About a two and a half years ago I took a class on technical writing to wrap up the requirements for my multi-decade batchelor's degree. For that technical writing class my idea was to detail some of the problems related to cyber security in the federal government. Since securing systems in the government space has been a significant portion of my job for most of the 21st century I wanted to bring light to places we can improve and create more effective cyber security. I wrote this paper on what I called deficiencies with the RHEL7 STIG. Since this time I've tried my best to address many issues through official channels but I've officially given up trying to get DISA to embrace technical merit. My hope is that putting this information here for others can start a wider discussion about real security rather than relying on the DISA STIG bureaucratic process.
A link to the PDF that is roughly what I originally wrote is available here. However a slightly trimmed version is available below.
Deficiency Analysis of the Defense Information Systems Agency Security Technical Implementation Guide for Redhat Enterprise Linux 7
Abstract
The Department of Defense spends countless hours into security configuration, assessment, and documentation. These security configurations are often derived from Security Technical Implementation Guides (STIGs) that are published for various hardware and software. One of the most time consuming and problematic STIGs is related to operating systems for the Redhat Enterprise Linux system that is created by the Defense Information Systems Agency. This STIG is of vast complexity due to the complex nature of the operating system. As a result, there is often outdated information incorporated into this document. In addition, many changes recommended in this STIG have far reaching effects on performance and usability. In this report many deficiencies with the current DISA RHEL 7 STIG are investigated and approximately 40% of them require additional consideration.
Introduction
Cyber security is the fastest growing focus area of information technology systems in the Department of Defense. The goal of security is to provide Confidentiality, Integrity, and Availability also known in the security community as the CIA triad. To achieves these goals operating systems must continually be evaluated as new features are integrated to ensure that the security is not compromised. In addition, security often creates hurdles for usability and performance and therefore most operating systems are not shipped in a “secure by default” configuration. To mitigate this problem the DoD creates Security Technical Implementation Guides with coordination from operating system vendors to provide a security posture they consider to be secure. In this report I plan to evaluate the STIG for the Redhat Enterprise Linux (RHEL) 7 operating system to determine areas that have failed to properly account for usability, performance, Denial of Service (DoS), and other issues. This is report is based off of the latest STIG published by the Defense Information Systems Agency Version 2 release 7 dated April 24, 2020 referenced as [1]. This report is targeted at technical evaluators and decision makers to bring attention to the numerous issues created by the process of implementing security guidance without full understanding of the impacts. It is often joked that the only secure system is the one that isn’t plugged in. We must strive to provide effective use of tax payer dollars by squeezing all the performance we can get out of hardware and without hindering our operations and maintenance staff with burdensome requirements that affect their ability to do their jobs. I this assessment I will evaluate the following areas as it applies to the DISA STIG for RHEL 7 [1]:
- Outdated Items
- Performance Impacts
- Potential DoS
- Usability Impacts
- Other Issues
There are several areas in this report where small excerpts of source code are provided from the RHEL/Centos 7.8 packages. These are included to provide assistance to technical staff who would like to validate the details in this report. Each inclusion of source code will have several surrounding lines to provide context, the line numbers, and source file identified. Also, every item detailed in this report also details the STIG ID numbers from [1] at the end of the section. This is to provide clarity and also assistance to any further review necessary
Outdated Items
Password Outdated Items
The current release for the RHEL 7 STIG [1] was released on April 24, 2020. Despite this recent release it is still out of date with the guidance from the National Institute of Standards and Technology (NIST) Special Publication 800-63 Digital Identity Guidelines [2]. There are several criteria that have been outdated with the latest recommendations and are detailed below. These guidelines apply to human passwords and not stored machine level passwords. Password Expiration No Longer Recommended According to [3] users tended to choose weaker passwords when they know password changes were imminent in the near future. The password changes often used “common transformations such as increasing a number in the password” [3]. The result is that security is not increased by these changes. While recommendations for temporal password expirations are no longer recommended event-based expirations still remain in place. Event-based password expirations are the result of some sort of breach of the password database.
Affected STIG IDs:
RHEL-07-010230, RHEL-07-010240, RHEL-07-010250, RHEL-07-010260
Password Composition Rules No Longer Recommended
The current requirements in [1] specify the following password composition rules:
- At least one each of class (uppercase letters, lowercase letters, numbers, and symbols)
- No more than 3 repeating characters
- No more than 4 repeating characters in the same class
All these requirements have been removed as of [2]. The reasoning is again based on human factors. Users tended to use predictable methods of fulfilling these requirements which minimized any security gain. Another determined weakness of the old scheme is that these rules encouraged people to use the same password across multiple systems. This is particularly damaging for DoD users where accounts can exist on multiple networks. These multiple networks are very common in DoD environments where users may have an account on unclassified, secret, and top-secret systems.
Affected STIG IDs:
RHEL-07-010120, RHEL-07-010130, RHEL-07-010140, RHEL-07-010150, RHEL-07-010160, RHEL-07-010170, RHEL-07-010180, RHEL-07-010190, RHEL-07-010280
Additional Password Restrictions
The new mechanism of preventing weak passwords is to utilize a blacklist of common passwords. A specific list of potential candidates is included below from [2]:
- Passwords obtained from previous breach corpuses
- Dictionary words
- Repetitive or sequential characters (e.g. ‘aaaaaa’, ‘1234abcd’).
- Context-specific words, such as the name of the service, the username, and derivatives thereof
Password databases are included with the cracklib package that comes with the operating system. Any of these databases of blacklisted passwords can be added to the pam_pwquality module.
Password Based Key Derivation Functions (PBKDFs)
The updated recommendation for memorized secret verifiers should use time and memory hard key derivation functions. The only NIST evaluated algorithm that supports this requirement is Balloon [3]. This particular rule only affects one item in [1] related to PBKDFs which is for the grub2 boot loaded. Unfortunately, there is no support in grub2 for the Balloon algorithm. Given the lack of a current technical implementation this item should be prioritized to develop a solution.
Mount Options for /dev/shm
The temporary file system used for shared-memory applications is setup by the system init process, systemd. Several STID IDs require this file system to be mounted with the nodev and nosuid options. However, these options are already defined in the source code for systemd used by the operating system. The specific lines are highlighted below: Note: Some white-space has been trimmed from the following source code for readability.
Source file: systemd-219/src/core/mount-setup.c
76 static const MountPoint mount_table[] = {
77 { "sysfs", "/sys", "sysfs", NULL, MS_NOSUID|MS_NOEXEC|MS_NODEV,
78 NULL, MNT_FATAL|MNT_IN_CONTAINER },
79 { "proc", "/proc", "proc", NULL, MS_NOSUID|MS_NOEXEC|MS_NODEV,
80 NULL, MNT_FATAL|MNT_IN_CONTAINER },
81 { "devtmpfs","/dev", "devtmpfs", "mode=755", MS_NOSUID|MS_STRICTATIME,
82 NULL, MNT_FATAL|MNT_IN_CONTAINER },
83 { "securityfs", "/sys/kernel/security", "securityfs", NULL, MS_NOSUID|MS_NOEXEC|MS_NODEV,
84 NULL, MNT_NONE },
85 #ifdef HAVE_SMACK
86 { "smackfs", "/sys/fs/smackfs", "smackfs", "smackfsdef=*", MS_NOSUID|MS_NOEXEC|MS_NODEV,
87 mac_smack_use, MNT_FATAL },
88 { "tmpfs", "/dev/shm", "tmpfs","mode=1777,smackfsroot=*", MS_NOSUID|MS_NODEV|MS_STRICTATIME,
89 mac_smack_use, MNT_FATAL },
90 #endif
91 { "tmpfs", "/dev/shm", "tmpfs","mode=1777", MS_NOSUID|MS_NODEV|MS_STRICTATIME,Affected STIG IDs:
RHEL-07-021022, RHEL-07-021023
Disable Promiscuous Mode
The current recommendation to disable promiscuous mode is unnecessary as enabling it through the init scripts has been deprecated in RHEL 7 as detailed in [4].
Affected STIG ID: RHEL-07-040670
Performance Impacts
Auditing Performance
Auditing accounts for 71 of the 248 STIG items in [1]. Given the large footprint there should be some significant research into the impact of auditing on performance. However, there is minimal information to document the impact but the most comprehensive appears to be [5] from 2015 and [6] from 2018. [5] discussed the impact of audit overhead based on the frequency of audit events per second and is captured in Figure 1. [6] details the overhead of common programs and breaks down the performance by area of the audit system. Figure 2 details their findings
Figure 1 Audit Performance Overhead Source: [5]
Figure 2 Audit Overhead by Program Source: [6]
It is clear from both figures that auditing can add unacceptable overhead. In Figure 2 the overhead added by the kernel is minimal but the Netlink socket used to flush data to the audit daemon, and the audit daemon overhead itself is significant for various programs. Especially affected are programs like Firefox and Apache that generate many audit records. Further analysis should be conducted to determine how auditing performance affects scalability of a single system.
Affected STIG IDs:
RHEL-07-030000, RHEL-07-030010, RHEL-07-030300, RHEL-07-030310, RHEL-07-030320, RHEL-07-030330, RHEL-07-030340, RHEL-07-030350, RHEL-07-030360, RHEL-07-030370, RHEL-07-030380, RHEL-07-030390, RHEL-07-030400, RHEL-07-030410, RHEL-07-030420, RHEL-07-030430, RHEL-07-030440, RHEL-07-030450, RHEL-07-030460, RHEL-07-030470, RHEL-07-030480, RHEL-07-030490, RHEL-07-030500, RHEL-07-030510, RHEL-07-030520, RHEL-07-030530, RHEL-07-030540, RHEL-07-030550, RHEL-07-030560, RHEL-07-030570, RHEL-07-030580, RHEL-07-030590, RHEL-07-030610, RHEL-07-030620, RHEL-07-030630, RHEL-07-030640, RHEL-07-030650, RHEL-07-030660, RHEL-07-030670, RHEL-07-030680, RHEL-07-030690, RHEL-07-030700, RHEL-07-030710, RHEL-07-030720, RHEL-07-030740, RHEL-07-030750, RHEL-07-030760, RHEL-07-030770, RHEL-07-030780, RHEL-07-030800, RHEL-07-030810, RHEL-07-030820, RHEL-07-030830, RHEL-07-030840, RHEL-07-030870, RHEL-07-030880, RHEL-07-030890, RHEL-07-030900, RHEL-07-030910, RHEL-07-030920, RHEL-07-030321, RHEL-07-030871, RHEL-07-030872, RHEL-07-030873, RHEL-07-030874, RHEL-07-030819, RHEL-07-030821, RHEL-07-030200, RHEL-07-030201, RHEL-07-030210, RHEL-07-030211
Firewall Performance
The firewall functionality of RHEL is implemented using iptables. There have been numerous performance issues identified in iptables and so careful attention must be paid to the number of rules used. However, [1] requires “that the firewall must be configured to “system access control program must be configured to grant or deny system access to specific hosts and services”. As hardware continues to grow in capability it can support more and more services. As the number of services supported from a single machine grows so will the to enforce these requirements. Figure 3 shows the impact on throughput as the number of rules increase while Figure 4 details the impact on throughput of supporting more ports. The purple line in these graphs represent iptables used by RHEL 7.
Figure 3: iptables performance impact of rules Source: [8]
Figure 4 iptables performance impact with increasing ports Source [8]
Affected STIG ID:
RHEL-07-040100, RHEL-07-040520, RHEL-07-040810
Potential Denial of Service (DoS) Changes
There are several recommendations from [1] that open possible avenues of DoS.
Login DoS
[1] requires that accounts be locked out for a minimum of 15 minutes after three unsuccessful logon attempts within a 15-minute timeframe. Given the weakened security posture from using outdated password guidelines may be some justification for this rule but this setting opens user accounts to a simple DoS. Furthermore, this rule ultimately compromises availability of the CIA triad. The PCI-DSS standard [7] requires a user id to be locked out after no more than 6 attempts. The PCI-DSS standard seems more reasonable of a balance between confidentiality and availability. Another rule from [1] requires the root (superuser) account to also be locked out for 15 minutes after three unsuccessful logon attempts within a 15-minute time frame. This policy creates the possibility of a DoS to all system users with no ability to resolve it from within since only a superuser can unlock accounts.
Rather than lockout the root account, the pam_securetty module can be used to limit access to a console through out-of-band access using a Backplane Management Console (BMC) or Integrated Lights Out (ILO). With this additional security measure, the root account could be exempted from account lock out while still preserving security.
Affected STIG ID:
RHEL-07-010320, RHEL-07-010330, RHEL-07-010430, RHEL-07-040670
System DoS
There are several requirements in [1] that require home directories, /var, and /tmp file systems to all exist on separation partitions. The justification for this requirement is that it could protect the system if the partition “became full or failed”. The failure of the /var or /tmp file systems would prevent many system level daemons from functioning properly. Failures in /home would only cause user access DoS. However, these fail to account for the impact to system performance as a disk is partitioned up. On Solid State Devices (SSDs) there is no significant impact, but when utilizing Hard Disk Drives (HDDs) the impact can be dramatic. HDDs only support approximately 100-150 Input Output Operations (IOPs) per second where SSDs tend to support around 4000+ IOPS. When disks are partitioned many data transfers that could have been merged into a single IOP can no longer be merged due to their non-contiguous nature. The result is that a system disk can become saturated causing multiple second delays in responsiveness. Therefore, this recommendation should be considered only when the partitions are on SSDs or separate HDDs.
Affected STIG ID:
RHEL-07-021310, RHEL-07-021320, RHEL-07-021340
Other Items
There are several rules from [1] that are inaccurate in their justification and therefore should be corrected or removed.
Removing System Accounts
[1] states that “If the accounts on the system do not match the provided documentation, or accounts that do not support an authorized system function are present, this is a finding”. The recommendation is to remove several system accounts. However, these accounts come suitably protected by providing an unusable login shell and having a locked password. Therefore, there is no risk of unauthorized use of these accounts. In addition, these conflict with vendor guidance in [5] that states: It is recommended to keep UIDs/GIDs of the system service accounts as default. The UIDs/GIDs of some system service accounts are hard-coded in the application itself, changing the uids/gids may potentially break the functionality of an application. These accounts exist for compliance with the Linux Standard Base [9].
Affected STIG ID:
RHEL-07-020270
Interactive Users Must Have Home Directory in /etc/passwd
The discussion in [1] states that “This could create a Denial of Service because the user would not be able to access their logon configuration files, and it may give them visibility to system files they normally would not be able to access”. The first part of the statement is true but there is no risk of visibility into system files they would not normally have. This is because the Linux Discretionary Access Control (DAC) is still in use. As can be seen below from the login source code that if changing directory to the user’s home directory fails then the “/” directory is used:
util-linux-2.23.2/login-utils/login.c
1330 /* wait until here to change directory! */
1331 if (chdir(pwd->pw_dir) < 0) {
1332 warn(_("%s: change directory failed"), pwd->pw_dir);
1333
1334 if (!getlogindefs_bool("DEFAULT_HOME", 1))
1335 exit(0);
1336 if (chdir("/"))
1337 exit(EXIT_FAILURE);
1338 pwd->pw_dir = "/";
1339 printf(_("Logging in with home = \"/\".\n"));
1340 }Also, relevant is the DEFAULT_HOME attribute can be set to “no” in the /etc/login.defs file to prevent login in the event that the user’s home directory is inaccessible.
Affected STIG ID:
RHEL-07-020620
Usability Items
Impact on Usability
Security must be a balance between the impact on performance and usability. Different groups have different numbers of personnel to be able to operate and maintain their systems and so a discussion is necessary about the STIG rules that impact efficient system administration. Efficient administration is the ability for a small number of administrators to operate and maintain a large number of systems. The following items detail various STIG rules and their impact on efficient administration. Delay between failed login attempts
[1] requires the addition of a 4 second delay be added to the Pluggable Authentication Modules (PAM) configuration for pam_faillock. This restriction however, ignores the fact that configuration provided by the vendor already provides a two second delay on failure on local password failures through the pam_unix module. The following source code section captures the respective section for pam_unix and highlights the fail delay setting.
Linux-PAM-1.1.8/modules/pam_unix/support.c
721 int _unix_verify_password(pam_handle_t * pamh, const char *name
722 ,const char *p, unsigned int ctrl)
723 {
724 struct passwd *pwd = NULL;
725 char *salt = NULL;
726 char *data_name;
727 int retval;
728
729
730 D(("called"));
731
732 #ifdef HAVE_PAM_FAIL_DELAY
733 if (off(UNIX_NODELAY, ctrl)) {
734 D(("setting delay"));
735 (void) pam_fail_delay(pamh, 2000000); /* 2 sec delay for on failure */This setting combined with the idle connection closing only creates a nuisance for users and administrators logging into the system and has a negligible impact on security. To capture the additional security, see the following two approximate equations where the values are represented by the following:
900 is the number of seconds an account is locked after 3 failures
3 failures before an account is locked
1000 attempts to guess the password
6 is the number of seconds with the pam_faildelay requirement
2 is the number of seconds removing the pam_faildelay requirement
time no fail delay=((1000/3)*900)+((1000/3)*2)=300366 seconds or 3.476 days
time with fail delay=((1000/3*900)+((1000/3)*6)=301698 seconds or 3.49 days
The addition of the failure delay is clearly insubstantial and therefore should be removed from the requirements.
Affected STIG ID:
RHEL-07-010430
Remote privileged user access without password prompts
There are number of tools developed by High Performance Computing (HPC) administrators to effectively administer a large number of systems efficiently. These tools are often developed based on the need to administer thousands of systems by a small administration team. Two of the most common of these tools are Parallel Distributed Shell (pdsh) and Clustered Shell (clush). These tools work by opening secure shell connections to each server and running a command across hundreds of thousands of servers in just seconds. In order to make administrative changes efficiently administrators must be able to login as root (the superuser) or have the ability to run commands with superuser privileges using su or sudo. [1] requires that both remote root logins be disabled and su/sudo be disabled without a password. Preventing direct access to the root account is a considered a good security practice. This prevents users from logging in directly as a user other than their own and preserving non-repudiation. However, additional security restrictions can be added to restrict su/sudo to only users of a specific group. Using these productions allow privileged escalation while maintaining non-repudiation and increased security. To make these changes the su command can be restricted to the wheel group by adding highlighted line to /etc/pam.d/su.
#%PAM-1.0
auth sufficient pam_rootok.so
auth required pam_wheel.so use_uid group=wheel
auth substack system-auth
auth include postlogin
account sufficient pam_succeed_if.so uid = 0 use_uid quiet
account include system-auth
password include system-auth
session include system-auth
session include postlogin
session optional pam_xauth.so
Restricting sudo access to the wheel group can be done by adding the following to the /etc/sudoers file.
%wheel ALL=(ALL) NOPASSWD: ALL
Affected STIG ID:
RHEL-07-010340, RHEL-07-010350
Conclusion
Summary of findings
There are many issues identified in this report which should cause alterations of the DISA RHEL 7 STIG. To highlight the percentages of issues we have discussed here the following chart shows the percentage of those issues based on the 248 Rules in [1]. It is clear here that much consideration needs to be addressed to ensure that the Confidentiality, Integrity, and Availability are evaluated in a balanced manner. It appears as though the Availability portion of the three is not adequately being considered in many of the STIG rules identified here. Figure 5 captures the percentage of issues and acceptable rules identified in [1].
Figure 5 DISA RHEL 7 STIG Rules Breakdown
Recommendations
Based on the findings from source code reviews, updated guidance, and the various performance issues the following actions are recommended for updating the DISA RHEL 7 STIG:
- Update all outdated password rules
- Develop a memory and time hard PBKDF implementation (Balloon) for the GRUB2 bootloader
- Develop a tool to monitor performance impacts of compliant auditing
- Develop a tool to monitor performance impacts of compliant firewall rules
- Utilize provided information for DoS issues to fix STIG rules
- Remove the password fail delay requirement
- Allow documented exceptions for privileged escalation without password for admin groups utilizing the documented security enhancements here
References
[1] Red Hat Enterprise Linux 7 Security Technical Implementation Guide, Version 2 Release 7, 2020.
[2] Digital Identity Guidelines, NIST SP 800-63, 2017.
[3] NIST SP 800-63 Digital Identity Guidelines-FAQ, pages.nist.gov. [Online]. Available: https://pages.nist.gov/800-63-FAQ/. [Accessed: 26-Apr-2020].
[4] “How do you set an interface to permanent promiscuous mode in RHEL 7?,” Red Hat Customer Portal, 18-Mar-2019. [Online]. Available: https://access.redhat.com/solutions/3525641. [Accessed: 09-May-2020].
[5] H. Chen, Y. Xiao, L. Zeng, “Auditing overhead, auditing adaptation, and benchmark evaluation in Linux,” Security and Communication Networks 2015, pp. 3523-2534, June 2015.
[6] S. Ma, J. Zhai, Y. Kwon, K. H. Lee, X. Zhang, G. Ciocarlie, A. Gehani, V. Yegneswaran, D. Xu, and S. Jha, “Kernel-supported cost-effective audit logging for causality tracking,” in Proc. 2018 USENIX Annual Technical Conference (ATC), Boston, MA, Jul. 2018
[7] “Official PCI Security Standards Council Site - Verify PCI Compliance, Download Data Security and Credit Card Security Standards,” PCI Security Standards Council. [Online]. Available: https://www.pcisecuritystandards.org/document_library. [Accessed: 09-May-2020].
[8] P. Sutter, “Benchmarking nftables,” Red Hat Developer, 18-Oct-2018. [Online]. Available: https://developers.redhat.com/blog/2017/04/11/benchmarking-nftables/. [Accessed: 08-May-2020].
[9] “Is it safe to remove/change system user account on Red Hat Enterprise Linux ?,” Red Hat Customer Portal, 22-Oct-2019. [Online]. Available: https://access.redhat.com/solutions/31669. [Accessed: 09-May-2020].
[10] Users and Groups, Linux Standard Base Core Specification 5.0, 2015.
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