Episode 156

Posted on Friday, Apr 8, 2022
This week we bring you the TL;DL (too-long, didn’t listen 😉) version of Camila’s recent 4-part Ubuntu hardening series, plus we look at security updates for Twisted, rsync, the Linux kernel, DOSBox, Tomcat and more.

Show Notes


This week we bring you the TL;DL (too-long, didn’t listen 😉) version of Camila’s recent 4-part Ubuntu hardening series, plus we look at security updates for Twisted, rsync, the Linux kernel, DOSBox, Tomcat and more.

This week in Ubuntu Security Updates

48 unique CVEs addressed

[USN-5354-1] Twisted vulnerabilities [00:47]

  • 2 CVEs addressed in Bionic (18.04 LTS), Focal (20.04 LTS), Impish (21.10)
  • No limit to the amount of memory consumed when parsing the SSH version string from a client/server - malicious client/server could then send an infinite length version and crash the corresponding server/client - limited to 4KB
  • Cookies and auth headers exposed in cross-origin redirects - leak sensitive info

[USN-5355-1, USN-5355-2] zlib vulnerability [01:34]

  • 1 CVEs addressed in Trusty ESM (14.04 ESM), Xenial ESM (16.04 ESM), Bionic (18.04 LTS), Focal (20.04 LTS), Impish (21.10)
  • Announced by Tavis Ormandy on oss-security in last week of March - reproducible crash when compressing certain inputs
  • Reported upstream only to be told was already fixed upstream back in 2018 but no CVE was ever assigned and since no release had been made since then no distros had picked it up
  • Shows the importance of CVEs to distros security patching workflows - in general no CVE, no patch
  • New upstream release has now been made as well

[USN-5359-1] rsync vulnerability [03:09]

  • 1 CVEs addressed in Bionic (18.04 LTS), Focal (20.04 LTS)
  • Same as above - rsync contains a vendored copy of zlib code

[USN-5357-1, USN-5357-2] Linux kernel vulnerability [03:24]

  • 1 CVEs addressed in Xenial ESM (16.04 ESM), Bionic (18.04 LTS)
  • 4.15 18.04 LTS, 16.04 ESM
  • Episode 155 - heap buffer overflow in handling IPsec ESP transformations -> local user crash / codeexec
  • UAF in network traffic control subsystem - requires an attacker to be root OR to be able to use unprivileged user namespaces - which is the default for Ubuntu (but it is often suggested to disable this in most hardening guides)

[USN-5358-1, USN-5358-2] Linux kernel vulnerabilities [04:45]

  • 2 CVEs addressed in Bionic (18.04 LTS), Focal (20.04 LTS), Impish (21.10)
  • 5.13 (21.10), 5.4 (20.04 LTS, 18.04 LTS HWE)

[USN-5356-1] DOSBox vulnerabilities [05:01]

  • 2 CVEs addressed in Bionic (18.04 LTS)
  • Heap buffer overflow when parsing BAT files with very long lines
  • Incorrectly would allow access to files under /proc (e.g. /proc/self/mem) - could then allow an application to get code execution - added checks on file access to prevent this

[USN-5360-1] Tomcat vulnerabilities [05:47]

[USN-5361-1] Linux kernel vulnerabilities [06:46]

[USN-5362-1] Linux kernel (Intel IOTG) vulnerabilities [07:03]

[USN-5364-1] Waitress vulnerability [07:17]

  • 1 CVEs addressed in Focal (20.04 LTS), Impish (21.10)
  • Pure python impl of Webserver Gateway Interface (WSGI) standard - interface spec on how server an application communicate
  • Request smuggling vuln due to differences in how waitress and a possible frontend proxy interpret HTTP requests

[USN-5365-1] H2 vulnerabilities [07:47]

  • 2 CVEs addressed in Focal (20.04 LTS), Impish (21.10)
  • Database engine
  • Deserialisation of untrusted data -> code execution
  • Crafted connection URLS -> code execution

Goings on in Ubuntu Security Community

TL;DL of recent series on Ubuntu hardening [08:11]

  • Various listener requests for a summary on the recent series of episodes discussing Ubuntu hardening


Hello listener! This is going to be a quick episode and so I will make a quick introduction for it. During our last four episodes we talked about how to harden our Ubuntu systems, making it more robust…and dare I say it? More secure! However, four episodes is quite a lot, and not everyone is willing to listen to several minutes of my awesome voice, so I am here today to fix that and give you an episode that is a summary, a “Too Long, Didn’t Listen” if you will, of those previous four episodes. So let’s get going because today is no day for delays, and let’s talk, shortly and succinctly, about things you can do to harden your Ubuntu system.

Let’s start with hardening measures you can apply to your system whilst installing it:

(1) Encrypt your disk. Input and output operations might take a little while longer to happen, but if your hardware can take it, that might not even be an issue. Do remember though that everytime the device is shutdown, when you turn it on, you will have to decrypt the disk before using the operating system, which in turn means: inputting a password to get things going. So maybe only do this if you have a system where this won’t be a hindrance. Oh, and don’t lose your password, or else, you’ll end up with a disk full of pretty, but uninterpretable, characters and no functional operating system at all!

(2) Create a swap partition or a swap file to get the most out of your RAM. Availability is also a cyber security concern you should have, and providing your system with some swap space not only buffs it by giving it more RAM memory to work with - even it if is only a wannabe RAM - but it also allows you, as a system administrator, to be better prepared to face memory issues that might come to haunt your system, since you can monitor your swap space usage and use this as reference to know if your system might be feeling a little bit overloaded. Avoiding unnecessary crashes just got a whole lot easier. Choo OOM manager! A side note, though: check your system requirements so that you setup swap in a way that fits your system’s needs, or else, instead of making your device work better, you will only make it work harder.

(3) Partition your system! Put /var and /home in different disk partitions and avoid all that log file backup or those kitty videos from flooding your disk space and forcing your critical processes to stop execution because there is no space left in the system. Ooops. Maybe we should also take some time to update the log backup script and to remind users that the server is no place to store videos…even if they are adorable. And while you are at it, maybe also add /tmp to its own partition. World writable /tmp is a well known attacker target and grounding it and sending it to the corner to think about its bad behavior might be a good way to avoid possible attacks. Especially considering that different partitions can be set to have different permissions.

(4) Strong passwords. This shouldn’t even be in this list because you already use strong passwords when setting up your users during install, right? What? I’m not nervous because I definitely need to change my password from ‘security2022’ to something a lot better, you are!

With an installed system, our hardening journey is far from over, as we now need to set everything up securely before getting our service and its related applications running. How to proceed?

(1) Ubuntu does not enable a password for the ‘root’ user for a reason, and so, recommendation number one is: just leave ‘root’ and its password alone. Leave it there hibernating with all of its amazing and yet destructive power over the system. No ‘root’ user password, no successful brute force attacks, not even through SSH. An attacker in a regular user shell is a lot less scary than an attacker in a ‘root’ shell. Use ‘sudo’ instead, and configure ‘sudo’ permissions for your users appropriately in the /etc/sudoers file. YOU get to CHOOSE what commands each user can run as the superuser, so take your time to set these up. Give each user the least they need to perform their tasks, and stay safe. I know…it’s amazing…you get to CONTROL what your users are allowed to do in your system. “What? Has this always existed?” Yes, my friend, yes it has, so it’s about time you start configuring it properly.

(2) Use SSH instead of Telnet for remote login, because you are NOT a caveman that requires your data to be transmitted over the network in plain text. Yes, cavemen knew not to use Telnet and they also knew that even when using SSH they had to properly configure it before using it, or else, not even encryption would save them. If you doubt me, go do your research…this is 100% historically accurate, my fingers are definitely not crossed behind my back as I say this. Disable root access through SSH, use SSH2 instead of SSH1, setup allowlists and denylists for users and IP addresses, and set a maximum number of login attempts were all of the basic things cavemen in our planet did when setting up their SSH servers whilst sitting around their very cozy and newly discovered fires. Plus, they also setup private key login for their SSH servers, not because they were too lazy to type in their passwords - …nooooo, they had passphrases for their keys - but instead because it is a well-known and trusted way to verify the identity of whoever is trying to connect to the server. Passwords by themselves sometimes just aren’t enough. So…if cavemen were able to discover fire AND properly set up their SSH servers…then it is more than your obligation to at least do the same, if not more. Oh, and don’t forget to properly set permissions in the ‘authorized_keys’ file…I mean…come on guys…properly setting permissions in a very important file in your OS is a lot easier than hunting, foraging, surviving in the menacing prehistoric Earth environments, and that’s why cavemen did it as well.

(3) Can we really call it hardening of the system if we don’t consider hardening of the one and only, the star of the show, the kernel itself? The ‘sysctl’ command in your Ubuntu system is there to attend to all of your kernel hardening needs, allowing you to define kernel configurations, but not requiring you to reboot the machine to get them to stick. With ‘sysctl’ you can do so many things that I wouldn’t be able to summarize it all here, and I am already in a pinch because I am very bad at making my scripts short, and I need to keep this one short. So, for now, I will give you a little taste of what ‘sysctl’ can do to get those curiosity juices flowing! Restrict users allowed to read kernel logs and block IP packet forwarding for devices that are not routers. Was I able to make you interested? Well, I know I wont get the answer to that, but what I do know is that both those measures I mentioned can already take you a long way when you think of hardening your system, and they are two amongst many available…sooo, get those fingers typing and those kernel options researched and you, my friend, are in the right path to get your system hardened!

(4) Setup a host based firewall. They are efficient in blocking unwanted network traffic, they can be configured to your host’s specific needs and they are portable, since, when the host migrates, its firewall goes with it. Plus, it’s very easy to set up, you can use the Ubuntu tool known as the Uncomplicated Firewall (‘ufw’) to help you, and it gets you started on protecting yourself against the harsh, harsh Internet ecosystem that lies out there. Oh, and don’t even try to argue with me and tell me about your network based firewall and how it already does the job for you, because I just discussed it in the long version of this series, so to make it short, I will say one simple word to get my point across once again: layers!

(5) Remember when we were talking about partitioning your disk/filesystem? Well, let’s kick that up a notch and configure each partition individually, setting permissions and defining usage configurations for each one of the different partitions in our disks. We are all unique in this huge world we live in, and so are our partitions. Treat them with the love, care and individuality that they deserve and they shall return all of your efforts in the form of a more secure system. If you have a network shared partition, for example, why not set the ’noexec’ option for this partition, and avoid executables to be run from an area in your device that could be considered untrustworthy at best and devastatingly dangerous at worst? Don’t trust users, I always say, specifically when they come for your files through the network. Another good option would be to set a partition as read-only, if it is a partition that requires no more permissions than this. The /etc/fstab file is the one you can go to in order to set all of these configurations, which will be applied at mount time, exactly during system boot.

(6) Don’t ignore your logs. Setup a nice logging system for your device. Use syslog or journal to do so, and yeah…sure…thank me later, I won’t complain if you do. But seriously though, how can you expect to maintain and troubleshoot a device if you don’t know what is happening with that device? And how do you expect to keep a system secure if you can’t maintain and troubleshoot it? Yes, logs can be annoying to look at and analyze sometimes, but that is why utilities such as ‘syslogd’ and ‘journald’ exist to help you search through those logs. Syslog even allows you to send all of your data to a centralized server, which can then focus exclusively on processing log data that it gets from various network devices. You have all of that goldmine of data at your feet and all you need to do now is use it. Ubuntu has the tools that allow you to do that, but it doesn’t have the will…that, my friend, needs to come from you. So to show how important it is to set up and use logs, I will end this suggestion with a quote, because everything that includes quotes is usually considered important, right? “Knowing yourself is the beginning of all wisdom” - Aristotle. There, now go get some logging setup.

Ok. Next step is installing your applications so that you can get your service up and running. I am not even going to go into detail about using secure software, setting up this software including security configurations, and using encryption when sending data through the network, because that is obvious enough, right? If not…then I am sorry to tell you, you might need to listen to the long version of this. I will go into detail though, not much, but a little bit (if you want ‘much’, go listen to parts 1, 2, 3 and 4), on what you can do after you set up your service, and on what you can do until forever to keep your hardened system from going soft on you. So let’s jump in.

(1) One or two network services per device!!! Don’t make your server a jack of all trades, because that is a recipe for a hack of all spaces. If you are going to use the network to expose your service, maybe incorporate it as a part of the service’s architecture as well. Have more than one device running server software which makes up a part of the entire provided service, and have those devices communicate with each other through the network. Different server applications in different devices will isolate each relevant component and avoid a complete meltdown of the service in general, in case something is compromised. Divide and conquer. It’s like we don’t say this enough.

(2) Close unused ports in your system and disable unnecessary services and daemons. By not doing so you are only increasing the attack surface for your system, meaning, you are giving more possibilities for an attacker on how to attack you. Less is more, and the bare minimum should be enough. Be sure new installs and new updates don’t open up ports you don’t want to be opened and don’t bring in new files, scripts or executables that might compromise you. Keep a continuous eye on everything that is running in the background. Just because you can’t see it, it does not mean it can’t be hacked.

(3) Check your file permissions and change them if necessary! Defaults were made to be overwritten and you don’t need 777 files lying around in your system anymore, do you? Know your resources and set permissions accordingly. Correctly setting up users and groups is implied here, especially considering that users and groups will define who can and can’t access a file in the system. Plus, disable the setuid and the setgid bit for executables that don’t need it. When researching for privilege escalation techniques in Linux, “Find setuid binaries in the system” is the first technique to show up, so that should be enough of a warning to you that an executable should only be allowed to run as another user in case it is extremely…and let me say that again with emphasis: EXTREMELY necessary, or else, it might just be another day, another root shell for some random attacker.

(4) Install some third party software to help you keep your system safe. “We are all in this together”, a quote from a song in a teen musical I am totally not ashamed to admit I watched too much when I was a bit younger, used to say, and that applies for the cyber security community. Software that can help you better the security in your devices is plenty out there, and here, today, I will mention a few of them that you can check out and possibly use in order to harden your Ubuntu OS even more. Obviously, since this is a summary, we are doing this the fast way, so let’s get listing: Fail2ban, Snort and Suricata for intrusion detection and prevention; the Google PAM package, which allows implementation of 2-factor-authentication for your Ubuntu users; ClamAV, for malware detection; the Mozilla TLS configuration generator, to help you securely generate configuration files for well known applications; and finally, AppArmor, or possibly SELinux, for Mandatory Access Control that will complement the Discretionary Access Control you already set up with your file permissions earlier.

To finish this all off, don’t forget to keep your packages up-to-date, to use shred instead of remove to get rid of files containing sensitive data in your system, and to continuously go back and reconsider all of the previously mentioned points, so that your system can securely keep up with changes that are being made around it. The world won’t stop spinning and technology won’t stop evolving, so your server cannot afford to not be maintained and updated on a regular basis, or else, all of your initial hardening will be for naught.

That is all for today friends, and I hope you enjoyed it. It was a quick one, but it was an episode made with love. Feel free to share your thoughts on any of our social media platforms and for now I bid you farewell and until next time! Bye!

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