Hacker News Reader

Comment by dannyw | Hacker News Reader

The actual exploit (simplified for brevity):

    if (validSignature == signatureFromGuardian)
        approve()

If you send an invalid signature from a non-guardian, the condition also resolves to true.

View on HN

Maybe I'm having a dumb day here but I'm not following... is the code block there the intended behavior or the exploit? In the case you mention, both conditions are false...

Apologies, == instead of &&.

false == false

Can you link to the code. This seems bizarre to me that someone would write it this way.

Microbugs always look like that. Weird stuff pops out of our brains when transcribing ideas about rules into logic. The overwhelming majority of such thinkos are clearly wrong and don't work, or often even build. And sometimes you get bugs that take a while to find because the case where it would fail is an untested edge case.

And then, once in a while, one of those edge cases...

The longer it takes to find the cause of a bug, the more likely the bug is a thinko.

Wait.. are you saying that the conditional was actually:

    if (isSignatureValid == isSignatureFromGuardian)

As variable names, “validSignature” and “signatureFromGuardian” sound like they’re storing the literal signature values, but from your description these are actually holding boolean results _describing_ the signatures, and not the signatures themselves?

Geez, if you name variables that it’d be amazingly easy to make a mistakes like what you describe, and then just read straight over them when you’re reviewing the code.

> If you send an invalid signature from a non-guardian, the condition also resolves to true.

Huh? That resolves to false && false, which equals false.

I think dannyw meant something like this:

  # This function is defined by the system
  def validSignature(sig):
    return false

  # This function is defined by the system
  def isSignaturefromGuardian(sig):
    if sig.validator == "guardian":
      return true
    return false

  # A bug in the system compared both return values
  if validSignature(tx.sig) == signatureFromGuardian(tx.sig):
    approve()

```

I looked at your post, then at these Twitter threads recommended in the other comments https://news.ycombinator.com/item?id=30187898

I think "something about not checking signatures right" is about as deep as I'm going to understand this.

There is a parameter passed in as part of a function call that basically says “here’s where you can find the function to validate the signature”. The attacker was able to point that to a function they wrote instead of using the real one, so the function returned saying “yep, that’s a valid signature” when it wasn’t.

I understand the first part, "There is a parameter passed in as part of a function call that basically says “here’s where you can find the function to validate the signature", but I'm having trouble understanding the second part, "The attacker was able to point that to a function they wrote instead of using the real one". In other words I (relatively) understand the 'what', but I don't understand how was the attacker able to do the redirect?

Something something system addresses.

Reply on news.ycombinator.com