CVE-2011-1739 in FreeBSD
Summary
by MITRE
The makemask function in mountd.c in mountd in FreeBSD 7.4 through 8.2 does not properly handle a -network field specifying a CIDR block with a prefix length that is not an integer multiple of 8, which allows remote attackers to bypass intended access restrictions in opportunistic circumstances via an NFS mount request.
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Analysis
by VulDB Data Team • 11/05/2021
The vulnerability described in CVE-2011-1739 resides within the mountd daemon of FreeBSD operating systems version 7.4 through 8.2. This issue specifically targets the makemask function located in the mountd.c file, which serves as a critical component in handling Network File System (NFS) mount requests. The mountd daemon operates as a server-side process that manages NFS client access permissions and network access controls, making it a prime target for privilege escalation and unauthorized access attempts.
The technical flaw manifests when the makemask function processes a -network field that specifies a CIDR block with a prefix length that is not an integer multiple of 8. In standard networking practices, CIDR notation typically uses prefix lengths that are multiples of 8, corresponding to traditional octet boundaries. However, the flawed implementation fails to properly validate or handle cases where the prefix length falls between these standard boundaries, creating a parsing inconsistency that can be exploited by malicious actors. This vulnerability falls under CWE-129, which addresses improper validation of the length or size of a buffer, and specifically relates to inadequate input validation in network access control mechanisms.
The operational impact of this vulnerability allows remote attackers to bypass intended access restrictions through opportunistic circumstances during NFS mount requests. When an attacker crafts a malicious NFS mount request with a specially formatted CIDR block containing a non-standard prefix length, the flawed makemask function fails to correctly process the network mask, potentially allowing unauthorized access to NFS shares that should be restricted. This creates a significant security risk in environments where NFS services are exposed to untrusted networks, as attackers can exploit this weakness to gain access to resources that should remain protected.
This vulnerability aligns with ATT&CK technique T1071.004, which covers protocol tunneling and network protocol manipulation, as the attack exploits weaknesses in network protocol handling rather than direct system exploitation. The flaw represents a classic case of improper input validation in network services, where the system fails to properly sanitize network access control parameters. Organizations using affected FreeBSD versions should immediately implement mitigations including updating to patched versions, implementing network segmentation, and monitoring NFS traffic for suspicious CIDR block usage patterns. The vulnerability demonstrates the importance of proper validation in network access control implementations and highlights the need for comprehensive testing of edge cases in security-critical network services.