Solidity Optimizer and ABIEncoderV2 Bug Announcement
By way of the Ethereum bug bounty program, we acquired a report a few flaw throughout the new experimental ABI encoder (known as ABIEncoderV2). Upon investigation, it was discovered that the element 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 remains to be marked as experimental, however we nonetheless 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, considered one of which was mounted 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 launch 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 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 must be involved
When you’ve got deployed contracts which use the experimental ABI encoder V2, then these may be affected. Because of this solely contracts which use the next directive throughout the supply code will 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.
examine if contract is weak
The bug solely manifests itself when all the following circumstances 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’s 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 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
- in case you solely use integer sorts (that could 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’ve got a contract that meets these circumstances, and need to confirm whether or not the contract is certainly weak, you may attain out to us by way of safety@ethereum.org.
stop a majority of these flaws sooner or later
As a way 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 take a look at 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 top quality, and have not too long ago began engaged on ‘semantic’ fuzzing of sure elements on OSS-Fuzz (we’ve got beforehand crash-fuzzed the compiler, however that didn’t take a look at compiler correctness).
For builders — bugs throughout 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 might be launched solely into the compiled bytecode.
One of the simplest ways to guard towards 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 doubtless are usually not “silent” and as a substitute manifest in invalid information.
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 methodology invocations to different contracts.
Timeline
2019-03-16:
- Report by way of bug bounty, about corruption brought about 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 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:
- Determination to make data public.
- Reasoning: It might 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 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 information will be exchanged with contracts from the skin (a Dapp) or when interacting between contracts. It helps quite a lot of varieties of information, together with easy values like numbers, bytes and strings, in addition to extra advanced information sorts, together with arrays and structs.
When a contract receives enter information, it should decode that (that is achieved by the “ABI decoder”) and previous to returning information or sending information to a different contract, it should encode it (that is achieved 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 contemporary implementation named “ABI encoder V2” with the purpose of getting a extra versatile, secure, performant and auditable code generator. This experimental code generator, when explicitly enabled, has been provided to customers because the 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 perform calls or in occasion information with out prior project to an area variable. Utilizing return doesn’t set off the bug. The categories bytesNN and bool will lead to corrupted information whereas enum would possibly result in an invalid revert.
Moreover, arrays with parts shorter than 32 bytes is probably not dealt with accurately even when the bottom sort is an integer sort. Encoding such arrays in the best way described above can result in different information within the encoding being overwritten if the variety of parts encoded shouldn’t be a a number of of the variety of parts that match a single slot. If nothing follows the array within the encoding (notice 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 concern 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 latest 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 elements 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