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Security Policy

This section describes the security policy of Photon OS.

1 - Default Firewall Settings

The design of Photon OS emphasizes security. On the minimal and full versions of Photon OS, the default security policy turns on the firewall and drops packets from external interfaces and applications. As a result, you might need to add rules to iptables to permit forwarding, allow protocols like HTTP, and open ports. You must configure the firewall for your applications and requirements.

The default iptables on the full version have the following settings:

iptables --list
Chain INPUT (policy DROP)
target     prot opt source               destination
ACCEPT     all  --  anywhere             anywhere
ACCEPT     all  --  anywhere             anywhere             ctstate RELATED,ESTABLISHED
ACCEPT     tcp  --  anywhere             anywhere             tcp dpt:ssh

Chain FORWARD (policy DROP)
target     prot opt source               destination

Chain OUTPUT (policy DROP)
target     prot opt source               destination
ACCEPT     all  --  anywhere             anywhere

For more information on how to change the settings, see the man page for iptables.

Although the default iptables policy accepts SSH connections, the sshd configuration file on the full version of Photon OS is set to reject SSH connections. See Permitting Root Login with SSH.

If you are unable to ping a Photon OS machine, check the firewall rules. To verify if the rules allow connectivity for the port and protocol, change the iptables commands by using lsof commands to see the processes listening on ports:

    lsof -i -P -n

2 - Default Permissions and umask

The umask on Photon OS is set to 0027.

When you create a new file with the touch command as root, the default on Photon OS is to set the permissions to 0640–which translates to read-write for user, read for group, and no access for others. Here’s an example:

touch newfile.md
stat newfile.md
  File: 'newfile.md'
  Size: 0               Blocks: 0          IO Block: 4096   regular empty file
Device: 801h/2049d      Inode: 316454      Links: 1
Access: (0640/-rw-r-----)  Uid: (    0/    root)   Gid: (    0/    root)

When you create a directory as root, Photon OS sets the permissions to 0750:

mkdir newdir
stat newdir
  File: 'newdir'
  Size: 4096            Blocks: 8          IO Block: 4096   directory
Device: 801h/2049d      Inode: 316455      Links: 2
Access: (0750/drwxr-x---)  Uid: (    0/    root)   Gid: (    0/    root)

Because the mkdir command uses the umask to modify the permissions placed on newly created files or directories, you can see umask at work in the permissions of the new directory. Its default permissions are set at 0750 after the umask subtracts 0027 from the full set of open permissions, 0777.

Similarly, a new file begins as 0666 if you were to set umask to 0000. But because umask is set by default to 0027, a new file’s permissions are set to 0640.

So be aware of the default permissions on the directories and files that you create. Some system services and applications might require permissions other than the default. The systemd network service, for example, requires user-defined configuration files to be set to 644, not the default of 640. Thus, after you create a network configuration file with a .network extension, you must run the chmod command to set the new file’s mode bits to 644. For example:

chmod 644 10-static-en.network

For more information on permissions, see the man pages for stat, umask, and acl.

3 - Disabling TLS 1.0 to Improve Transport Layer Security

Photon OS includes GnuTLS to help secure the transport layer. GnuTLS is a library that implements the SSL and TLS protocols to secure communications.

On Photon OS, SSL 3.0, which contains a known vulnerability, is disabled by default.

However, TLS 1.0, which also contains known vulnerabilities, is enabled by default.

To turn off TLS 1.0, perform the following steps:

  1. Create a directory named /etc/gnutls.
  2. In /etc/gnutls create a file named default-priorities.
  3. In the default-priorities file, specify GnuTLS priority strings that remove TLS 1.0 and SSL 3.0 but retain TLS 1.1 and TLS 1.2.
  4. After adding a new default-priorities file or after modifying it, you must restart all applications, including SSH, with an open TLS session for the changes to take effect.

The following is an example of a default-priorities file that contains GnuTLS priorities to disable TLS 1.0 and SSL 3.0:
console cat /etc/gnutls/default-priorities SYSTEM=NONE:!VERS-SSL3.0:!VERS-TLS1.0:+VERS-TLS1.1:+VERS-TLS1.2:+AES-128-CBC:+RSA:+SHA1:+COMP-NULL

In this example, the priority string imposes system-specific policies. The NONE keyword means that no algorithms, protocols, or compression methods are enabled, so that you can enable specific versions individually later in the string. The priority string then specifies that SSL version 3.0 and TLS version 1.0 be removed, as marked by the exclamation point. The priority string then enables, as marked by the plus sign, versions 1.1 and 1.2 of TLS. The cypher is AES-128-CBC. The key exchange is RSA. The MAC is SHA1. And the compression algorithm is COMP-NULL.

On Photon OS, you can verify the system-specific policies in the default-priorities file as follows:

  1. Concatenate the default-priorities file to check its contents:
root@photon-rc [ ~ ]# cat /etc/gnutls/default-priorities
SYSTEM=NONE:!VERS-SSL3.0:!VERS-TLS1.0:+VERS-TLS1.1:+VERS-TLS1.2:+AES-128-CBC:+RSA:+SHA1:+COMP-NULL
  1. Run the following command to check the protocols that are enabled for the system:
root@photon-rc [ /etc/gnutls ]# gnutls-cli --priority @SYSTEM -l
Cipher suites for @SYSTEM
TLS_RSA_AES_128_CBC_SHA1                                0x00, 0x2f      SSL3.0

Certificate types: none
Protocols: VERS-TLS1.1, VERS-TLS1.2
Compression: COMP-NULL
Elliptic curves: none
PK-signatures: none

For information about the GnuTLS priority strings, see https://gnutls.org/manual/html_node/Priority-Strings.html.

For information about the vulnerability in SSL 3.0, see SSL 3.0 Protocol Vulnerability and POODLE Attack.

For information about the vulnerabilities in TLS 1.0, see Guidelines for the Selection, Configuration, and Use of Transport Layer Security (TLS) Implementations.