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Jun 9, 2015 - 14 minute read - Guide

Debian GNU/Linux security checklist and hardening

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Debian GNU/Linux security checklist and hardening


  1. About this doc

  2. Security updates

  3. Vulnerability Assessment

    2.1 GCC mitigation

    2.2 0ld sch00l *nix file auditing

    2.3 GNU/Linux’s auditd

    2.4 T00ls

  4. Kernel security

    3.1 Apparmor

    3.2 SELinux

    3.3 Mempo kernel

    3.3.1 PaX\/Grsecurity
  5. SSL/TLS Checklist

    4.1 Ciphersuites in Apache2/Nginx

    4.2 OpenSSH

    4.2.1 OpenSSH in post-prism era
  6. Web security

    5.1 Web server( Apache/Nginx?)

    5.2 WAF( Web Application Firewall)

  7. Security standard

    6.1 STIGs for Debian

  8. Reference

##–[ 0. About this documentation

GNU/Linux already become one of most important fundamental element in *modern* IT platform. Almost every important applications heavily rely on the core component of GNU system: GCC, Glibc and linux kernel. GNU/Linux is totally free/libre and open source software( FLOSS). Many people thinks free/libre and open source software is secure because its open to many eyes. Yes, that’s true. According to Coverity’s report.

The source code quality of FLOSS project are better than closed software systems. But the FLOSS is not unbreakable. This documentation is going to discuss something we should know about GNU/Linux security operations. These examples in this doc has been tested only on Debian GNU/Linux 7.5.

##–[ 1. Security update

Follow the minimal installation principle: Debian is providing mini installation iso.

To check which packages need security updates:

#sudo apt-get upgrade -s | grep -i security

##–[ 2. Vulnerability Assessment

Know your GNU/Linux system as your *enemy* does. Your enmey might hide in the shadow and watch and learn the ways you’ve been using the system. As a defender, some philosophical ideas( thanks to Bruce Schneier) should be kept in mind

Security is NOT:

  • Security is NOT installing a firewall

  • Security is NOT a Product or Service

  • Security is Not a Product; It’s a Process

Security is:

  • Security is a Process, Methodology, Costs, Policies and People

  • Security is only as good as your “weakest link”

  • Security is 24x7x365 … constantly ongoing .. never ending

A security system is only as strong as its weakest link. Defense-in-depth seems the only option we have. You should be the best professional paranoia and also need a proper threat model. “Who’s gonna attack your system” would be daily bread for your mind;-)

Those two articles are good examples of assessing GNU/Linux distros: Assessing I, Assessing II

“checksec.sh” is very useful tool from trapkit:

Be very careful with those binaries, which has suid( or sgid?) bit:

###—-[ 2.1 GCC mitigation

setuid binaries are highly risks if the program had the bug that can be exploitable. The setuid binaries should be protected under GCC’s mitigation. We only examine 4 mitigation options here ( some GCC mitigation description from one Phrack paper, thanks pi3..dude, did I owe you a beer?)

*) NX

This feature can prevent shellcode execution on the stack. This mechanism can be implemented by hardware or software emulation.

In GCC’s options, NX is enable by default. If you want to turn it off, use “-z execstack”.

Check method:

*) Stack canaries (canaries of the death)

This is a compiler mechanism, in contrast to previously kernel-based described techniques. When a function is called, the code inserted by the compiler in its prologue stores a special value (the so-called cookie) on the stack before the metadata. This value is a kind of defender of sensitive data. During the epilogue the stack value is compared with the original one and if they are not the same then a memory corruption must have occurred. The program is then killed and this situation is reported in the system logs. Details about technical implementation and little arm race between protection and bypassing protection in this area will be explained further.

GCC options: -fno-stack-protector, do not add any canary onto any functions

-fstack-protector, only add the canary onto a few functions in compile time

-fstack-protector-all , add the canary onto all functions, be cautions about this one. It’d be triggered the heavily performance hit.

-fstack-protector-strong, add the canary onto those functions, which the stack buffers would be used. This is a smart one. But its only supported by GCC 4.9.x. Kees Cook shared a very good writing here.

Check the symbols in an elf file:


RELocation Read-Only.

Turn it on: -z norelro Turn it off: -z now

Check elf header to find partial-RELRO:

*) PIE

PIE enforces every process’s code segment is mmap()’d, it begins at a different base address at each execution of the application.

Note: mmap()’ is always used no matter what the type of the executable is (PIE vs. non-PIE). For non-PIE binaries the kernel uses an internal flag equivalent to MMAP_FIXED when mapping program headers.

GCC option: -pie, it only work for sec mitigation when kernel enables ASLR.

These exploit mitigations provided by GCC will definitely increase the cost of attackers. We all did believed so…until shit happened( as always?). Hector Marco released a method that can bypass NX/ASLR/PIE/CANARY mitigations locally/remotely easily. After these years of debating and bragging about how secure of GNU/Linux is/was and we finally ended up in*One mem infoleak can rule the fuc*ing GNU/Linux*!!! Damn, PaX/Grsecurity will be our last hope again, like a decade ago………..

###—-[ 2.2 0ld sch00l *nix file auditing

There are a bunch of files that could be exploited by attackers in the specific scene. Fortunately, FOSS( Free & Open Source) community is providing a lot of methods for the security audit work. They should be a defender’s daily bread, which being part of defense-in-depth model.

WildCards is a powerful feature in UNIX-like platform, but it can be exploited by attackers:

World-writable file audit:

World-readable file audit, correct permission: chmod 640 /var/log/:

Check if files were orphange:

List avaiable users, be cautions about who’s the “user”:

Check which files belong to whom. Then delete the user correctly:

List which users are unavailable:

List expired passwords:

The correct permission should be 644 at least. 600 would be better:

Files with suid or sgid flags:

Check if some stupid mistakes has been made( Thanks to Tim Brown):

Note: The main stream GNU/Linux distro( Debian, Gentoo, OpenSUSE, CentOS) won’t have big chance to do stupid things, but it’s worth to look at GNU/Linux platform with 3-rd party applications. Some commercial applications may do something stuipid like this one:


—-[ 2.3 GNU/Linux’s auditd

One particular scene is that some m41wares( or human attackers) might be interested in change some file’s metadata for some *interesting* reasons and then change it back to the original. Let’s show time:

Install auditd and make sure its on boot startup:

Config file:

Store log file:

Add one policy to /etc/audit/audit.rules:

Use this test program to change the permission:

#include <stdio.h> #include <sys/stat.h> #include <stdlib.h>

int main(int argc, char *argv[]) { struct stat sb;

if( stat(argv[1], &sb) == -1){

	if( chmod(argv[1], sb.st_mode) == -1)
			return ;


The date of “Modify” and “Change” should be different: shawn@shawn-fortress ~ $ stat change-test

Check *who* did it:

shawn@shawn-fortress ~ $ ausearch -i -k identify

###—-[ 2.4 T00ls


–[ 3. Kernel security

  • Anti-DoS related:

** SYN cookies is a syn flood attack protection, the default is enable( 1):

** TCP FIN-WAIT-2 status lifetime, it’d be an DoS attack risk if the value is too big. It’d be cause remote machine doesn’t have enough time to close the connection if the value is too small. Default is 60( seconds). 15 is better, you think? Further reading:

** SYN queue length, the bigger value can handle more connections, the default is 1024:

** Device queue length, this value should be bigger than syn queue? The default is 1000

** listen()’s backlog, the default is 128:

** TIME_WAIT status TCP connections, the system will empty the connection if the number is exceed the value,

** TIME-WAIT status can be reuse, the default is disable( 0):

** fast recycle of TIME-WAIT status connection, the default is disable( 0):

** TCP KEEPALIVE probe frequency,the default is 7,200 seconds:

** TCP KEEPALIVE probe packets, the default is 9:

** how many times of SYN and SYN+ACK can be re-transimit, the default is 5:

** the bigger value of TCP ORPHAN would prevent simple DoS attack, each ORPHAN cost 64KB memory, so 65535 is about 4GB:

** How many pages( 4KB each page in x86) can be used in TCP connection:

Be careful about this one, it’d be triggered OOM if the TCP connection consume all pages.

** The maximum send and receive window, you can set 64MB for a 10G NIC:

** Each TCP connection’s read buffer( X bytes):

If default paging 8kb * 2 = 16kb/connection, 4GB memory can be used for: (4 * 1024 * 1024) / 16 = 262144

Oracle DB server’s best practice

  • Networking

Ref: https://www.suse.com/documentation/sles11/singlehtml/book_hardening/book_hardening.html

** Source Routing is used to specify a path or route through the network from source to destination. This feature can be used by network people for diagnosing problems. However, if an intruder was able to send a source routed packet into the network, then he could intercept the replies and your server might not know that it’s not communicating with a trusted server.

** ICMP redirects are used by routers to tell the server that there is a better path to other networks than the one chosen by the server. However, an intruder could potentially use ICMP redirect packets to alter the hosts’s routing table by causing traffic to use a path you didn’t intend.

** Turn it off if this is not a router:

** IP spoofing protection, the default is disabled( 0):

** If you want to ignore all ICMP package, you can enable it. The default is disabled( 0):

** Ignore ICMP broadcast, the default is enabled( 1):

** To alert you about bad message, the default is disabled( 1):

** To turn on logging for Spoofed Packets, Source Routed Packets, and Redirect Packets, the default is disabled( 0):

Exploit mitigation

** Randomize the memory area. 0: disable ASLR. 1: Randomize the stack, VDSO page, shared memory regions. 2: (1) + also randomize the data segment.

** Not allow the user to read kernel address symbol tables:

** dmesg info can be read by root-only:

** the minimal memory map address, 65536 is good at least:

** Now allow the debugger trace the process by ptrace. The current Debian stable kernel doesn’t have this feature. 0: All process can be debugged. 1: Only one father process can be debugged. 2: Only root can do debug( with CAP_SYS_PTRACE) 3: no process can be debugged

###—-[ 3.1 Apparmor

Why Apparmor? It’s easy to deploy. More importantly, it’s easy to audit the polices. Everyone can write your own MAC/RBAC policy. Debian/OpenSuSE shipped with Apparmor by default.

Install Apparmor and MAC polices from community: #apt-get install -y apparmor-profiles apparmor

Check the status: #aa-status

###—-[ 3.2 SELinux

S0rry. I barely use SELinux for reasons. The 1st one is I don’t trust NSA, even the source code is GPL’ed. NSA is professional about implant backdoors, which would be very hard to audit. People has been discussing it for years:


NSA has inserted its code into Android

NSA linux/android kernel

Sebastian Krahmer found a exploitable bug from SELinux recently. It looks like a backdoor more than a “vulnerablity”, isn’t it?;-) Another important reason people don’t like SELinux because it’s hard to use and cause other application troubles from time to time. Stop disabling SELinux movement won’t work in the near future:

S0rry, Mr.Walsh. It’s nothing personal:-)

###—-[ 3.3 Mempo kernel

““⌘ Mempo project aims to provide most secure and yet comfortable out-of-the-box Desktop and Server computer, for professionals, business, journalists, and every-day users avoiding PRISM-like spying. ⌘”

Mempo is a FLOSS project for protect user’s digital freedom. Let the massive surveillance cry;-)

The Debian Mempo repo is not working for the internet user now and it’ll be back soon. So I’ll write how to use PaX/Grsecurity from offical Mempo repository on Debian.

###—-[ 3.3.1 PaX/Grsecurity

PaX/Grsecurity is the cutting-edge kernel protection in past 14 years. But they don’t have the credit what they supposed to have. Almost every main stream OS kernel security mechanism has influenced by PaX/Grsecurity in past decade. Lionel tells a little story about PaX/Grsecurity better than myself:

PaX/Grsecurity treat the kernel security as a whole. They’ve been inventing many innovations( SEGMEXEC, PAGEEXEC, MPROTECT, UDEREF, RANDSTRUCT*, etc), while hardening the kernel in source code level( make important struct read-only, etc). PaX/Grsecurity is one of most respected 0ld sch00l hacker community. The main contributor( Spender) was even been through a very badly economic situation back in 2004

Thanks to the G0d of techn0logy, PaX/Grsecurity is still alive…. I personally agree with some ideas:

“The “better than none” point of view is actually a nice way to false sense of security for those who don’t know better. We got better-than-none apparmor, selinux, tomoyo, some poorly maintained and crippled ports of grsec features or alikes, namespaces and containers, rootkit-friendly LSM, the dumb and useless kernel version of SSP, etc. What’s the sum of all this shit for end users? False sense of security…”

“without Grsecurity/PaX, linux security is like monkey can never perform a regular masturbation cu’z lacking of giant pennis;-)”

Too many better-than-none product or solutions, which only makes you feel safe, maybe for a while. Feel safe is not equal to secure. Fuc*ing cargo cult shitty security only makes things worse.

I’ll show you how to install PaX/Grsecurity manually( still waiting Mempo back online):

** Download kernel ( Pick one): https://www.kernel.org/pub/linux/kernel/v3.x/

** Download PaX/Grsecurity patch ( you can download the latest version from ):

** Decompress the kernel and patch the kernel with grsecurity:

** Do “make menuconfig” to customize your kernel, or you can use my test config

** Compile

** Install the new kernel

##–[ 4. Crypto

“Encryption works. Properly implemented strong crypto systems are one of the few things that you can rely on. Unfortunately, endpoint security is so terrifically weak that NSA can frequently find ways around it.” — Edward Snowden

Damn, we should treat the crypto engineering very carefully. Because it may be the last outter-heaven we have;-)

—-[ 4.1 SSL/TLS Checklist

Very good writing

SSL/TLS has been through BEAST/CRIME/LUCKY-13/HEARTBLEED/POODLE in past few years. and it’s already become one of hottest topic in cyber security. There are a set of vulnerable protocols and ciphersuites are worth to do audit. There are a few open source tools would make your audit work easier. Try this one:


If succeed, it’s risk to FREAK.

—-[ 4.2 OpenSSH

Config file:/etc/ssh/ssh_config

###——[ 4.2.1 OpenSSH in post-prism era

Well, plz read this

###—-[ 4.2 Ciphersuites in Apache2/Nginx

The explanation is here



–[ 5. Web security

You’re not reading this article for learning pentest, are you? Let’s just consider the fence of defender’s;-)

OWASP code review: https://www.owasp.org/images/2/2e/OWASP_Code_Review_Guide-V1_1.pdf https://www.owasp.org/index.php/OWASP_Code_review_V2_Project

OWASP testing guide: https://www.owasp.org/images/5/52/OWASP_Testing_Guide_v4.pdf https://www.owasp.org/index.php/OWASP_Guide_Project

—-[ 5.1 Web server( Apache/Nginx ?)






Blacklist/whitelist IP/networks

Blacklist/whitelist web contents

Options FollowSymLinks

Hide info

Limit risky HTTP methods






—-[ 5.2 WAF

ModSecurity is an open source, cross-platform web application firewall (WAF) module. Known as the “Swiss Army Knife” of WAFs, it enables web application defenders to gain visibility into HTTP(S) traffic and provides a power rules language and API to implement advanced protections. The web malicious signatures( including OWASP Top 10) are maintained by ModSecurity community. You can deploy it on Debian.

Anti-DoS: mod_evasive

About anti DoS solution, I personally don’t get used to mod_evasive. Iptables would be much easier to maintain, eg:

–[ 6. Security standard

Well, there are a bunch of crazy security standards in the planet. Some are compliance in some contries. FIPS-140-2/3, CC( EAL 7? damn, it’d be an incarnation of the organge book;-)), PCI-DSS are very popular terms you might hear from your security consultant. But… due to lack of engineering implementation, these crazy( & creepy?) security standards are not our concerns here.

—-[ 6.1 STIGs for Debian

Once there’s history, there’s story about offense & defense. Once we’ve heard fascinating stories from Mr.Sn0wden about how NSA fuck the world, there should be some open information about how BIG-BROTHER do the defense. STIGs is one of them.

I think I’m not the right person to write this section…..plz check this out.

–[ 7. Reference

[1] Back To The Future: Unix Wildcards Gone Wild http://www.defensecode.com/public/DefenseCode_Unix_WildCards_Gone_Wild.txt

[2] SYNPROXY http://lwn.net/Articles/563151/

[3] DDoS protection Using Netfilter/iptables http://people.netfilter.org/hawk/presentations/devconf2014/iptables-ddos-mitigation_JesperBrouer.pdf

[4] INTERNET PROTOCOL http://tools.ietf.org/html/rfc791

[5] A simple TCP spoofing attack http://www.citi.umich.edu/u/provos/papers/secnet-spoof.txt

[6] ICMP Attacks Illustrated http://www.sans.org/reading-room/whitepapers/threats/icmp-attacks-illustrated-477

[7] SUSE Linux Enterprise Server 11 SP3 - Security and Hardening https://www.suse.com/documentation/sles11/singlehtml/book_hardening/book_hardening.html

[8] Securing Debian Manual https://www.debian.org/doc/manuals/securing-debian-howto/

[9] A Brief Introduction to auditd http://security.blogoverflow.com/2013/01/a-brief-introduction-to-auditd/

[10] Apparmor RBAC http://wiki.apparmor.net/index.php/Pam_apparmor_example

[11] Hardening PHP from php.ini http://www.madirish.net/199

[12] CVE-2014-0196 exploit http://bugfuzz.com/stuff/cve-2014-0196-md.c

[13] Secure Secure Shell https://stribika.github.io/2015/01/04/secure-secure-shell.html

[14] STIGs http://iase.disa.mil/stigs/Pages/index.aspx http://iase.disa.mil/stigs/os/unix-linux/Pages/index.aspx