Solidity Optimizer and ABIEncoderV2 Bug Announcement
By means of 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 element suffers from a number of totally different variations of the identical kind. 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, considered one of which was mounted 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 release comprises the fixes to all bugs defined on this weblog put up.
All of the bugs talked about right here must be simply seen in assessments that contact the related code paths, a minimum of when run with all mixtures of zero and nonzero values.
Credit to Melonport staff (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 may be affected. Which means that 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 data.
So far as we are able to inform, there are about 2500 contracts reside on mainnet that use the experimental ABIEncoderV2. It isn’t clear what number of of them comprise the bug.
The best way to test if contract is susceptible
The bug solely manifests itself when all the following situations are met:
- Storage information involving arrays or structs is shipped on to an exterior perform name, to abi.encode or to occasion information with out prior project to an area (reminiscence) variable AND
- there may be an array that comprises parts with measurement lower than 32 bytes or a struct that has parts 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
- should you solely use integer sorts (which may be shorter) and solely encode at most one array at a time
- should you solely return such information and don’t use it in abi.encode, exterior calls or occasion information.
You probably have a contract that meets these situations, and need to confirm whether or not the contract is certainly susceptible, you may attain out to us through security@ethereum.org.
The best way to forestall a majority of these flaws sooner or later
So as to be conservative about adjustments, the experimental ABI encoder has been obtainable 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-about steady.
We do our greatest to make sure prime quality, and have lately began engaged on ‘semantic’ fuzzing of sure elements on OSS-Fuzz (we have now beforehand crash-fuzzed the compiler, however that didn’t take a look at 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.
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 assessments to 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 circulate, 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 immediately 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 data public.
- Reasoning: It will 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 stop additional proliferation of susceptible 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 information could be exchanged with contracts from the surface (a Dapp) or when interacting between contracts. It helps a wide range of forms of information, together with easy values like numbers, bytes and strings, in addition to extra complicated information sorts, together with arrays and structs.
When a contract receives enter information, it should decode that (that is performed by the “ABI decoder”) and previous to returning information or sending information to a different contract, it should encode it (that is performed 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 named the “ABI encoder”.
In mid-2017 the Solidity staff 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 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 project to an area variable. Utilizing return doesn’t set off the bug. The kinds bytesNN and bool will lead to corrupted information whereas enum may result in an invalid revert.
Moreover, arrays with parts shorter than 32 bytes is probably not dealt with appropriately even when the bottom kind is an integer kind. Encoding such arrays in the way in which described above can result in different information within the encoding being overwritten if the variety of parts encoded just isn’t a a number of of the variety of parts 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 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, except inline meeting is used.
These two bugs have been recognized by 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 have now included a number of totally different fuzzers testing totally 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 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
