet timeouts. The attack requires no special privileges and can be executed from any network location capable of sending the malicious packets. The resulting denial of service impacts the entire network stack, potentially causing complete system crashes or rendering network services unavailable. This vulnerability directly maps to the ATT&CK technique T1499.004 for Network Denial of Service, where adversaries leverage system vulnerabilities to disrupt network availability.
The impact of this vulnerability extends beyond simple service disruption as it represents a fundamental flaw in the kernel's network processing capabilities. When the kernel encounters the invalid pointer dereference condition, it typically results in a kernel panic or system crash, requiring manual intervention and system reboot to restore normal operation. This makes the vulnerability particularly dangerous in mission-critical environments where network availability is paramount. The exploitability of this issue is relatively high given that it requires only the ability to send crafted packets to the target system, making it a preferred attack vector for network-based denial of service campaigns.
Mitigation strategies for CVE-2011-1927 focus primarily on kernel version upgrades to 2.6.39 or later, which contain the necessary patches to address the improper ICMP packet construction. System administrators should prioritize updating their kernel versions as a first-line defense against this vulnerability. Additionally, network administrators can implement packet filtering rules to limit the volume of fragmented packets or restrict certain fragment patterns that could trigger the vulnerable code path. The fix implemented in the patched kernel versions properly validates memory references and ensures correct ICMP packet construction, eliminating the possibility of invalid pointer dereferences during fragment expiration processing. Organizations should also consider implementing intrusion detection systems that can identify and alert on suspicious fragmented packet patterns that may indicate exploitation attempts against this vulnerability.