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 completely 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, one among 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 incorporates the fixes to all bugs defined on this weblog submit.
All of the bugs talked about right here ought to be simply seen in checks that contact the related code paths, not less than when run with all mixtures 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 by way of the Ethereum bug bounty program!
Who ought to be involved
When you have deployed contracts which use the experimental ABI encoder V2, then these is perhaps affected. Because of this solely contracts which use the next directive throughout 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 will 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.
Tips on how to verify if contract is susceptible
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’s an array that incorporates 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
- in the event you solely use integer sorts (which may be shorter) and solely encode at most one array at a time
- in the event 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 susceptible, you’ll be able to attain out to us by way of security@ethereum.org.
Tips on how to stop all these flaws sooner or later
To be able to be conservative about adjustments, the experimental ABI encoder has been out there 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 secure.
We do our greatest to make sure top quality, and have just lately began engaged on ‘semantic’ fuzzing of sure components on OSS-Fuzz (we now have 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 are launched solely into the compiled bytecode.
The easiest way to guard towards all these flaws is to have a rigorous set of end-to-end checks to your contracts (verifying all code paths), since bugs in a compiler very probably are usually not “silent” and as an alternative manifest in invalid knowledge.
Doable penalties
Naturally, any bug can have wildly various penalties relying on this system management circulation, however we anticipate that that is extra prone to result in malfunction than exploitability.
The bug, when triggered, will underneath sure circumstances ship corrupt parameters on methodology invocations to different contracts.
Timeline
2019-03-16:
- Report by way of bug bounty, about corruption induced 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:
- Choice to make info 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 forestall additional proliferation of susceptible contracts on mainnet.
2019-03-26:
- New compiler launch, model 0.5.7.
- This submit 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 accomplished by the “ABI decoder”) and previous to returning knowledge or sending knowledge to a different contract, it should encode it (that is accomplished 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 is known as the “ABI encoder”.
In mid-2017 the Solidity crew began to work on a recent 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 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 task to a neighborhood variable. Utilizing return doesn’t set off the bug. The kinds bytesNN and bool will lead to 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 sort is an integer sort. 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 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 (word 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 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 latest addition of Solidity to OSS-Fuzz – a safety toolkit for locating discrepancies or points in quite a lot of tasks. For Solidity we now have 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 submit was collectively composed by @axic, @chriseth, @holiman