CVE-2001-0323 in Solaris
Summary
by MITRE
The ICMP path MTU (PMTU) discovery feature in various UNIX systems allows remote attackers to cause a denial of service by spoofing "ICMP Fragmentation needed but Don t Fragment (DF) set" packets between two target hosts, which could cause one host to lower its MTU when transmitting to the other host.
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Analysis
by VulDB Data Team • 10/07/2025
The vulnerability described in CVE-2001-0323 represents a critical flaw in the Internet Control Message Protocol (ICMP) path MTU discovery mechanism implemented across various UNIX operating systems. This weakness specifically targets the PMTU discovery process that is designed to dynamically determine the maximum transmission unit size for network paths, allowing systems to optimize packet delivery while avoiding fragmentation issues. The flaw exploits the trust model inherent in ICMP processing, where systems accept certain ICMP messages as legitimate indicators of network conditions without sufficient validation of message authenticity or source legitimacy.
The technical implementation of this vulnerability stems from the lack of proper authentication mechanisms within the ICMP PMTU discovery protocol. When a host receives an ICMP "Fragmentation needed but Don't Fragment (DF) set" message, it automatically adjusts its transmission MTU size for subsequent communications with the source of that message. This automatic adjustment occurs without verifying that the ICMP packet originates from a legitimate source or that the message content accurately reflects actual network conditions. The vulnerability becomes exploitable when a remote attacker can forge or spoof these ICMP messages, particularly targeting the PMTU discovery process between two communicating hosts.
The operational impact of this vulnerability extends beyond simple denial of service conditions, as it fundamentally compromises the network communication integrity between affected systems. Attackers can manipulate network performance by artificially reducing MTU sizes, which forces systems to fragment larger packets into smaller ones, increasing network overhead and potentially causing significant performance degradation. The attack can be particularly devastating in environments where large data transfers are common, as reduced MTU sizes can lead to substantial bandwidth inefficiencies and increased latency. This vulnerability can be exploited to create persistent network degradation conditions that may persist until manual intervention occurs or the affected systems are restarted.
This vulnerability aligns with CWE-209, which addresses improper handling of ICMP messages, and demonstrates characteristics consistent with ATT&CK technique T1498, specifically "Network Denial of Service," where adversaries manipulate network conditions to disrupt service availability. The attack vector represents a classic example of how trust-based protocols can be exploited when authentication mechanisms are insufficient, as it leverages the implicit trust that systems place in ICMP responses without validating their source. Organizations implementing network security measures should consider this vulnerability when assessing their exposure to ICMP-based attacks, particularly in environments where network traffic monitoring is limited and ICMP processing is not properly restricted. The vulnerability underscores the importance of implementing proper ICMP filtering and source validation mechanisms to prevent unauthorized manipulation of network path characteristics. Mitigation strategies should include implementing strict ICMP message filtering, employing network segmentation to limit ICMP propagation, and configuring systems to validate ICMP message sources through additional verification mechanisms.