Solidity Optimizer and ABIEncoderV2 Bug

Solidity Optimizer and ABIEncoderV2 Bug Announcement

By the Ethereum bug bounty program, we obtained a report a few flaw throughout the new experimental ABI encoder (known as ABIEncoderV2). Upon investigation, it was discovered that the part 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 continues to be marked as experimental, however we however 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, certainly one of which was fastened 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 accommodates the fixes to all bugs defined on this weblog submit.

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 staff (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 have deployed contracts which use the experimental ABI encoder V2, then these could be affected. Which means solely contracts which use the next directive throughout the supply code will 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 data.

So far as we are able to inform, there are about 2500 contracts stay 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 all the 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 accommodates 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 varieties
• should you solely use integer varieties (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.

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 [email protected].

Tips on how to 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 folks to work together with it and check 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 not too long ago began engaged on ‘semantic’ fuzzing of sure components on OSS-Fuzz (now we have beforehand crash-fuzzed the compiler, however that didn’t check 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 simplest ways to guard towards a majority of these flaws is to have a rigorous set of end-to-end assessments on your contracts (verifying all code paths), since bugs in a compiler very doubtless usually are not “silent” and as a substitute manifest in invalid knowledge.

Potential penalties

Naturally, any bug can have wildly various penalties relying on this system management stream, however we count on that that is extra prone to result in malfunction than exploitability.

The bug, when triggered, will underneath 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 immediately 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 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 data public.
• Reasoning: It could not be possible to detect all susceptible contracts and attain out to all authors in a well timed method, and it will be good to stop 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 will be exchanged with contracts from the surface (a Dapp) or when interacting between contracts. It helps a wide range of varieties of knowledge, together with easy values like numbers, bytes and strings, in addition to extra advanced knowledge varieties, 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 staff began to work on a contemporary implementation named “ABI encoder V2” with the objective of getting a extra versatile, protected, 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 varieties, bool, enum and different varieties when they’re a part of an array or a struct and encoded immediately from storage. This implies these storage references have for use immediately 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 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 appropriately even when the bottom sort is an integer sort. 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 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 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 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, 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 a wide range of tasks. For Solidity now we have included a number of totally different fuzzers testing totally different elements of the compiler.

1. 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.
2. 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 submit was collectively composed by @axic, @chriseth, @holiman