Solidity Optimizer and ABIEncoderV2 Bug Announcement
By means of the Ethereum bug bounty program, we obtained a report a couple of flaw inside 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 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 assume 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 publish.
All of the bugs talked about right here must be simply seen in checks that contact the related code paths, at the least when run with all mixtures of zero and nonzero values.
Credit to Melonport workforce (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
In case you have deployed contracts which use the experimental ABI encoder V2, then these may be affected. Because of this solely contracts which use the next directive inside the supply code could be affected:
pragma experimental ABIEncoderV2;
Moreover, there are a variety of necessities for the bug to set off. See technical particulars additional under for extra info.
So far as we are able to inform, there are about 2500 contracts reside on mainnet that use the experimental ABIEncoderV2. It’s not clear what number of of them comprise 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 shipped on to an exterior operate name, to abi.encode or to occasion knowledge with out prior task 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
- 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 knowledge and don’t use it in abi.encode, exterior calls or occasion knowledge.
In case you have 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 [email protected].
How one can forestall most of these flaws sooner or later
To be able to be conservative about modifications, 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 of steady.
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 take a look at compiler correctness).
For builders — bugs inside 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.
The easiest way to guard in opposition to most of these flaws is to have a rigorous set of end-to-end checks on your contracts (verifying all code paths), since bugs in a compiler very probably aren’t “silent” and as a substitute manifest in invalid knowledge.
Doable penalties
Naturally, any bug can have wildly various penalties relying on this system management move, however we anticipate that that is extra prone 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 triggered 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 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:
- 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 could be good to forestall additional proliferation of weak contracts on mainnet.
2019-03-26:
- New compiler launch, model 0.5.7.
- This publish launched.
Technical particulars
Background
The Contract ABI is a specification how knowledge could 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 complicated knowledge sorts, together with arrays and structs.
When a contract receives enter knowledge, it should decode that (that is achieved by the “ABI decoder”) and previous to returning knowledge or sending knowledge 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 operate in a contract (and likewise 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 workforce began to work on a recent implementation named “ABI encoder V2” with the objective 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 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 task to a neighborhood variable. Utilizing return doesn’t set off the bug. The categories bytesNN and bool will end in corrupted knowledge whereas enum would possibly result in an invalid revert.
Moreover, arrays with parts shorter than 32 bytes might not be dealt with accurately even when the bottom kind is an integer kind. Encoding such arrays in the way in which 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 (be aware that dynamically-sized arrays are at all times 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 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 quite a lot of initiatives. For Solidity we now have 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 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 publish was collectively composed by @axic, @chriseth, @holiman
