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Solidity Optimizer and ABIEncoderV2 Bug Announcement
By means of the Ethereum bug bounty program, we acquired 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 just a few 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 suppose 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 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 put up.
All of the bugs talked about right here needs to be simply seen in exams that contact the related code paths, no less than when run with all combos 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 needs to be involved
In case you have deployed contracts which use the experimental ABI encoder V2, then these could be affected. Because of this solely contracts which use the next directive throughout the supply code might 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 reside on mainnet that use the experimental ABIEncoderV2. It isn’t clear what number of of them include the bug.
The right way to test if contract is weak
The bug solely manifests itself when the entire following situations 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 project to a neighborhood (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
- 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 knowledge and don’t use it in abi.encode, exterior calls or occasion knowledge.
In case you have a contract that meets these situations, and need to confirm whether or not the contract is certainly weak, you may attain out to us through security@ethereum.org.
The right way to forestall these kind of flaws sooner or later
With a view to be conservative about adjustments, the experimental ABI encoder has been accessible 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-about secure.
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 towards these kind of flaws is to have a rigorous set of end-to-end exams to your contracts (verifying all code paths), since bugs in a compiler very seemingly are usually not “silent” and as an alternative manifest in invalid knowledge.
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 about when studying from arrays of booleans straight 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 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 stop 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 might be exchanged with contracts from the skin (a Dapp) or when interacting between contracts. It helps a wide range 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 finished by the “ABI decoder”) and previous to returning knowledge or sending knowledge to a different contract, it should encode it (that is finished by the “ABI encoder”). The Solidity compiler generates these two items of code for every outlined operate in a contract (and likewise 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 crew began to work on a recent 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 supplied 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 straight from storage. This implies these storage references have for use straight inside abi.encode(…), as arguments in exterior operate calls or in occasion knowledge with out prior project to a neighborhood variable. Utilizing return doesn’t set off the bug. The categories bytesNN and bool will end in corrupted knowledge whereas enum may result in an invalid revert.
Moreover, arrays with parts shorter than 32 bytes will not be 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 knowledge within the encoding being overwritten if the variety of parts encoded isn’t 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 knowledge 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, except inline meeting is used.
These two bugs have been recognized via the current 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 completely different fuzzers testing completely different facets 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 will 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
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