Solidity Optimizer and ABIEncoderV2 Bug Announcement
By 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 a couple of completely 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 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, one in every 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 incorporates 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 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 needs to be involved
When you have deployed contracts which use the experimental ABI encoder V2, then these is perhaps 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 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 dwell on mainnet that use the experimental ABIEncoderV2. It isn’t clear what number of of them include the bug.
Easy methods to verify 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 operate name, to abi.encode or to occasion information with out prior project to an area (reminiscence) variable AND
- there’s an array that incorporates components with measurement lower than 32 bytes or a struct that has components 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 varieties
- for those who solely use integer varieties (which may be shorter) and solely encode at most one array at a time
- for those who solely return such information and don’t use it in abi.encode, exterior calls or occasion information.
When you have a contract that meets these circumstances, and wish to confirm whether or not the contract is certainly weak, you possibly can attain out to us by way of security@ethereum.org.
Easy methods to stop these kinds of flaws sooner or later
With a purpose 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 check 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 prime quality, and have just lately began engaged on ‘semantic’ fuzzing of sure components on OSS-Fuzz (we have now beforehand crash-fuzzed the compiler, however that didn’t check 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 can be launched solely into the compiled bytecode.
One of the simplest ways to guard in opposition to these kinds of flaws is to have a rigorous set of end-to-end exams in your contracts (verifying all code paths), since bugs in a compiler very possible usually are 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 technique invocations to different contracts.
Timeline
2019-03-16:
- Report by way of bug bounty, about corruption precipitated 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 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:
- Determination 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 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 information could be exchanged with contracts from the surface (a Dapp) or when interacting between contracts. It helps a wide range of kinds of information, together with easy values like numbers, bytes and strings, in addition to extra complicated information varieties, 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 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 group 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 varieties, bool, enum and different varieties 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 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 may result in an invalid revert.
Moreover, arrays with components 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 information within the encoding being overwritten if the variety of components encoded is just not a a number of of the variety of components 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 difficulty 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 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 varieties 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