Network File Services  «Prev  Next»

Lesson 5 NFS security
Objective List potential NFS security problems and resolutions.

List potential NFS Security Problems and Resolutions

NFS evolved in an era when security was not a primary concern. Consequently, there is little mechanism inside NFS to protect against misuse.
Question: What are potential NFS security problems and their corresponding resolutions? When using NFS, there are several potential security problems that can arise. Here are some of the most common security problems and their corresponding resolutions:
  1. Unauthorized access: If NFS is not configured with proper access controls, unauthorized users may be able to access sensitive files on the NFS server. To resolve this issue, use the "exportfs" command to specify which hosts are allowed to access which directories on the NFS server. Additionally, use NFSv4 or Kerberos authentication for secure authentication and access control.
  2. Man-in-the-middle attacks: If the NFS traffic is not encrypted, it may be susceptible to interception and tampering by attackers. To resolve this issue, use NFS over Transport Layer Security (TLS) or Secure Sockets Layer (SSL) to encrypt the NFS traffic.
  3. Denial-of-service attacks: If the NFS server is not configured to limit the number of concurrent connections or requests, attackers may be able to overload the server with requests and cause a denial-of-service (DoS) attack. To resolve this issue, configure NFS with appropriate limits on the number of concurrent connections and requests, and use a firewall to block suspicious traffic.
  4. File corruption: If multiple clients access the same file simultaneously, file locking issues may occur, leading to file corruption or data loss. To resolve this issue, configure NFS with appropriate file locking mechanisms such as NFSv4 locking or Distributed Lock Manager (DLM) to ensure that only one client can access a file at a time.
  5. Weak authentication: If NFS is configured with weak authentication settings, such as the use of weak passwords, attackers may be able to compromise user accounts and gain access to sensitive data on the NFS server. To resolve this issue, use strong authentication mechanisms such as NFSv4 or Kerberos authentication, and enforce strong password policies.

Overall, the key to addressing potential NFS security problems is to configure NFS with appropriate access controls, authentication mechanisms, encryption, and other security features. It's important to stay up-to-date with security best practices and to seek assistance from qualified support resources as needed to ensure that your NFS system is secure and protected from potential security threats.

Common problems

Common security problems associated with NFS include:
  1. Incorrectly specifying the tcpd access information
    Red Hat Linux wraps portmap (and therefore NFS) access with tcpd[1], allowing the administrator to identify particular hosts or networks that have access. Incorrectly specifying the tcpd access information is a common exposure.
  2. User and group IDs on the NFS client and server are not the same
    Suppose a user with an ID of 242 owns some files on your NFS server. Any NFS client with a user ID of 242 can access these files, regardless of whether it's the same user 242 or not.

Solutions to common problems

It is much easier to prevent security problems from arising than to try to resolve them once they appear. Some thought beforehand, coupled with the following suggestions, will help prevent any intrusions:
  1. Create sensible access restrictions in /etc/exports. Think about your NFS users and only give the appropriate minimum access to users.
  2. Never export the root (/) filesystem because this exposes too much of your system's configuration. If you absolutely must export the root directory, export it read-only.
  3. Use wildcards only when absolutely necessary. Miscreants could gain access to your files by spoofing[2] DNS, and a wildcard only increases their chances of being successful.
  4. In /etc/hosts.deny, deny all access to the portmap service. In /etc/hosts.allow, allow access only to those hosts and networks to which you want to give NFS services. (See the tcpd man page for more information about these files.)
  5. Ensure user and group IDs match on both the NFS server and clients.

Question: Why should user and group IDs match on both the NFS client and server?
Answer: Because NFS grants access based on user and group ID.
Explanation: A user should have the same user and group ID on both client and server
[1] Tcpd: A program that provides host-based security for many Linux Internet applications.
[2] Spoofing: Faking a hostname to bypass one or more security mechanisms.