Nicolas Seriot

Security is a concern for every layer. It's not magic pixie dust that' can be sprinkled on top of software to renders it secure!

A while ago I read a great article about how the Adobe PDF serialization format is nearly impossible to secure because it allows inherently unsafe constructs.

For example, it allows cross-references that are basically just arbitrary unaligned pointers. It uses many different alignment and padding algorithms. It has length-prefixed and not-length prefixed sections. Etc, etc...

Apparently it was a serious research exercise to make a safe PDF parser, and they only covered a fraction of the full spec!

To put things in perspective: Originally, PDF allowed arbitrary code execution as a core feature, allowing the output of shell commands to be used as document content.

Most people like the Chromium and Firefox teams have just given up and now parse PDF using a sandboxed JavaScript VM because it's too hard to do it safely with C++. They parse HTML and JavaScript with C++, but not PDF. Think about that.

A similar issue caused Log4j, where a "format string parser" contained a vulnerability because it was too flexible and allowed network requests to be triggered by user-controlled data.

Even trivial, "surely it must be safe" formats like XML and JSON are riddled with security issues, such as different layers in a microservice architecture having different handling semantics for duplicate keys, null values, etc... This can result in exploits such as…

Again, I think this is based on some starting assumptions about what is an ideal state, and a lot of people would argue they are flawed assumptions. It's easy to get trapped into a context when considering what the ideal solution would look like.

People forget this, but ASCII (and therefore UTF-8) already has characters in it for field/record/etc. separation. One increases complexity significantly (and introduces inefficiency) by overloading other characters with their semantics.

There are much more simple & efficient ways to demarcate ordered arrays in particular (e.g. simple length prefixed structures).

Nulls are highly problematic for any number of reasons that are widely documented, and JSON unfortunately compounds this by having two ways of representing them (explicitly and by omission).

One might quibble with the notion that a format that didn't even exist in 2000 doesn't really have two decades worth of applications working on it, but setting that aside, the design constraints of JavaScript and the challenges that have emerged from it are far from ideal. The work arounds that have emerged (the whole Number fiasco, the "standardizing" on UTF-8 while using UTF-16 when escaping, line terminators, the backslash escapign of control characters, the inconsistencies between JSON & JSONP, the binary64 "good interoperability" rule, and comments being disallowed, and of course the various JSON Schema efforts).

While I agree that an ideal serialization standard would not…

JSON gets the most important thing right: it's simpler than XML. I probably should have rephrased my comment. I literally mean that it's a decent format, but not a great one.

One issue is that it's surprisingly poorly specified: http://seriot.ch/parsing_json.php.

As a data-interchange format, it lacks first class dates/timestamps, the issues about specification are a pain when dealing with numbers (depending on language, you may not be able to use the full range of 64 bit ints), and has no builtin concept of schemas. That said, I'm not sure what I'd recommend above JSON, though you clearly wouldn't use it for really high performance servers.

It's also used as a configuration language, where more issues surface. It doesn't accept trailing commas or comments (I can see the argument for requiring all keys to be quoted, but that's a pain to write as well). For configuration, I think I prefer TOML to JSON. Dhall looks cool, but I've never used it.

Unfortunately, the problem here is programmers moreso than formats. It literally doesn't matter what you specify, programmers will not implement it to a T. Most programmers simply don't know that every single detail matters. Many of those who may have some idea don't really care, since they can't imagine how something like this could happen.

It's not just XML. It's every ecosystem I've ever used. Push it around the edges and you will find things.

This is neat, not because it is special to JSON in particular but because it's an example of examining a good chunk of a large ecosystem: https://seriot.ch/projects/parsing_json.html Consider this is likely to be true in any ecosystem that doesn't make it a top priority to avoid.

It used an off-the-shelf library, I don't remember which one anymore. I don't think it really matters though -- YAML is a format, not a parser.

Per the YAML docs [0], "Creating a fully compliant parser has proven almost impossible." Pretty much every YAML parser out there is going to have its own idiosyncratic subset that it handles. That doesn't make the _files_ not-YAML.

(This does make YAML a horrible data exchange format. No argument there!)

Also, JSON has a similar condition [1].

[0]: https://github.com/yaml/yaml-grammar
[1]: http://seriot.ch/projects/parsing_json.html

Ignoring S-expr based configs for a moment, of the configuration languages, JSON parsing is probably the most straightforward. YAML is a superset of JSON, TOML is probably on par, TS and JS are obviously more complex than JSON, and XML is...XML. All of that is to say, I'd rather write a JSON parser than any of the above.

And JSON parsing is a minefield. http://seriot.ch/projects/parsing_json.html

I don't know of any notable S-expr based config languages, probably because they are so easy to parse, people just roll their own, leading to balkanization.