Billion laughs attack

Is there a way to disable anchors and aliases or cap the number of characters that can be created through expansions? Right now PyYAML seems to be susceptible to billion laughs attacks. @guyskk created a new version in #37 that prevents that but it also uses OrderedDict and SafeLoader, so it might be a good idea to implement just this functionality like the ignore_aliases=True flag in #104 for yaml.load/yaml.safe_load.

[nitzmahone]

Off the top of my head, customizing a Composer to completely disable anchor support and propagation would probably be pretty straightforward even in the current code. Any other kind of memory limiting behavior would be better accomplished by the runtime environment (eg, OS memory quotas, cgroups), as the details can vary significantly between Python versions, implementations (eg CPython vs PyPy), and even build parameters. Attempting to track and limit memory usage within a library with so many variables in play doesn’t seem like a recipe for success.

If it is indeed as easy as I think it might be to disable anchor support at the Composer level, that might be a customization we could include in the box with some of the new work that’s going on… I’ve added this to the 6.0 project as something to consider and play with (but no promises).

EDIT: and TBC, what I’m thinking of here would not address the ask in #37- anchors and refs would still be syntactically significant to the parser, they’d just be errors when actually composing the final representation of the doc.

[pantoniou]

For what is worth, this attack is harmless (and perhaps a valid use case) if one were to do lazy anchor resolution. That is, one would never resolve references, merely follow them until the resulting end node.

Admittedly this is perhaps a hard thing to do in many languages.

Doing it that way, makes YAML suitable for representing any arbitrary hierarchy of objects, including ones with cyclic references.

For future versions of the spec I would argue that this is the default behavior.