domingo, 19 de abril de 2020

ASIS CTF Quals 2015 - Sawthis Writeup - Srand Remote Prediction


The remote service ask for a name, if you send more than 64 bytes, a memory leak happens.
The buffer next to the name's is the first random value used to init the srand()


If we get this value, and set our local srand([leaked] ^ [luckyNumber]) we will be able to predict the following randoms and win the game, but we have to see few details more ;)

The function used to read the input until the byte \n appears, but also up to 64 bytes, if we trigger this second condition there is not 0x00 and the print shows the random buffer :)

The nickname buffer:



The seed buffer:



So here it is clear, but let's see that the random values are computed with several gpu instructions which are decompiled incorrectly:







We tried to predict the random and aply the gpu divisions without luck :(



There was a missing detail in this predcitor, but there are always other creative ways to do the things.
We use the local software as a predictor, we inject the leaked seed on the local binary of the remote server and got a perfect syncronization, predicting the remote random values:




The process is a bit ugly becouse we combined automated process of leak exctraction and socket interactive mode, with the manual gdb macro.




The macro:



















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Quando eu te falei em amor

Quando os meus olhos te tocaram
Eu senti que encontrara
A outra, metade de mim
Tive medo de acordar
Como se vivesse um sonho
Que não pensei em realizar
E a força do desejo
Faz me chegar perto de ti

Quando eu te falei em amor
Tu sorriste para mim
E o mundo ficou bem melhor
Quando eu te falei em amor
Nos sentimos os dois
Que o amanha vem depois
E não no fim

Estas linhas que hoje escrevo
São do livro da memória
Do que eu sinto por ti
E tudo o que tu me das
É parte da história que eu ainda não vivi
E a força do desejo
Faz me chegar de ti

Quando eu te falei em amor
Tu sorriste para mim
E o mundo ficou bem melhor
Quando eu te falei em amor
Nos sentimos os dois
Que o amanha vem depois e não no fim

André Sardet

Collide

The dawn is breaking
A light shining through
You're barely waking
And I'm tangled up in you
Yeah

But I'm open, you're closed
Where I follow, you'll go
I worry I won't see your face
Light up again

Even the best fall down sometimes
Even the wrong words seem to rhyme
Out of the doubt that fills my mind
I somehow find, you and I collide

I'm quiet, you know
You make a first impression
I've found I'm scared to know
I'm always on your mind

Even the best fall down sometimes
Even the stars refuse to shine
Out of the back you fall in time
I somehow find, you and I collide

Don't stop here
I've lost my place
I'm close behind

Even the best fall down sometimes
Even the wrong words seem to rhyme
Out of the doubt that fills your mind

You finally find, you and I collide
You finally find You and I collide
You finally findYou and I collide

Howie Day


Everything

You're a falling star, You're the get away
car.
You're the line in the sand when I go too
far.
You're the swimming pool, on an August day.
And You're the perfect thing to see.

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Ah, When you smile at me you know exactly what you
do.
Baby don't pretend, that you don't know it's
true.
Cause you can see it when I look at you.

And in this crazy life, and through these crazy
times
It's you, it's you, You make me sing.
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everything.

You're a carousel, you're a wishing well,
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And I get to kiss you baby just because I
can.
Whatever comes our way, ah we'll see it
through,
And you know that's what our love can do.

And in this crazy life, and through these crazy
times
It's you, it's you, You make me sing
You're every line, you're every word, you're
everything.

So, La, La, La, La, La, La, La
So, La, La, La, La, La, La, La

And in this crazy life, and through these crazy
times
It's you, it's you, You make me sing.
You're every line, you're every word, you're
everything.
You're every song, and I sing along.
Cause you're my everything.
yeah, yeah

So, La, La, La, La, La, La, La
So, La, La, La, La, La, La, La

Michael Bublé