Exploiting the Jackson RCE: CVE-2017-7525

Earlier this year, a vulnerability was discovered in the Jackson data-binding library, a library for Java that allows developers to easily serialize Java objects to JSON and vice versa, that allowed an attacker to exploit deserialization to achieve Remote Code Execution on the server. This vulnerability didn’t seem to get much attention, and even less documentation. Given that this is an easily exploited Remote Code Execution vulnerability with little documentation, I’m sharing my notes on it.

What To Look For

There are a couple of ways to use Jackson, the simplest, and likely most common, is to perform a binding to a single object, pulling the values from the JSON and setting the properties on the associated Java object. This is simple, straightforward, and likely not exploitable. Here’s a sample of what that type of document looks like:

What we are interested in, is a bit different – in some cases1 you are create arbitrary objects, and you will see their class name in the JSON document. If you see this, it should raise an immediate red flag. Here’s a sample of what these look like:

To determine if this really is Jackson that you are seeing, one technique is (if detailed error messages are available) to provide invalid input and look for references to either of these:

  • com.fasterxml.jackson.databind
  • org.codehaus.jackson.map

Building An Exploit

The ability to create arbitrary objects though, does come with some limitations: the most important of which is that Jackson requires a default constructor (no arguments), so some things that seem like obvious choices (i.e. java.lang.ProcessBuilder) aren’t an option. There are some suggestions on techniques in the paper from Moritz Bechler, though the technique pushed in the paper is interesting (the focus is on loading remote objects from another server), it didn’t meet my needs. There are other, simple options available.

Helpfully, the project gave us a starting point to build an effective exploit in one of their unit tests:

This code leverages a well-known ‘gadget’ to create an object that will accept a compile Java object (via transletBytecodes) and execute it as soon as outputProperties is accessed. This creates a very simple, straightforward technique to exploit this vulnerability.

We can supply a payload to this to prove that we have execution, and we are done.

Building The Payload

In this case, the goal is to prove that we have execution, and the route I went is to have the server issue a GET request to Burp Collaborator. This can be done easily with the following sample code:

This code can be compiled with the javac compiler, and then the resulting .class file should be Base64 encoded, and provided to the transletBytecodes field in the JSON document. As soon as the document is processed, it will create the object, load the code, and execute it. You may still see errors from code failing after the code executes, such as from type-mismatches or the like.

Limiting Attack Surface

This is just one technique to exploit this flaw, there are many others available. To mitigate the issue, at least in part, Jackson has been modified with a blacklist of types known to be useful gadgets for this type of attack:

  • org.apache.commons.collections.functors.InvokerTransformer
  • org.apache.commons.collections.functors.InstantiateTransformer
  • org.apache.commons.collections4.functors.InvokerTransformer
  • org.apache.commons.collections4.functors.InstantiateTransformer
  • org.codehaus.groovy.runtime.ConvertedClosure
  • org.codehaus.groovy.runtime.MethodClosure
  • org.springframework.beans.factory.ObjectFactory
  • com.sun.org.apache.xalan.internal.xsltc.trax.TemplatesImpl
  • org.apache.xalan.xsltc.trax.TemplatesImpl
  • com.sun.rowset.JdbcRowSetImpl
  • java.util.logging.FileHandler
  • java.rmi.server.UnicastRemoteObject
  • org.springframework.aop.support.AbstractBeanFactoryPointcutAdvisor
  • org.springframework.beans.factory.config.PropertyPathFactoryBean
  • com.mchange.v2.c3p0.JndiRefForwardingDataSource
  • com.mchange.v2.c3p0.WrapperConnectionPoolDataSource

There are likely others that can be used in similar ways to gain code execution that haven’t become well-known yet, so this doesn’t eliminate the problem, it just makes it less likely.

Required Reading & References

To fully understand this vulnerability, there are a few things that you should read:


  1. To exploit this issue, the user of the library must have enabled Default Typing (mapper.enableDefaultTyping), if this hasn’t been done, then the exploit here doesn’t work, as you aren’t able to create arbitrary objects. 

Breaking the NemucodAES Ransomware

The Nemucod ransomware has been around, in various incarnations, for some time. Recently a new variant started spreading via email claiming to be from UPS. This new version changed how files are encrypted, clearly in an attempt to fix its prior issue of being able to decrypt files without paying the ransom, and as this is a new version, no decryptor was available1. My friends at Savage Security contacted me to help save the data of one of their clients; I immediately began studying the cryptography related portions of the software, while the Savage Security team was busy looking at other portions.

The Code

The code that really matters is in a PHP file2, named after the Bitcoin address that the victim is to pay the ransom to, and stored under the user’s %TEMP% directory. Here’s the bit that matters to us:

There are some important things that we see immediately:

  • They generate a unique encryption key for each file.
  • They are using AES-128 in ECB mode.
  • They are using RSA to encrypt the AES-128 key and store it in a .db file (also named after the Bitcoin address).
  • They encrypt the first 2,048 bytes of the file, and then replace it with random data.
  • The .db file contains the path, encrypted AES-128 key, and the encrypted data removed from the file.

The Critical Mistake(s)

If you’ve been to any of my talks on cryptography, you should see an immediate issue with this code. If not, let me point this line out:

This line creates a 128 byte key to be used to encrypt the file (it seems the developers don’t know bits from bytes), using PHP’s mt_rand function. This function generates random numbers using the Mersenne Twister algorithm, which happens to use a small (32-bit) seed – this is where the fun begins.

Because of this small seed, if we can observe the initial output of mt_rand, we can brute-force the seed and then predict its future output. Thankfully, the developers of Nemucod made this easy for us. If you recall, the first 2,048 bytes of each file are replaced with random data from mt_rand, then the encryption key is generated immediately after. This means that they have given us everything we need.

Using the first few bytes (4 or 5), we can brute-force the seed that mt_rand used3, and by running mt_rand the appropriate number of times, we can create the exact output that the PHP script did when it encrypted the files, revealing the file encryption keys and allowing us to decrypt all of the files.

Cracking the Seed

To get the seed, we need to brute-force all 2^32 possible values, thankfully there’s a handy tool to do this – and do it within a matter of seconds. A few years ago the always impressive Solar Designer released just what we need. This is a simple command-line tool that takes output (in this case the first few bytes of the first file encrypted) and provides the seed that was used.

Using php_mt_seed, it takes only about a minute to test all of the possible seeds, and identify the correct one. Once we have that, decryption is simple, and we have all of the data back without paying a single cent to the extortionists.

Why Randomness Matters

When it comes to key generation (and many other aspects of cryptography), the use of a secure random number generator is critical. If you look at the documentation for mt_rand, you’ll see this very clear warning:

This function does not generate cryptographically secure values, and should not be used for cryptographic purposes. If you need a cryptographically secure value, consider using random_int(), random_bytes(), or openssl_random_pseudo_bytes() instead.

Had the developers heeded this warning, and used a more appropriate method for generating the file encryption keys, this method would not have worked. Had the developers not been so kind as to provide us with output from mt_rand in the files, this would not have worked. It is the developers of Nemucod that made recovering the data trivial, due to the lack of understanding of proper secure techniques4. While I don’t want to aid ransomware authors, this is a well known aspect of cryptography – if you write crypto code without a full understanding of what you are doing, and what you are using, this is what happens.


  1. In the hours before this post was published, Emsisoft released a decryption tool for those hit by this version of Nemucod. 
  2. This ransomware targets Windows users, though the core is written in PHP. The downloader, written in JavaScript, downloads the Windows version of PHP (version 5.6.3) and uses that to execute the PHP file. Yes, this is as crazy as it sounds. 
  3. If mt_rand is not seeded explicitly via mt_srand, PHP will select a random seed, and seed it automatically. In this case, the developers did not explicitly select a seed. 
  4. There may be additional methods here that could be used, but those are beyond the scope of this article. 

30 Days of Brave

Brave is a web browser available for multiple platforms that aims to provide additional security and privacy features – plus a novel monetization scheme for publishers. I gave it 30 days to see if it was worth using. I switched on all platforms I use to give it a fair shot, I normally use Chrome which made the switch less painful, though the results were very much mixed. There are some things I honestly liked about it, some things I really disliked, and at least one thing that just made me mad.

The Good

There are some truly good things about Brave, here are a few that are important to me.

Based on Blink (Chromium)

Brave is built on the Blink engine, the same engine that powers Chromium & Chrome – this gives Brave some of the better security properties of Chrome, and Brave actually uses the Chrome user-agent to pretend to be Chrome. This means that Brave has similar performance and rendering quality to the other Blink browsers, which gives it an edge over Firefox and keeps it on par with Chrome.

The use of Blink is a key to making the switch reasonable, there are no issues with sites breaking, as is so common when switching from one browser to another.

HTTPS Everywhere

Brave integrates HTTPS Everywhere to force connections to use TLS when possible, this is great, though the same can be achieved by using the HTTPS Everywhere plugin. During my time using Brave, it reports having performed over 15,000 TLS upgrades – just on my personal laptop.

Ad Blocking & Payments

Brave takes an interesting view of ads, it includes ad blocking, but also includes Brave Payments (disabled by default), which allows you to give something to the sites you visit most often. I put $5 into it, and let it run for a month – it tracks how much time you spend on each site, and splits up the money between them.

Of all the sites that Brave lists in my top sites, only two are setup to actually receive the payments – this site (which I setup during the testing process), and Archive.org. There are a number of sites that are included that really make little sense – for example, sites like RSA.com, PizzaHut.com, TeeSpring.com, Namecheap.com, Oracle.com, and Eventbrite.com all made the list to be paid. These aren’t content sites, but they all got a share of the money. You can selectively disable certain sites from being included, but that requires watching the list, and making sure that it’s maintained. You don’t have the opportunity to confirm who gets paid before the payment takes place, so make sure you check the list often.

When a payment is made, the money (Bitcoin actually), is transferred to accounts that Brave Software controls, and when (if) a site receives $100 in payments, one of two things happens:

  • If the site has already been setup for payments, the money is transferred to the site’s Bitcoin wallet.
  • If the site isn’t setup, they will attempt to contact them to set up the site so they can receive their money, if they don’t after a period of time, the money is distributed to other sites that are properly setup.

It’s an interesting setup, and somewhat cool to be honest – though does leave a decent amount of money in the control of Brave Software. Will this site ever get $100 in revenue from Brave users? I’m not holding my breath. That means that the money will stay in the control of Brave Software essentially forever.

The ad blocking itself works well, roughly the same you would get from uBlock Origin.

Performance

It’s hard to quantify just how much time is actually saved by using Brave; it’s not just general performance, but the integrated ad blocking that saves bandwidth and processing time. It claims to have saved me 18 minutes on my laptop and 5 minutes on my iPhone.

It does feel a bit faster, but the placebo effect may explain it.

The Bad

There are some things about Brave that just didn’t live up to my expectations, some of these are from a lack of polish – things that will likely be fixed as time goes on, others were more fundamental.

Private Browsing

Like pretty much every major browser today, Brave offers a private browsing feature, but it’s implemented in a way that I find troubling. Typically when you using Private Browsing, a new window is created, and everything in that window is held to a private scope. In Brave, a tab is private – so you mix the scopes, and can easily cross that boundary. When you right-click a link in a private tab, you can open the link in a new private tab, or in a new normal tab. This makes it extremely easy to cross the line, and expose activity that was meant to be isolated.

For me, I often use this feature to separate session scopes, logging into the same site in a normal window, and a different account (or no account) in a private window. This design makes it trivial to take an action under the wrong account. I think they were trying to make things easier, but what they did was make it easy to make mistakes.

Memory Leaks

Brave is leaky, like Titanic kind of leaky. I once left a Twitter tab open over a weekend, when I came back on Monday Brave had consumed every available byte of RAM. So much so, that even killing the process turned out to be impossible and I had to perform a hard reboot. Chrome is known for its high RAM usage, though Brave has pushed it too far.

PDF Handling

Built-in PDF handling is essentially a must these days, and Brave tries here – but ultimately fails. The integrated PDF viewer works well in most cases, unless the PDF is behind a login. In these cases, it fails and requires that the feature be disabled to be able to download them. As changing this setting requires restarting the application, so I eventually just left it off.

Rough Edges

Brave is a perfect setup for a death by a thousand cuts, from oddness with tab management, painful auto-complete in the search / address bar, the inability to search for anything with a period as Brave treats it as a URL, and many others. Much of this will improve as Brave matures, though for now the rough edges are a constant annoyance that make me want to switch back to Chrome. Some I’ve learned to work around, others are still painful every time I run into them.

The Mad

Brave recently published a highly misleading article that painted a very negative view of the standard QUIC protocol, trying to accuse Google of using QUIC as a way to circumvent ad blocking. The article was built on, at best, a significant misunderstand on the part of Brave. The article was later updated, though the update was entirely insufficient to set the record straight, leaving users with a misunderstanding of the technology, and how it applies to Chrome and other browsers.

Whether purely from a lack of understanding, or something else, the issue was poorly handled – they attacked a competitor (one which makes their product possible) without understanding the details they were talking about, mislead users of Chrome and Brave, and failed to accurately update their article to undo their misstatements. There are some people at Brave Software that I greatly respect, so this was shocking for me, and I lost a great deal of respect for the organization as a result.

Overall

Brave is an interesting experiment in how a browser can address privacy concerns, and provide an avenue for monetization; I hope that others in the market look at it and learn from what they do right. The application for iOS feels a lot more polished than the desktop version, and while I’m going to switch back to Chrome as my primary browser, I may keep the iOS version handy.

Confide, Screenshots, and Imaginary Threats

Recently Vice published a story about a lawsuit against the makers of the ‘secure’ messaging application Confide. This isn’t just a lawsuit, it’s a class-action lawsuit and brought by Edelson PC – an amazingly successful (and sometimes hated1) law firm – this isn’t a simple case. The complaint includes a very important point:

Specifically, Confide fails to deliver on two of the three requirements that it espouses as necessary for confidential communications: ephemerality and screenshot protection. […] Confide represents, in no uncertain terms, that its App blocks screenshots. But that isn’t true. Any Confide user accessing the platform through the Windows App can take screenshots of any and all received messages.

This article isn’t about the lawsuit though, it’s about threat modeling and screenshots.

Of Screenshots and Cameras

Preventing screenshots, or at least attempting to, has been around for some time, and was made popular thanks to Snapchat – their client would detect that a screenshot was captured, and using an API call, notify the server that this had happened (unsurprisingly, this API was rarely documented, as none of the third-party clients wanted it), so the sender could be alerted. When this feature was added, technical attacks were discussed by many that were following Snapchat’s attempts at living up to their word – from modifying the binary to not make the API call, to using a proxy server that would block the call to the server.

Yet for all of the technical solutions, there was an easier answer: grab another device with a camera and take a picture.

Many people that work in corporate or other environments that have strict security requirements often carry two mobile devices – if you know (or suspect) that a messaging application will block or report a screenshot, just take a picture of it with your other device. This completely bypasses the technical restriction on screenshots2. It’s simple, it works, and it’s undetectable. Then there are virtual machines – you could run an application in a virtual machine, and capture the screenshot from the host, instead of the guest operating system. Once again, this is effective and undetectable. Then there are numerous others.

Trust Violated

If you can’t trust the person you are talking to, don’t talk to them. If send a message to somebody that contains sensitive statements, and you can’t trust them to keep it private – the only way to ensure that it’s not shared is to simply not send it. There is no technical solution to ensure that a message displayed on a screen is actually ephemeral. This is why high security environments where sensitive (i.e. classified) information may be on display don’t allow cameras or devices with cameras at all.

If a user wants to capture information about a conversation, and there are numerous ways that they are able to do just that, they will get it. If they take a little care, nobody will know that they’ve done it. If they have technical ability (of know somebody that does), then it’ll be effortless and undetectable. Confide may, and should, make changes to address the technical issues with their screenshot protection feature to behave in a more effective way; that said, they will never be able to actually prevent all screenshots.

Screenshot protection is a vain effort; if it’s displayed on screen, it can be captured. People may think that they need it because it’s how they would capture a conversation, but it doesn’t actually provide any effective security. Features like this, and claims that applications implement them, are little more than snake-oil aimed at making consumers believe that an application provides a level of security that isn’t actually possible.


  1. Jay Edelson & team may be hated by some in Silicon Valley – but they have done a lot to protect consumers, and suing over security claims is an important avenue to ensure that companies live up to their promises. My research on Snapchat was cited by the FTC in their action against Snapchat, which I am still very proud of. Needless to say, I favor whatever action is necessary to ensure that companies live up to their promises, and consumers aren’t being sold snake-oil. 
  2. There are theoretical means to complicate this, by using a controlled “flicker” to make it more difficult to capture a useable photo. This comes with various downsides, not the least of which is the high likelihood of giving users a constant headache. 

Shadow Brokers, Equation Group, Oh My…

NSA / Equation Group

Yet again, a group known as The Shadow Brokers is in the news, with yet another leak from what is widely accepted as the NSA (Equation Group1 in APT terms). This release is, to many, the most important release of this leaked stolen material from the most elite and secretive hacking operation in the world.

This is a collection of a few notes on this highly unusual operation. If you haven’t read this excellent overview of the most recent release by Dan Goodin, you should do that now.

A Brief Timeline

The Shadow Brokers have released material on five occasions2:

August 13, 2016: Equation Group Cyber Weapons Auction – Invitation

A group of files were uploaded to Github (and quickly removed), including a file (“eqgrp-auction-file.tar.xz.gpg”) containing exploits and implants for Linux and similar operating systems, and a file (“eqgrp-free-file.tar.xz.gpg” with a password of theequationgroup) containing exploits and implants for a variety of networking equipment; the latter used as a sample.

October 20, 2016: Message#5 — Trick or Treat?

A second, smaller sample file named “trickortreat.tar.xz.gpg” with a password of payus.

January 13, 2017: MESSAGE FINALE

In what was purported at the time to be their final message, Shadow Brokers released a new file, “equation_drug.tar.xz.gpg” that included files that had been identified by Kaspersky AntiVirus as being Trojan.Win32.EquationDrug.

April 8, 2017: Don’t Forget Your Base

In a only somewhat surprising move, they posted a rambling treatise on US politics and Donald Trump, which included the password (CrDj"(;Va.*NdlnzB9M?@K2)#>deB7mN) for “eqgrp-auction-file.tar.xz.gpg” – the file they tried and failed to sell when they started. This contain the Linux exploits and implants that had been promised, though much of the content was rather old and of little interest.

April 14, 2017: Lost in Translation

Perhaps the most exciting release, as it included the Windows exploits and the Equation Group’s equivalent of Metasploit called FuzzBunch – but the story doesn’t end there, there was a big surprise included. The three files are “windows.tar.xz.gpg“, “odd.tar.xz.gpg“, and “swift.tar.xz.gpg” – with the last containing an unexpected surprise.

Burning Gold

As the community works to analyze the latest dump, going through the exploits trying to identify what they are, and if they are known some very interesting things have been found. ETERNALBLUE appears to be an 0day against Windows XP, Vista, 2003, 2008, 2008R2, and Windows 7 (it’s being reported that Windows 10 is vulnerable as well3) – one can imagine that there is panic in Redmond as Microsoft works to analyze it and prepare a patch. This is just one exploit, there’s also ETERNALCHAMPION, EASYBEE, EASYPI, ECLIPSEDWING, EDUCATEDSCHOLAR, EMERALDTHREAD, EMPHASISMINE, ENGLISHMANSDENTIST, ERRATICGOPHER, ESKIMOROLL, ESTEEMAUDIT, ETERNALROMANCE, ETERNALSYNERGY, EWOKFRENZY, EXPLODINGCAN, and ZIPPYBEER – there’s a lot of work to be done to fully understand the impact and figure out what needs to be fixed. A number of versions of Windows targeted by these exploits are not supported, such as Windows XP, Windows 2003, and Vista – meaning that there’s no patch coming, these systems will remain vulnerable forever.

What’s clear here is, there is some real value in the exploits that have been released (estimated at $2M or more) – and are likely very important to NSA intelligence operations (or at least they would have been till they learned that they had been compromised).

As NSA doesn’t talk about anything if it can be avoided, it’s unlikely that we will ever know what the impact is to their operations. We will likely see just how effective these exploits are though, as criminals work to leverage these exploits in exploit-kits and the like – you can be sure, just because these exploits are known, this certainly isn’t the last we’ll see of them.

The SWIFT Files

While it was known that they had various Windows exploits, they dropped something that is of less interest from a technical perspective, though fascinating from an operational perspective: there’s a collection of operator’s notes relating to attacks on SWIFT. These files include detailed system configurations, passwords, and step by step notes of what was done on the devices that were attacked.

Matt Suiche has done a great job of documenting what’s exposed by this cache, so I won’t repeat that here. If you want a better understanding of how they work, and what they’ve done, I highly suggest reading it.

The Origin

There’s been a great deal of debate about the source of these files, some have suggested that it was an insider, possibly even Harold Thomas Martin, though an insider makes little sense. The data is most likely from a jump-server – a server that NSA operators would push their files to, and connect to targets from. This would explain the type of files and documentation found – and would explain the files that aren’t included, such as source code, training material, and similar files that an insider would have access to, but wouldn’t be stored on a server outside of their network.

One likely scenarios is that a jump-server was captured by another intelligence agency, and the leaks and bizarre rants were part of a political play – though their choices of how to release information has greatly reduced their effectiveness. The SWIFT files could have been positioned as a WikiLeaks-styled bombshell, though was dropped quietly without the fanfare to make news outside of the normal technology publications – while the exploits have a substantial impact, they are of little interest to most people outside of the industry, the SWIFT work on the other hand, could be of much larger significance.

The level of detail about targets, and details from operator logs should allow the NSA to narrow the possible sources; I’m hoping that at some point there’s an official statement about who they believe is releasing these files – though the odds of that happening don’t seem good.


  1. The name Equation Group was coined by Kaspersky, while they didn’t directly state that Equation Group is the NSA (or possibly, just NSA’s TAO group), a close look at the evidence is quite clear. The material released fits with both the signatures of the Equation Group and with a US Government operation. There is some small chance that the attribution is wrong, but the odds of that appear to be quite low. 
  2. From the start of these releases, I’ve maintained an archive of the files, so that they are searchable and browsable. This has made it easy to review the files, and quickly find related files. It’s interesting to say the least. 
  3. Update 2017-04-15: Microsoft has released an update on their review of these exploits; ETERNALBLUE, ETERNALROMANCE, and ETERNALSYNERGY were all addressed in MS17-010 released in March 2017 (the source of the report was not listed). They confirmed the issue impacted Windows 10, and pretty much every other supported version of Windows.