Solidity Optimizer and ABIEncoderV2 Bug Announcement
By the Ethereum bug bounty program, we obtained a report a couple of flaw throughout 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 however suppose that this deserves a outstanding announcement since it’s already used on mainnet.
Moreover, two low-impact bugs within the optimizer have been recognized over the previous two weeks, one in every of which was fastened with Solidity v0.5.6. Each have been launched with model 0.5.5. See the second a part of this announcement for particulars.
The 0.5.7 release comprises the fixes to all bugs defined on this weblog submit.
All of the bugs talked about right here must be simply seen in checks that contact the related code paths, not less than when run with all mixtures of zero and nonzero values.
Credit to Melonport group (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
When you have deployed contracts which use the experimental ABI encoder V2, then these is perhaps affected. Which means solely contracts which use the next directive throughout the supply code could be affected:
pragma experimental ABIEncoderV2;
Moreover, there are a selection of necessities for the bug to set off. See technical particulars additional beneath 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.
Tips on how to examine if contract is weak
The bug solely manifests itself when the entire following situations are met:
- Storage information involving arrays or structs is distributed on to an exterior perform name, to abi.encode or to occasion information with out prior task to a neighborhood (reminiscence) variable AND
- there’s an array that comprises 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 varieties
- in case you solely use integer varieties (which may be shorter) and solely encode at most one array at a time
- in case you solely return such information and don’t use it in abi.encode, exterior calls or occasion information.
When you have a contract that meets these situations, and need to confirm whether or not the contract is certainly weak, you’ll be able to attain out to us through security@ethereum.org.
Tips on how to stop some of these flaws sooner or later
To be able to be conservative about modifications, the experimental ABI encoder has been accessible 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-about secure.
We do our greatest to make sure prime quality, and have just lately began engaged on ‘semantic’ fuzzing of sure elements on OSS-Fuzz (we now have beforehand crash-fuzzed the compiler, however that didn’t check compiler correctness).
For builders — bugs throughout the Solidity compiler are tough to detect with instruments like vulnerability detectors, since instruments which function on supply code or AST-representations don’t detect flaws which might be launched solely into the compiled bytecode.
The easiest way to guard towards some of these flaws is to have a rigorous set of end-to-end checks in your contracts (verifying all code paths), since bugs in a compiler very seemingly will not be “silent” and as a substitute manifest in invalid information.
Attainable penalties
Naturally, any bug can have wildly various penalties relying on this system management circulation, however we count on that that is extra prone to result in malfunction than exploitability.
The bug, when triggered, will underneath sure circumstances ship corrupt parameters on methodology invocations to different contracts.
Timeline
2019-03-16:
- Report through bug bounty, about corruption induced when studying from arrays of booleans immediately from storage into ABI encoder.
2019-03-16 to 2019-03-21:
- Investigation of root trigger, evaluation of affected contracts. An unexpectedly excessive depend of contracts compiled with the experimental encoder have 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 have been checked, and none have been discovered to be affected.
- A bugfix to the ABI encoder was made.
2019-03-20:
- Resolution to make info public.
- Reasoning: It will not be possible to detect all weak contracts and attain out to all authors in a well timed method, and it will be good to stop additional proliferation of weak contracts on mainnet.
2019-03-26:
- New compiler launch, model 0.5.7.
- This submit launched.
Technical particulars
Background
The Contract ABI is a specification how information could be exchanged with contracts from the surface (a Dapp) or when interacting between contracts. It helps a wide range of sorts of information, together with easy values like numbers, bytes and strings, in addition to extra complicated information varieties, together with arrays and structs.
When a contract receives enter information, it should decode that (that is carried out by the “ABI decoder”) and previous to returning information or sending information to a different contract, it should encode it (that is carried out by the “ABI encoder”). The Solidity compiler generates these two items of code for every outlined perform in a contract (and in addition for abi.encode and abi.decode). Within the Solidity compiler the subsystem producing the encoder and decoder is known as the “ABI encoder”.
In mid-2017 the Solidity group began to work on a recent implementation named “ABI encoder V2” with the aim of getting a extra versatile, protected, 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 varieties, bool, enum and different varieties when they’re a part of an array or a struct and encoded immediately from storage. This implies these storage references have for use immediately inside abi.encode(…), as arguments in exterior perform calls or in occasion information with out prior task to a neighborhood variable. Utilizing return doesn’t set off the bug. The kinds bytesNN and bool will end in corrupted information whereas enum may result in an invalid revert.
Moreover, arrays with components shorter than 32 bytes is probably not dealt with appropriately even when the bottom kind is an integer kind. Encoding such arrays in the best way described above can result in different information within the encoding being overwritten if the variety of components encoded is just not a a number of of the variety of components that match a single slot. If nothing follows the array within the encoding (be aware that dynamically-sized arrays are all the time encoded after statically-sized arrays with statically-sized content material), or if solely a single array is encoded, no different information is overwritten.
Unrelated to the ABI encoder difficulty 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 via the latest addition of Solidity to OSS-Fuzz – a safety toolkit for locating discrepancies or points in a wide range of tasks. For Solidity we now have included a number of completely different fuzzers testing completely different features 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 may occur when performing index entry on bytesNN varieties with a compile-time fixed worth (not index) of 31 or when utilizing the byte opcode in inline meeting.
This submit was collectively composed by @axic, @chriseth, @holiman
