# Breaking Smart Contracts (Devcon5)

- Channel: [Devcon](https://streameth.org/devcon)
- Date: 2020-10-09
- Duration: 53:50
- Topics: blockchain, smart contracts, distributed ledger, ethereum, Science & Technology
- Watch: https://streameth.org/watch/yt-Zs-P5FSJvGQ
- YouTube: https://www.youtube.com/watch?v=Zs-P5FSJvGQ

## Description

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https://archive.devcon.org/archive/watch/5/breaking-smart-contracts

Some of the most financially devastating hacks in recent years have happened on the blockchain. In this workshop, we will walk through a series of simple Solidity coding challenges and common mistakes, where participants are asked read code, understand and try to break them. Regarding each challenge, we will talk about the history of the hacks involving that family of bug. We will talk about the attacks and possible solutions.

Speaker(s): Shayan Eskandari, Sergii Kravchenko, John Mardlin, Bernhard Mueller
Skill level: Intermediate
Track: Security
Keywords: technical

Follow us: https://twitter.com/efdevcon, https://twitter.com/ethereum
Learn more about devcon: https://www.devcon.org/
Learn more about ethereum: https://ethereum.org/ 

Devcon is the Ethereum conference for developers, researchers, thinkers, and makers. 
Devcon 5 was held in Osaka, Japan on Oct 8 - 11, 2019.
Devcon is organized and presented by the Ethereum Foundation, with the support of our sponsors. To find out more, please visit https://ethereum.foundation/

## Transcript

[Applause] everyone this is the workshop there are more people here but I'm trying Dean John we are but a constant diligence we do small contracts acuity and fidelity if you have any consultation or you want to work on downs you can go to a website with all the excitement no more and also which is totally everyone and see if your code has like their common mistakes or not so there is a game that we do this quick intro go through that high-level introduction to eat alone just to know you're in the same space and you go through some of the tools we have the premix some of the plans that you can start playing around with eating and more details on how eating works or the memory words didn't want you to make it easier for you to actually get through these challenges and also some of their capitalize or CPS and we have a few challenges at the end that we try to advocate you are walking to our to our workshop is a desk from proper setups so we're gonna just read everything one hour and probably do more question are saying traffic so according to your background nice job talk about diversity and so like where which with legasus from the area the group here likes everybody tritanus her contract at some point more or less do you feel like we understand like you understand what the state looks like and like blogs and codes and things like that okay although super fast is a big find great but it's good to review and somebody probably didn't want to admit that they don't know it so okay so but here what is it here iam your even is like it's all these accounts they have these four properties every single account has announced a balance code may be heavy it and a mazy storage so the state is made up of this great big list of accounts there are contract accounts and external accounts sternal accounts don't have code don't have storage there's op codes to things and the VM there you go yes this is useful to be this stuff in the context is like security that there are these these two types of transactions you can just send money or send eath from one externally own account POA to another or you can send me contracted with magic happens when you send you to another track send this transaction the contract can do like any other sort of internal transactions message calls sometimes all this creates a state-transition contracts can call each other and this is like it gets worse right like I've heard recently that we're had like there are you know the average call theft on the main net is like at like getting deeper all the time and contracts are more like increasing composability if someone we stop saying interoperability 2.0 hooks Interop away and now we say composability I just figured that out but and so so this composability is creating a lot more complexity things are having deeply in the call stack and that interesting implications and a lot about can everybody hear me at the back to do I need to projector more okay gasps I mean there's guests you all but a lot gasps limit that's really important actually gasses who here subscribe to my newsletter were aliens smart contract security newsletter right so I can't stop talking about like I want to do a newsletter that's not about gas but it's the most interesting thing to be real right now so the block cast limit that can get you in a lot of trouble if you run out of all the possible gas that and you might eleven give you and then if you're opcodes just change in price and you were counting on them not doing that sorry there's a table of opcodes with gas prices I made these slots there are a few things there that one was like for the gas price yeah so if you increase that's why seven but one way - with my nothing yeah and prioritize that over the other one so you can drop down and get ahead of the transaction we're just increasing the gas price with one way or the fact that that blocks are the limit is that because Bluffs are limited or scars so you can do some cryptic honors plays or like some attacks based on that it was it just over all things programming is if you want to talk front right go deep on that sock [Applause] and there's a lucky duck tomorrow at 4:00 but nothing rather overall funding taxonomy so so as I said this was supposed to be a more hands-on workshop but you're gonna be I'm going to be hands-on and he can also follow you by HSN so you can go to this link there's also on the white board if you go to this thing [Music] they're going to see this just here it's supportive challenges yeah a fun challenge be fun challenge one you can see a challenge here like this if these are not challenges I just want to make sure we are on the same level before that one big thing I do just to know exactly when you're gonna be on the same level but this is solidity or guests step on here I'm here beside the source already but it doesn't do anything special here it just have a radical game owner and just when you say high pitch and 70 puts the name in the name and its high think nothing for your specialty so there's a few things I want you to know what is like but this is like that things are public is that if you go to Rob's pen which is in here it's kind of Austin as you can see the transformation of blockchain and what is happening so let's say you want to store a value so give me ahead of yourself I'm going to the end here so you wanna start value and love change tournaments or secrets or anything you won't know that but you might not know how that works and what that means so a lot of times in smoke on charter the family starts at here so it's called slot 0 so what I can see that you can actually see the memory the memory stored so we can actually go here and it's a beauty of NSE the memory that that thing that constructor with you is so you can actually read the text that was passed to the contract through these day guys they have gibberish - but they stayed up here - so I'm working on this other tool called Regents for now my child is really better on send the link on Twitter like there aren't but this is something to be able to like easily poke around you know even small contracts so like in this case I just want to show you that what I mean by nice so let's say this is smart this is my contract I just copy the address and so everyone is too [Music] so the school has like nice interface and he tells interlock all the information that there is so in this case what we need is to connect to a node which is now I use Austin and I think you brought us to a user on node or connect to any known but you call this kind of maintenance trying to test it to just be able to write great things so if you see a bunch of options like so this tool you can check it out later is not important you're just going to show you how to restore it like in this case someone reads and when you do query you can read the balance details of a block both of this much on track its storage so in this case I want to be the soil or at least address that I just copied so imagine that you have a secret in that small car track so when I click it goes query the first 10 storage there so this is what's smart contracts of the PMC's but this tool also charged to like cast at the scene so it reads the first 10 storage and you can see that even though that store should not be visible from outside I mean they subtraction software development it is so we can read hello there front here there are other tools you can use destroy the set is not part of this works unnecessarily but for a plate if you wanna add function trees fortunately it's like perfect and as I said that I really like this for the canal I am nice so anything back to every mirror yet if you go to this link that that link kicked up taeho you're gonna have go through a bunch of challenges one is is challenge one that if you look instantly simple we has it balance that you look open the ballast he has a ten you can find and whether you pass on if you have to balance and there whatever that yeah the specified it's got a berry you can see any way to have infinite balance here [Music] yeah do you want your trip normals now do you want to then showcase Oh easier there is a so you make this is how a code editor finds a spot and go through the code fine what do that is there are some tools so it's a dimension that makes it easier to move Barney's comment box come on so yeah people here use remix anyone remix - grapes so one of the great things that you've been working on completely it's as a effects platform which is a security of the service tool a little bit like you throw some our contracts at it and then it goes back a bunch of bugs of you so like it's pretty great another awesome thing because it's built right into remix so if you do plugins things like the thanks goes up you activate one and as it turns out if you load it in challenge one Williams with the expanding at the analyze button the screenshot has already set up it's all anyway so yeah so you see you at the Ln button and it spits up the agent of the bottom you something super convenient super neat good way to cheat on workshops yes very cool yeah also if you create an account on the next i/o you'll have a point for assistance if you just do this again with remix is it doesn't but kind of like a sample address and it goes away in regression but if you dream here on the account you can suddenly account with your credentials and that's kind of also if you want to pay money it's a freemium service so link you don't have to pay money but if you do take you get much universe standard like we have large contracts and actually put a long time on like goes a lot degrees it's great do that there's also these command-line tools at your great saver of index JavaScript stuff that's CLI and you like Python stuff they both have like the truffle immigration that's pretty good other other goodness [Music] so this is manual for more at someone but a variety drops and Tesla and I think and you they go to the faucet Alaska IO or me or I can just send it to you right now just like what he was saying so here like let's say for this challenge mythix is here that when I install it to and logins kinetics is here you can just press analyze and actually looks for you and spines all the issues on the air it says I can overcome even though that if you keep picking up your ransom message as value so that if someone forces send that much money to overflow which is less real estate but this one is a mountains using inclusive and easy on them flow as you said and [Music] [Music] all right so we talked some peas but gas runs out for any reason to function call three bears and the whole state change doesn't happen anymore there are followed by functions that will be invoked if you just don't mention any function names or you mention a functioning that doesn't exist so that that has been there for some of the attacks that way the transaction does not exist for rather part of that function so it's documentation is the best place to look at we going to do some other ones too so this is Mary add myself so I won't talk about that warehouse storage is stored and I already talked about these regions which this is a link if you want to download and look at already show that you but there is also this blog post from one of our colleagues if you just the link was long so it's the best way to find it if you google and understanding you to your contract storage and click on this program the blocks are in tough problems that's this is a boot block those there are different patterns of people at the carries 40 different things and if you have any questions there just hit also had a just in the off their works text so the other challenge we just talked about the conscious tour all not to store secrets right there on this so in the traditional main the best way to store smoke so passports is a passionate right so you can pass a password in solitude is a check so you can just hash save the kitchen and use that so it seems like a good way right if anyone looks at the very well they still cannot guess what the password is but you depends on how you store that so everything on if there is data means in your public so in this case [Music] and this Casey to go unchallenged to slipping up to USC [Music] [Music] so this is a simple look for people that have done so this might be too simple but so this is just contract again it's the password safe the password and then you're on you can guess the password and gets the mining happens project so in this case let you might say that anyone sending it gets password transactions you can see the password everybody but even before app we can just at the same way as you do in training before go to Roxanne and I just see the password and that's as easy as it gets like so the constructal can go there and look this is a transaction we did for another conference yes it simple not Sigma then you can see they come to contact there so it's the easiest way even still do tools and we filled with some other ways all right so [Music] the only thing that ever gets confused about that is an inch of your randomness so we can have can get some applications that you might need up randomness for [Music] again or in order yet down there's a lot of use and promotion of blockchain has been on these kind of applications too we don't see this much these names like that so you can really get randomness because early on people start to use different aspects even timestamp for the randomness but all those people can guess for it so they were very like what I'm just trying to use some of the things are not known before like the blood patch or the catch the boat ashore that some other a variation of that so let's see if that works or not so we have this other challenge vanished B which basically this facility just want if they will be there so you have to pass a number and put somebody in there because graphic a lottery ticket and this part is using the hash of the last block and in cash that and attention number and if your guess is the same as the number it transfers you this much of it so it seems to me like you randomly wouldn't know but how would you guess that so how would you make this worth nothing I would write like in order to attack this contract you have to try to contract employee continent and use that contract not Agnes contract so you go to the same you run your code the same environment that this code is running at the same time so here you notice my practice doctor right yeah do we actually have to write the first mantra since this is looking at the previous block you could just look if that wins the lottery and then a sentence reduction with a high playing basically get understand that that's possible you check risking because there's 15 seconds long so if you missed applause you miss that 92 7 Action Comics you can try to get in but you've seen attacks that's people selected thousand translation just to make sure they kept in but that means for this one compostela matter if you look at the appointees of the current block you'd have to brute-force this person in the previous block there's yeah but that's the race [Music] yeah there's no comparison - yeah yeah so that's why it's so let me say this practice together here so I'm cheating I have two conferences over here just to stay home yes so well what do you what you do is [Music] [Music] so one thing is that you don't need to oppose having full function here and just about the enterprise but this is easier for now for us and as I said this was supposed to be something you saw but I just saw it here so what you do what you have to do is [Music] [Music] and [Music] [Music] the devil tiny moment [Music] so now in this case what I need to do is deploy this before this transaction there's this much ether so what I got to do is just complete this transaction so it's this contract so I know why it is what is that I have a function one trick is to changes to constructor so you're not deploy and call the function again so in this case you have to deploy these parts in this contract call attack with the input of the target which is the target sadness and your kind some value in there so what it does is talk in called targets lottery function which is this function with some value which is the message of value this value is how much money of such a capacitor and the number that this contract is calculating here as well so if the number is the same it says that one easy way to make it more ethically good hack is itself unstrap self-destruct so you get the money back and be clear that water will here be compiled and just put that piece over using metal mask here just making sure about the technical maintenance [Music] yeah I said all that we have sent their souls right it was my time fighting the family's excitement and this constructional input in the name that state employee the right one [Music] did it not scripted he would come at the phone before the service I can construct the keyword comment and for error [Music] three juggling balls whoever yet get typos oh yeah yep alright so the address was it seven FTP port sometimes actually so news price economist who deployed BAM but this time and don't you forgot to folder sorry so it's not have to serve a new message a sentence mr. send it back - oh no what oh yeah over who has said who's hanging with a common target the visitor center mr. centers as me away [Music] [Music] but bloated sense oh right the sense and any value yes all right let's try this again just pitiful bit the problem was so here is this contract accepting us to send some money this time now we didn't have anyone yet attach the strap back so that's why I fashionable tale so send another transaction now yes so I can see you transferred what weight is happiness and he said to us this is at 13:37 bay but it's not scientifically so one the current one is unchecked except discharge after that so in order to see it much better maybe this is a weather vane so I especially density yes it said one way got this much either setting back to the message just send there and self-destruct alright so one of my main point of this was to say that you can attack the contract we take out of the contract because you get to save environment I think it's using the same I'm saying environment variables everything that that what the contract has access to is there any questions or comments about test the debugging was fine all right so this is like similar example but likenesses so that we need to contact technology contract so it goes back to the cow story and everyone here knows about our know what you learned about them from that connection so the Dow's Asik and you know the relation between velocity and up it started from that and it would say so attack patterns they must lawyer because I think I'm Rachel bye but yeah I'm going too fast [Music] so yeah so then down as I came in he was like there's two lines of code - yeah Phoenix is the other way around so here you can see that the value is fine then [Music] so is this gas left can say how much gas is left before they have to do another map there to see person to another fault or not there is not a way to say if the balance of it but if this balance here if this balance is how much left here check - you have to have a exit condition there because you can be you want square root of two thousand itself and if you know sort of gas as I said we believe it so you use all the balances that you try to run it there so as you said at the end 180 million dollars or something that were stolen with these two lights to the underground that's what makes like the safety unspoken I read apartments this seems fine but it should be at the other day on that visit pattern that's a check of that experimental interaction so we can check check a new get picked on negative interactions or deals in the other so for this you don't try to do it mean to step back and steal money from this charity for education so the content you have here is pretty simple the other man think that is a really common pattern another Pro tems contracts and other contracts so we have this entire pound vacuum balances that holds the balances there's an event that doesn't matter constructors should request to have one either in there they ask that when he gets the message has entered Center balance so anyone can donate its payable if you can do it through their own balance the direct society they get the balance can take your balance and you can be drawn as you can see it's using the same pattern as the same time as the down so it checks the balance it sends that out and if you succeed this is a convenient work for that without it certified if just any set contact means an event and it uses the Ducks the balance so you want to attack this I want to make it more exactly but as possible but exactly forgot to show you the solution and so the rest we like to do this like that starts working together for the Tollison is and how would you what would we have stopped condition here if you want have activity to attack I will try to like what stops you smaller than my balance so you want to get more than half of the time [Music] nothing is left my call will fail because it runs out of balance before that happens we need your balance just you tell yours just letting your balance my addresses bigger than the total balance of that just means like a kiss blonde and the total balance is 100 or say he has like zero by actually I'm just gonna quote here so this is like basically equivalent of using cyborg clothes what are things just coffee cold and ice in fact making exam so here it exactly the attract here we are using the Constructors but it's easier here to do but then you can thanks so here you have to let donate first also have it balanced in that charity to be able to attack because if you know it's really calling power we draw in checks if you have a balanced effector or not so the first step is to do that fans say question we passed 20 letters which is the challenge we need to have them in the same place and then we donate the message that value that passed this and the deployment and then they there after that you have to call attack so I'm going to just deploy it and then one walking with us with you [Music] show you here that there's a balance here the baby's much balance there [Music] [Music] all right so I would say this China exactly so event with any value years it doesn't matter like right here folks again [Music] all right so it's just boys fine so they're after this mind what it does is like it sends whatever you send us to today the charity and as you can see it created this a contract and sent any way to our charity so it we even here you should see that we have the balance for that contract if you want to check there's also this easy way to change [Music] this easily to check a lot of things here you can use challenge for which is the charity charity that's not a dress you have to deployed it and you can just give access to access to the functionality that there is so now you can check the balance of the response contract which is you can check the balance and it is 10 as you said 10 way of airplane you see some things have a margin see it should there's a practice and then we can experiment with them so so here you look at the code it's the same apart as we talked about so if you call attack attack you spend involved we draw with the total balance of that address and it's going to send that ball back expand get that ball back I mean to each other and call the ball back and address this that this colleges is more than one either you would try again so I guess to send some points that are dispersed it's already done that wouldn't want it there [Music] there's a sadness to make sure our [Music] so let's try this because that everybody trying to take succeed in tech - so there's no input the very positive are shocked at the target you just call attack [Music] [Music] [Music] [Music] we want to say I did that I did send that was prophesied to go [Music] and it's like I guess you've got the old part that I was really learning about this now but and see that is if they if that like is successful it should learn something like this [Music] or that is the right to come back [Music] and I wanted a work yeah it's a work but this is the transaction we sent out internally to artists and we confuse myself again [Music] when I start there's something cool and it started me facing it we take both of the sense of depth and esoteric there's an opportunity check just Chancellors that's the target bass has been more money pair and the problem here is our money so it has to be so if you want to do the community service check and we did it gasps gasps laughs it's the first time in ten to twenty three hundred twenty orders for a transfer increase the gas left and returned at don't continue something like this with works also as well but what can excite pleased to see how many things will find us and this one is an original one so it's a bit distracted transit a contract that is wonderful so we can buy matrix here charge for analyzed are you these artists are you locked in I think I bet that you have that moment sighs yeah [Music] I learned this trick from rain nice not joining your endless explaining that I get but it's like to be what what questions people have insecurities with anxieties do people have no like writing smart contracts like for the poetry experts what did you really get out of this anybody and be like deploy to contract to maintain like whatever yeah how'd that feel I had my way I guess my denim indefinite so it was super fast took like three seconds or something and it cost me $200 right $200 is like minimize the time oh you guys guys that contract went super famous Mike better than that and this is this is your Chris from a maker to separate yes yeah this is the first one my first time I I had from well personally I'd never do play anything of consequence in maintenance my stuff but I've never liked what anything that like matters a lot so I like that yeah [Music] hop away - yes and calculation is the same way ever late had a contract act when you were like lost money in some you know the many ways that you can lose money in this world yes I may know what do you know you're ready [Music] [Applause] done all you can't write there's a point of diminishing return when like there's just no there were like you know I don't think your o'clock post--and like you can't do anything you can't even see your own typos so you get somebody else to do it that's us you're prepared you're like well prepared a blog post about Aaron Chris mark contract audit great way but yeah so like there's a lot of things you can do to not get a very good result from an audit by just like dropping a code with no documentation on us or you can like do do your work internally up front to be really ready for it come to us like well in advance maybe maybe we decide that the best thing to do is work with you for a few days just to like give you like hey like fix this fix this delete this and then like I'm in a better audience like you know be able to focus just on like going deeper and I think that's like that's like the being well-prepared front on it can make like an audit more affordable effective for most people and I think like most even smaller teams that like our maker can afford to like write documentation if they're willing to take the time to do it usually if you write good documentation or specs for your code link you sure you just hacked up the codes maybe you wrote tests first in which case you're a link between grades where the curve but then if you like sitting you're like try to communicate in prose like just plain English to someone else what your code does you have to read it that guarantee you'll find like something you like be like what week does it actually do that thing when this happens and you'll read it again you like oh no it actually does something I do not think it does that's as waiters I highly recommend it yeah invited and writing in like comments and documentation and one overall what this have to do is sickness that the manual I explained you can see the line that was cold kind of comments like is it what I'm saying if they should do that helps a lot like we don't need to be graphs and charts letters like that apparently was so good action salts so we have free progression and with entrances actually limited to the premium version it very sadly deputies these two things that are most relevant for anything so whenever you make a low-level call my cute concretely and the DDM gets all the gas is still left execute that I Connor its home and if you do that that you should never change the world state possibilities because their contracts and trusted that this change might never happen because they had a clunker can be ended so let's to see be warned about these new things and if you move this time all termination is fixed and the variance will be detected as time already [Music] [Music] - CTF style things sweeter and of the few inches and equality in every buying reading an audit reports you can find all our blog post audit reports control on our public of their villages see the bottle bounce or the WebEx and they're all there and economic - - is that so thank you for being here like stickers and [Applause]