Solidity Storage Array Bug Announcement
This weblog submit is about two bugs linked 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 despite the fact that a contract containing them ought to very possible present malfunctions in assessments.
Daenam Kim with assist from Nguyen Pham, each from Curvegrid found a problem the place invalid information is saved in reference to arrays of signed integers.
This bug has been current since Solidity 0.4.7 and we think about it the extra severe of the 2. If these arrays use damaging integers in a sure scenario, it should trigger information corruption and thus the bug needs to be simple to detect.
Via the Ethereum bug bounty program, we obtained a report a couple of flaw throughout the new experimental ABI encoder (known as ABIEncoderV2). The brand new ABI encoder continues to be marked as experimental, however we however assume that this deserves a distinguished 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 accommodates the fixes to the bugs.
In the meanwhile, we don’t plan to publish a repair to the legacy 0.4.x collection of Solidity, however we would if there may be widespread demand.
Each bugs needs to be simply seen in assessments that contact the related code paths.
Particulars concerning the two bugs may be discovered under.
Signed Integer Array Bug
Who needs to be involved
If in case you have deployed contracts which use signed integer arrays in storage and both straight assign
- a literal array with at the least one damaging worth in it (x = [-1, -2, -3];) or
- an current array of a totally different signed integer kind
to it, this may result in information corruption within the storage array.
Contracts that solely assign particular person array parts (i.e. with x[2] = -1;) should not affected.
Methods to examine if contract is susceptible
In case you use signed integer arrays in storage, attempt to run assessments the place you employ damaging values. The impact needs to be that the precise worth saved is optimistic as an alternative of damaging.
If in case you have a contract that meets these situations, and wish 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.
Technical particulars
Storage arrays may be assigned from arrays of various kind. Throughout this copy and task operation, a sort conversion is carried out on every of the weather. Along with the conversion, particularly if the signed integer kind is shorter than 256 bits, sure bits of the worth must 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 kind. This results in too many bits being zeroed out. Specifically, the signal bit might be zero which makes the worth optimistic.
ABIEncoderV2 Array Bug
Who needs to be involved
If 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 throughout the supply code may be affected:
pragma experimental ABIEncoderV2;
Moreover, there are a selection of necessities for the bug to set off. See technical particulars additional under for extra info.
Methods to examine if contract is susceptible
The bug solely manifests itself when the entire following situations 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 task to an area (reminiscence) variable AND
- this information both accommodates an array of structs or an array of statically-sized arrays (i.e. at the least two-dimensional).
Along with that, within the following scenario, your code is NOT affected:
- in the event you solely return such information and don’t use it in abi.encode, exterior calls or occasion information.
Attainable 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 below sure circumstances ship corrupt parameters on technique invocations to different contracts.
Technical particulars
Through the encoding course of, the experimental ABI encoder doesn’t correctly advance to the following component in an array in case the weather occupy greater than a single slot in storage.
That is solely the case for parts which are structs or statically-sized arrays. Arrays of dynamically-sized arrays or of elementary datatypes should not affected.
The precise impact you will note is that information is “shifted” within the encoded array: If in case you have an array of kind uint[2][] and it accommodates 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 parts as an alternative of two.
This submit was collectively composed by @axic, @chriseth, @holiman
