Solidity Optimizer and ABIEncoderV2 Bug Announcement
By way of the Ethereum bug bounty program, we acquired a report a few flaw inside the new experimental ABI encoder (known as ABIEncoderV2). Upon investigation, it was discovered that the part suffers from a couple of completely different variations of the identical kind. The primary a part of this announcement explains this bug intimately. The brand new ABI encoder remains to be marked as experimental, however we nonetheless assume that this deserves a distinguished announcement since it’s already used on mainnet.
Moreover, two low-impact bugs within the optimizer have been recognized over the previous two weeks, considered one of which was fastened with Solidity v0.5.6. Each had been launched with model 0.5.5. See the second a part of this announcement for particulars.
The 0.5.7 launch incorporates the fixes to all bugs defined on this weblog publish.
All of the bugs talked about right here must be simply seen in exams that contact the related code paths, at the least when run with all mixtures of zero and nonzero values.
Credit to Melonport crew (Travis Jacobs & Jenna Zenk) and the Melon Council (Nick Munoz-McDonald, Martin Lundfall, Matt di Ferrante & Adam Kolar), who reported this through the Ethereum bug bounty program!
Who must be involved
You probably have deployed contracts which use the experimental ABI encoder V2, then these could be affected. Which means that solely contracts which use the next directive inside the supply code may be affected:
pragma experimental ABIEncoderV2;
Moreover, there are a selection of necessities for the bug to set off. See technical particulars additional under for extra info.
So far as we will inform, there are about 2500 contracts stay on mainnet that use the experimental ABIEncoderV2. It’s not clear what number of of them comprise the bug.
Find out how to test if contract is susceptible
The bug solely manifests itself when the entire following situations are met:
- Storage knowledge involving arrays or structs is shipped on to an exterior operate name, to abi.encode or to occasion knowledge with out prior task to an area (reminiscence) variable AND
- there’s an array that incorporates components with measurement lower than 32 bytes or a struct that has components that share a storage slot or members of kind bytesNN shorter than 32 bytes.
Along with that, within the following conditions, your code is NOT affected:
- if all of your structs or arrays solely use uint256 or int256 sorts
- in case you solely use integer sorts (which may be shorter) and solely encode at most one array at a time
- in case you solely return such knowledge and don’t use it in abi.encode, exterior calls or occasion knowledge.
You probably have a contract that meets these situations, and need to confirm whether or not the contract is certainly susceptible, you’ll be able to attain out to us through safety@ethereum.org.
Find out how to forestall a majority of these flaws sooner or later
With the intention to be conservative about modifications, the experimental ABI encoder has been obtainable solely when explicitly enabled, to permit individuals to work together with it and check it with out placing an excessive amount of belief in it earlier than it’s thought of secure.
We do our greatest to make sure prime quality, and have not too long ago began engaged on ‘semantic’ fuzzing of sure elements on OSS-Fuzz (we have now beforehand crash-fuzzed the compiler, however that didn’t check compiler correctness).
For builders — bugs inside the Solidity compiler are troublesome to detect with instruments like vulnerability detectors, since instruments which function on supply code or AST-representations don’t detect flaws which can be launched solely into the compiled bytecode.
One of the best ways to guard in opposition to a majority of these flaws is to have a rigorous set of end-to-end exams in your contracts (verifying all code paths), since bugs in a compiler very probably should not “silent” and as a substitute manifest in invalid knowledge.
Doable penalties
Naturally, any bug can have wildly various penalties relying on this system management move, however we anticipate that that is extra prone to result in malfunction than exploitability.
The bug, when triggered, will beneath sure circumstances ship corrupt parameters on technique invocations to different contracts.
Timeline
2019-03-16:
- Report through bug bounty, about corruption brought on when studying from arrays of booleans instantly from storage into ABI encoder.
2019-03-16 to 2019-03-21:
- Investigation of root trigger, evaluation of affected contracts. An unexpectedly excessive rely of contracts compiled with the experimental encoder had been discovered deployed on mainnet, many with out verified source-code.
- Investigation of bug discovered extra methods to set off the bug, e.g. utilizing structs. Moreover, an array overflow bug was present in the identical routine.
- A handful of contracts discovered on Github had been checked, and none had been discovered to be affected.
- A bugfix to the ABI encoder was made.
2019-03-20:
- Choice to make info public.
- Reasoning: It could not be possible to detect all susceptible contracts and attain out to all authors in a well timed method, and it might be good to forestall additional proliferation of susceptible contracts on mainnet.
2019-03-26:
- New compiler launch, model 0.5.7.
- This publish launched.
Technical particulars
Background
The Contract ABI is a specification how knowledge may be exchanged with contracts from the skin (a Dapp) or when interacting between contracts. It helps quite a lot of kinds of knowledge, together with easy values like numbers, bytes and strings, in addition to extra advanced knowledge sorts, together with arrays and structs.
When a contract receives enter knowledge, it should decode that (that is completed by the “ABI decoder”) and previous to returning knowledge or sending knowledge to a different contract, it should encode it (that is completed by the “ABI encoder”). The Solidity compiler generates these two items of code for every outlined operate in a contract (and in addition for abi.encode and abi.decode). Within the Solidity compiler the subsystem producing the encoder and decoder known as the “ABI encoder”.
In mid-2017 the Solidity crew began to work on a contemporary implementation named “ABI encoder V2” with the aim of getting a extra versatile, secure, performant and auditable code generator. This experimental code generator, when explicitly enabled, has been provided to customers for the reason that finish of 2017 with the 0.4.19 launch.
The flaw
The experimental ABI encoder doesn’t deal with non-integer values shorter than 32 bytes correctly. This is applicable to bytesNN sorts, bool, enum and different sorts when they’re a part of an array or a struct and encoded instantly from storage. This implies these storage references have for use instantly inside abi.encode(…), as arguments in exterior operate calls or in occasion knowledge with out prior task to an area variable. Utilizing return doesn’t set off the bug. The kinds bytesNN and bool will lead to corrupted knowledge whereas enum may result in an invalid revert.
Moreover, arrays with components shorter than 32 bytes will not be dealt with accurately even when the bottom kind is an integer kind. Encoding such arrays in the best way described above can result in different knowledge within the encoding being overwritten if the variety of components encoded just isn’t a a number of of the variety of components that match a single slot. If nothing follows the array within the encoding (notice that dynamically-sized arrays are at all times encoded after statically-sized arrays with statically-sized content material), or if solely a single array is encoded, no different knowledge is overwritten.
Unrelated to the ABI encoder situation defined above, two bugs have been discovered within the optimiser. Each have been launched with 0.5.5 (launched on fifth of March). They’re unlikely to happen in code generated by the compiler, until inline meeting is used.
These two bugs have been recognized by the current addition of Solidity to OSS-Fuzz – a safety toolkit for locating discrepancies or points in quite a lot of initiatives. For Solidity we have now included a number of completely different fuzzers testing completely different points of the compiler.
- The optimizer turns opcode sequences like ((x << a) << b)), the place a and b are compile-time constants, into (x << (a + b)) whereas not dealing with overflow within the addition correctly.
- The optimizer incorrectly handles the byte opcode if the fixed 31 is used as second argument. This could occur when performing index entry on bytesNN sorts with a compile-time fixed worth (not index) of 31 or when utilizing the byte opcode in inline meeting.
This publish was collectively composed by @axic, @chriseth, @holiman
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