Saturday, December 18, 2021

My take on the Log4J 2.x vulnerability

A week ago, a lot of Java application developers learned that their applications harbored a severe vulnerability, courtesy of the Log4J 2.x logging framework. The vulnerability, CVE-2021-44228 allowed execution of arbitrary code on any system that logged unsanitized user input using the Log4J 2.x library. This prompted a lot of people to scramble to protect their systems, and a lot of people to take to Twitter with opinions of What It All Means.

As the maintainer of an open-source library that integrates with Log4J, I spent my Saturday morning understanding the bug and updating my examples to use versions of the library that mitigated the problem. Fortunately, my library is not directly affected, as long as its consumers don't use unsanitized data to configure the logging framework.

Having done that, and reading as much as I could on the issue (the LunaSec writeups are excellent), I've decided to voice my own thoughts on the matter. Some of which I haven't seen other people say, so they may be similarly interesting to you.

First, I think that it's important to recognize that this vulnerability — like most things that end up in the news — is not the result of a single issue. Instead, it's a combination of features, some of which make perfect sense:

  1. Log4J 2.x provides string interpolation for configuration (aka “lookups”)

    This is a great idea; I implemented it for my own logging library. The idea is that you don't want your configuration files to contain hardcoded values, such as the address of your central syslog daemon (or, in my case, the name of a CloudWatch log group). Log4J 2.x provides a long list of lookups, ranging from environment variables to information from the Docker container running your application.

  2. One of the lookups retrieves data from the Java Naming and Directory Interface (JNDI) API

    In large deployments, it's nice to be able to centralize your configuration. There are lots of tools to do this, but Java Enterprise Edition settled on JNDI, also known as the javax.naming package. JNDI is an umbrella API for retrieving data from different sources, such as LDAP.

  3. javax.naming.spi.ObjectFactory supports loading code from a remote server

    This is a somewhat dubious idea, but the JNDI SPI (service provider interface) spec justifies it: it lets you defined resources (such as printer drivers) that can be retrieved directly from JNDI. They use the example of a printer driver, and it should be a familiar example to anyone who has installed a printer on their home computer (do you know that the driver you installed is trustworthy?).

    Note: this condition is necessary for remote-code execution, but not for data exfiltration.

  4. The Log4J 2.x PatternLayout also supports string interpolation, with the same set of lookups

    Here we're getting into questionable features in the library. I had no idea that this behavior existed, even though I dug deeply into the documentation and source code when implementing my appenders. The documentation for this layout class is quite long, and prior to the vulnerability, you had to infer the behavior based on the presence of the nolookups pattern option.

  5. Users log unsanitized data

    If users passed all of their logging messages through a sanitizer that looked for and removed the sequence ${, then the vulnerability wouldn't exist. Except nobody does that, because why would you have to? I call it out because I think it's important to consider what you're logging, and whether it might contain information that you don't want to log. As I say in my “effective logging” talk, passwords can hide in the darndest places.

These are the things that need to happen to make this vulnerability affect you. If you can prevent one of them from happening, you're not vulnerable … to this specific issue. And there are ways to do this, although they aren't things that you can easily implement once the vulnerability is discovered. I'm writing a separate post on how Cloud deployments can mitigate similar vulnerabilities.

For now, however, I want to focus on two of the responses that I saw online: pay for open-source maintainers, and inspecting code before using it. In my opinion, neither of these are valid, and they distract from preparing for the next serious vulnerability.

Response #1: inspect code before you use it.

For any non-trivial application, this is simply impossible.

The current “trunk” revision of log4j-core has 89,778 lines of code, excluding test classes (measured using find and wc). That doesn't count any add-on libraries that you might use to write to your preferred destination, such as my log4j-aws-appenders (which has over 10,000 lines of mainline code). And logging is a tiny part of a modern application, which is typically built using a framework such as Spring, and runs on an application server such as Tomcat or Jetty.

And even if your company is willing to pay for the staff to read all of that source code, what is the chance that they will discover a vulnerability? What is the chance that they will even learn all of its behaviors? I've spent quite a bit of time with the Log4J 2.x code and documentation, both to write my library and on a post describing how to write Log4J plugins, and I never realized that it applied string interpolation to the raw log messages. After I knew about the vulnerability, of course, it was easy to find the code responsible.

Response #2: we — or at least large companies — should be paying the maintainers of open-source projects.

I don't know the situation of the core Log4J developers, and whether or not they get paid for their work on the project. But I don't believe that this vulnerability was the result of an overworked, unpaid, solo developer. True, there was no discussion on the commit that introduced JNDI lookups (ignoring the tacky post-hoc commenters). But the code that applies string substitution to logging events has been in the library since the 2.0 release, and has been rewritten multiple times since then.

In fact, I think direct corporate sponsorship would lead to more unexpected behavior, because the sponsoring corporations will all have their own desires, and an expectation that those desires will be met. And if my experience in the corporate world is any guide, a developer that feels their paycheck is in jeopardy is much more likely to do something without giving it full thought.

So where does that leave us?

Unfortunately, I haven't seen very many practical responses (although, as I said, I'm writing a separate post to this effect). And I think that the reason for that is that the real answer puts the onus for vulnerabilities on us, the consumers of open-source software.

Linus's Law, coined by Eric S Raymond, is that “given enough eyeballs, all bugs are shallow.” And this played out in full view with this vulnerability: the Log4J team quickly found and patched the problem, and has been releasing additional patches since.² There have also been multiple third-parties that have written detailed evaluations of the vulnerability.³

But still, we, the consumers of this package, needed to update our builds to use the latest version. And then deploy those updated builds. If you didn't already have a process that allows you to quickly build and deploy a hotfix, then your weekend was shot. Even if you did get a hotfix out, you needed to spend time evaluating your systems to ensure that they weren't compromised (and beware that the most likely compromise was exfiltration of secrets!).

It's natural, in such circumstances, to feel resentment, and to look for external change to make such problems go away. Or maybe even to think about reducing your reliance on open-source projects.

But in my opinion, this is the wrong outcome. Instead, look at this event as a wake-up call to make your systems more robust. Be prepared to do a hotfix at a moment's notice. Utilize tools such as web application firewalls, which can be quickly updated to block malicious traffic. And improve your forensic logging, so that you can identify the effects of vulnerabilities after they appear (just don't log unsanitized input!).

Because this is not the last vulnerability that you will see.

1: You might be interested to learn that the Apache Software Foundation receives approximately $3 million a year (annual reports here). However, to the best of my knowledge, they do not pay stipends to core maintainers. I do not know how the core Log4J 2.x maintainers earn their living.

2: Log4J 2.x change report.

3: I particularly like the CloudFlare posts, especially the one that describes how attackers changed their payloads to avoid simple firewall rules. This post informed my belief that the goal of most attacks was to exfiltrate secrets rather than take over systems.

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