Preventing Log4j with Capabilities

Preventing Log4j with Capabilities

A capability-safe language would have minimized the impact of, or even
prevented, the log4j

If this is the first time you’re hearing about that vulnerability, you should go
read about it instead of this post! And also go patch it, if you have Java
software that uses or might transitively use log4j to log. It’s a doozy.

There are multiple issues surrouding this vulnerability that I’ll talk about in
this post:

  1. It’s doing string interpolation on user supplied strings.
  2. It’s accessing the network, without anyone realizing it might do that.
    (This is the part that capabilities would help with.)

The surface issue: String interpolation on user supplied strings

The first issue is that log4j was performing string replacement on user
supplied strings, not just on the template strings written by the developer.
For example, if the developer using log4j writes this:

logger.debug("user-name={}", userName);

Then log4j will substitute userName into {} as it should, but it will
also perform string replacements inside userName. So if someone picks the
user name {o}${o} because they think it looks like a pair of glasses, then
this line of code will attempt to expand ${o} by looking up what o expands
to in a developer provided configuration file.

But that’s non-sensical: a user name is a string, not a logging template.
The user that picked {o}${o} probably doesn’t even know how to program and
they were not attempting to write a log4j string template, they were drawing a
pair of glasses!

Contrast that to this code:

logger.debug("user-name=" + userName);

In this case, logger.debug() has no way of knowing that userName might
contain user data: it was simply just handed a single argument, and its first
argument is meant to be a template. Thus it is appropriate for it to try to
expand ${o} in the log message. Writing this code is a mistake by the
developer using log4j, whereas the behavior of the "user-name={}" code is
a bug in log4j.

If this bug wasn’t present then there would probably be a lot fewer vulnerable
applications in the wild, because many developers using log4j probably did use
the form "user-name={}" instead of the form "user-name=" + userName.

The deeper issue: Why is my logger using the network?

That was one issue. Another, deeper, issue is that log4j was fetching
arbitrary Java code off the network and executing it, when no one expected it

I have, in my head, a little person who was born and raised in a world where
capability-safe software is the default. And this person is yelling. He is

I hear this vulnerability affected pretty much everyone using log4j. But
why did everyone pass the network to the logger? Sure, maybe some people
wanted to use JNDI and LDAP or something, but most people didn’t, so why would
those people give the logger the network?

The answer, of course, is that no one “gave” the logger access to the network.
It just had access, because all code in Java has access to the network.
You can tell by these type signatures in the Java net library:

public class URL {
    // Make a new URL. Anyone can do it.
    public URL(String) ;

    // Turn a URL into a URLConnection. Anyone can do it.
    // (Despite the name, this doesn't actually open the connection,
    //  it just makes a URLConnection object.)
    public URLConnection openConnection();

public class URLConnection {
    // Actually open the connection. Anyone can do it.
    public abstract void connect();

(Links: new URL, openConnection, connect.)

By chaining these three methods together, arbitrary Java code can open a
connection to any URL it wants to. This may not look strange to you or me, but
it looks very problematic to the little capability-person in my head. He is

Wait, these three methods together let you create a network connection from
? That’s a violation of the integrity of your type system!

It’s like… say there is an authentication package, that all authentication
goes through, and you can try to authenticate a user, and if it passes you’ll
get an AuthenticatedUser, and then you can use the AuthenticatedUser to
perform more privileged actions.

For this to work well, it’s important that all authentication happens in the
authentication package, and that only it can create an AuthenticatedUser.
You can do this in Java, by making the constructors for AuthenticatedUser
non-public and ensuring that they are only called in the authentication
package, and only if the authentication succeeds. This can be a very useful
abstraction in a large codebase: it tells you that (certain kinds of)
authentication bugs can only happen inside the authentication package.

And this abstraction breaks if random code can conjure up an
AuthenticatedUser from nothing, and use it to perform privileged actions.

Likewise, you shouldn’t be able to conjure up a network connection from
. Any network connection must ultimately originate from the Network


Oh, you don’t have a Network object? So any random library code can just
access the network, on its own. And this is true not only of your
dependencies, but the dependencies of your dependencies. So the only way to
check if your application might transitively access the network would be to like…
search through the source code of all of your transitive dependencies? Wow.
Just wow. And you’re wondering why you have so many vulner—

Let’s cut off my imaginary capabilities-person there, he’s getting a little

What he’s making fun of us for not having is a different API that looks
like this (or something like it; there are a lot of ways to organize it):

public class URL {
    // Make a new URL. Anyone can do it.
    new URL(String);

// The ultimate source of all network access.
// This class is a capability. It grants access to all URLs.
class Network {
    // Turn a URL into a URLConnection.
    // You can only do this if you have a Network object.
    // Once it's done, the URLConnection grants the capability to open the
    // connection to that url.
    public URLConnection openConnection(URL);

// This class is a capability. It grants access to one particular URL.
class URLConnection {
    // Actually open the connection.
    public abstract void connect();

This begs the question: who can construct a Network object? If anyone can just
make one, then nothing substantial has changed. The log4j package (or really,
its JNDI dependency) would privately construct a Network and otherwise do the
same thing.

The trick is, there are no constructors for Network. Instead, there is
exactly one Network object ever in existence, and it is passed in to the
program at one location, perhaps as an argument to main. (Actually, if the
operating system was capability-safe and Java was cooperating with it, then
main would be given a Network if and only if the executable was given
network access.) And likewise for similar capabilities like a FileSystem

public static void main(
    String args[],
    Network network,
    Filesystem filesystem) {

The point is to use unforgeable Java objects to grant capabilities. Unforgable
means that arbitrary code can’t create one from nothing; this can be
accomplished in Java simply by it not having constructors. If you pass some code
a reference to one of these capability objects, directly or indirectly, you are
granting it access to the resource it represents. This is the essense of
capabilities: unforgeable objects that grant access to the resource they
. It’s very simple.

Let’s see how capability safety would influence log4j. First off, here’s what
log4j’s interface looks like currently:

import org.apache.log4j.Logger;

public class Incrementer {
    private static final Logger LOGGER
      = Logger.getLogger(Incrementer.class);

    public int increment(int number) {"Adding one");
        return number + 1;

Notice that Network isn’t passed to LOGGER. Thus log4j can’t access
the network! So when the log4j maintainers considered implementing the JNDI
feature that introduced the vulnerability, a few things could have happened

  1. They decide that it’s totally reasonable for a logging
    library to access the network all the time by default, and add a Network
    argument to the Logger.getLogger() method. Some users accept this and fall
    prey to the vulnerability, but more discerning users are concerned by this
    request for network access and switch to a simpler logging library that
    doesn’t require it, thus avoiding the vulnerability.
  2. They don’t think a logging library should be accessing the
    network at all, or feel that requiring a Network parameter would be
    frightening or inconvenient to users, and reject the feature.
  3. They think that the feature is worthwhile, but don’t want
    the breaking API change of modifying getLogger to require a Network. So
    instead they introduce a new method, perhaps called
    Logger.getLoggerWithNetwork(MyClass.class, network). Since most users don’t
    use this method and log4j can’t access the network without it, this
    prevents the great majority of vulnerabilities.

All three possibilities are better than what actually happened, which was that
log4j suddenly gained the ability to access the network, but its API did
not change to reflect this so users did not notice. Thus, a capability-safe
language would have saved, or at least mitigated, the day.

So future language designers, please consider making your language capability

I’m not actually sure what’s a good reference for more reading, besides Mark
Miller’s thesis if you really want to get into it. But here are some

December 26, 2021

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“Simplicity, patience, compassion.
These three are your greatest treasures.
Simple in actions and thoughts, you return to the source of being.
Patient with both friends and enemies,
you accord with the way things are.
Compassionate toward yourself,
you reconcile all beings in the world.”
― Lao Tzu, Tao Te Ching