Solidity Optimizer and ABIEncoderV2 Bug Announcement
By means of the Ethereum bug bounty program, we obtained 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 totally different variations of the identical sort. The primary a part of this announcement explains this bug intimately. The brand new ABI encoder continues to be marked as experimental, however we however 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, one in every 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 accommodates the fixes to all bugs defined on this weblog put up.
All of the bugs talked about right here needs to be simply seen in checks that contact the related code paths, no 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 by way of the Ethereum bug bounty program!
Who needs to be involved
When you have deployed contracts which use the experimental ABI encoder V2, then these is likely to be affected. Because of this solely contracts which use the next directive inside the supply code may be affected:
pragma experimental ABIEncoderV2;
Moreover, there are a variety of necessities for the bug to set off. See technical particulars additional beneath for extra data.
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 include the bug.
How one can verify if contract is weak
The bug solely manifests itself when the entire following circumstances are met:
- Storage knowledge involving arrays or structs is distributed 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 accommodates parts with dimension lower than 32 bytes or a struct that has parts that share a storage slot or members of sort 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
- when you solely use integer sorts (that could be shorter) and solely encode at most one array at a time
- when you solely return such knowledge and don’t use it in abi.encode, exterior calls or occasion knowledge.
When you have a contract that meets these circumstances, and need to confirm whether or not the contract is certainly weak, you’ll be able to attain out to us by way of safety@ethereum.org.
How one can forestall these kind of flaws sooner or later
To be able to be conservative about adjustments, the experimental ABI encoder has been accessible solely when explicitly enabled, to permit folks to work together with it and take a look at it with out placing an excessive amount of belief in it earlier than it’s thought of steady.
We do our greatest to make sure top quality, and have lately began engaged on ‘semantic’ fuzzing of sure components on OSS-Fuzz (we’ve got beforehand crash-fuzzed the compiler, however that didn’t take a look at compiler correctness).
For builders — bugs inside 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 are launched solely into the compiled bytecode.
The easiest way to guard towards these kind of 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 knowledge.
Attainable penalties
Naturally, any bug can have wildly various penalties relying on this system management movement, however we anticipate that that is extra more likely to result in malfunction than exploitability.
The bug, when triggered, will below sure circumstances ship corrupt parameters on technique invocations to different contracts.
Timeline
2019-03-16:
- Report by way of bug bounty, about corruption precipitated 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 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 data public.
- Reasoning: It could not be possible to detect all weak contracts and attain out to all authors in a well timed method, and it might be good to forestall additional proliferation of weak contracts on mainnet.
2019-03-26:
- New compiler launch, model 0.5.7.
- This put up 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 varieties 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 executed by the “ABI decoder”) and previous to returning knowledge or sending knowledge to a different contract, it should encode it (that is executed 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 group began to work on a recent implementation named “ABI encoder V2” with the purpose of getting a extra versatile, protected, performant and auditable code generator. This experimental code generator, when explicitly enabled, has been supplied 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 immediately from storage. This implies these storage references have for use immediately 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 would possibly result in an invalid revert.
Moreover, arrays with parts shorter than 32 bytes might not be dealt with appropriately even when the bottom sort is an integer sort. Encoding such arrays in the best way described above can result in different knowledge within the encoding being overwritten if the variety of parts encoded is just not a a number of of the variety of parts that match a single slot. If nothing follows the array within the encoding (word 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 knowledge 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, except inline meeting is used.
These two bugs have been recognized by way of the current addition of Solidity to OSS-Fuzz – a safety toolkit for locating discrepancies or points in quite a lot of tasks. For Solidity we’ve got included a number of totally different fuzzers testing totally 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 put up was collectively composed by @axic, @chriseth, @holiman
