Advisory Published
CVE Published

CVE-2024-24564: Vyper extract32 can ready dirty memory

First published: Mon Feb 26 2024(Updated: )

### Summary When using the built-in `extract32(b, start)`, if the `start` index provided has for side effect to update `b`, the byte array to extract `32` bytes from, it could be that some dirty memory is read and returned by `extract32`. ### Details Before evaluating `start`, the function `Extract32.build_IR` caches only: - The pointer in memory/storage to `b`: https://github.com/vyperlang/vyper/blob/10564dcc37756f3d3684b7a91fd8f4325a38c4d8/vyper/builtins/functions.py#L916-L918 - The length of `b`: https://github.com/vyperlang/vyper/blob/10564dcc37756f3d3684b7a91fd8f4325a38c4d8/vyper/builtins/functions.py#L920-L922 but do not cache the actual content of `b`. This means that if the evaluation of `start` changes `b`'s content and length, an outdated length will be used with the new content when extracting 32 bytes from `b`. ### PoC Calling the function `foo` of the following contract returns `b'uuuuuuuuuuuuuuuuuuuuuuuuuuu\x00\x00789'` meaning that `extract32` accessed some dirty memory. ```Vyper var:Bytes[96] @internal def bar() -> uint256: self.var = b'uuuuuuuuuuuuuuuuuuuuuuuuuuuuuu' self.var = b'' return 3 @external def foo() -> bytes32: self.var = b'abcdefghijklmnopqrstuvwxyz123456789' return extract32(self.var, self.bar(), output_type=bytes32) # returns b'uuuuuuuuuuuuuuuuuuuuuuuuuuu\x00\x00789' ``` ### Impact For contracts that are affected, it means that calling `extract32` returns dirty memory bytes instead of some expected output.

Credit: security-advisories@github.com security-advisories@github.com

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