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Solidity Storage Array Bug Announcement
This weblog publish is about two bugs related to storage arrays that are in any other case unrelated. Each have been current within the compiler for a very long time and have solely been found now regardless that a contract containing them ought to very probably present malfunctions in exams.
Daenam Kim with assist from Nguyen Pham, each from Curvegrid found a problem the place invalid knowledge is saved in reference to arrays of signed integers.
This bug has been current since Solidity 0.4.7 and we take into account it the extra critical of the 2. If these arrays use damaging integers in a sure state of affairs, it is going to trigger knowledge corruption and thus the bug must be simple to detect.
By way of the Ethereum bug bounty program, we acquired a report a couple of flaw throughout the new experimental ABI encoder (known as ABIEncoderV2). 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.
Credit to Ming Chuan Lin (of https://www.secondstate.io) for each discovering and fixing the bug!
The 0.5.10 release incorporates the fixes to the bugs.
In the intervening time, we don’t plan to publish a repair to the legacy 0.4.x collection of Solidity, however we would if there’s common demand.
Each bugs must be simply seen in exams that contact the related code paths.
Particulars concerning the two bugs will be discovered under.
Signed Integer Array Bug
Who must be involved
When you have deployed contracts which use signed integer arrays in storage and both immediately assign
- a literal array with a minimum of one damaging worth in it (x = [-1, -2, -3];) or
- an current array of a totally different signed integer sort
to it, it will result in knowledge corruption within the storage array.
Contracts that solely assign particular person array components (i.e. with x[2] = -1;) will not be affected.
examine if contract is weak
If you happen to use signed integer arrays in storage, attempt to run exams the place you utilize damaging values. The impact must be that the precise worth saved is optimistic as a substitute of damaging.
When you have a contract that meets these situations, and wish to confirm whether or not the contract is certainly weak, you’ll be able to attain out to us by way of security@ethereum.org.
Technical particulars
Storage arrays will be assigned from arrays of various sort. Throughout this copy and project operation, a kind conversion is carried out on every of the weather. Along with the conversion, particularly if the signed integer sort is shorter than 256 bits, sure bits of the worth need to be zeroed out in preparation for storing a number of values in the identical storage slot.
Which bits to zero out was incorrectly decided from the supply and never the goal sort. This results in too many bits being zeroed out. Particularly, the signal bit can be zero which makes the worth optimistic.
ABIEncoderV2 Array Bug
Who must be involved
When you have deployed contracts which use the experimental ABI encoder V2, then these may be affected. Which means that 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 info.
examine if contract is weak
The bug solely manifests itself when all the following situations are met:
- Storage knowledge involving arrays or structs is shipped on to an exterior perform name, to abi.encode or to occasion knowledge with out prior project to a neighborhood (reminiscence) variable AND
- this knowledge both incorporates an array of structs or an array of statically-sized arrays (i.e. a minimum of two-dimensional).
Along with that, within the following state of affairs, your code is NOT affected:
- if you happen to solely return such knowledge and don’t use it in abi.encode, exterior calls or occasion knowledge.
Doable penalties
Naturally, any bug can have wildly various penalties relying on this system management circulate, however we anticipate that that is extra more likely to result in malfunction than exploitability.
The bug, when triggered, will beneath sure circumstances ship corrupt parameters on methodology invocations to different contracts.
Technical particulars
Throughout the encoding course of, the experimental ABI encoder doesn’t correctly advance to the following aspect in an array in case the weather occupy greater than a single slot in storage.
That is solely the case for components which are structs or statically-sized arrays. Arrays of dynamically-sized arrays or of elementary datatypes will not be affected.
The particular impact you will note is that knowledge is “shifted” within the encoded array: When you have an array of sort uint[2][] and it incorporates the info
[[1, 2], [3, 4], [5, 6]], then it will likely be encoded as [[1, 2], [2, 3], [3, 4]] as a result of the encoder solely advances by a single slot between components as a substitute of two.
This publish was collectively composed by @axic, @chriseth, @holiman
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