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GHSA-r56x-j438-vw5m
No affected components available
Summary
Using the slice builtin can result in a double eval vulnerability when the buffer argument is either msg.data, self.code or <address>.code and either the start or length arguments have side-effects.
A contract search was performed and no vulnerable contracts were found in production. Having side-effects in the start and length patterns is also an unusual pattern which is not that likely to show up in user code. It is also much harder (but not impossible!) to trigger the bug since 0.3.4 since the unique symbol fence was introduced (https://github.com/vyperlang/vyper/pull/2914).
Details
It can be seen that the _build_adhoc_slice_node function of the slice builtin doesn't cache the mentioned arguments to the stack: https://github.com/vyperlang/vyper/blob/4595938734d9988f8e46e8df38049ae0559abedb/vyper/builtins/functions.py#L244
As such, they can be evaluated multiple times (instead of retrieving the value from the stack).
PoC
with Vyper version 0.3.3+commit.48e326f the call to foo passes the asserts:
l: DynArray[uint256, 10]
@external
def foo(cs: String[64]) -> uint256:
for i in range(10):
self.l.append(1)
assert len(self.l) == 10
s: Bytes[64] = b""
s = slice(msg.data, self.l.pop(), 3)
assert len(self.l) == 10 - 2
return len(self.l)
Patches
Patched in https://github.com/vyperlang/vyper/pull/3976.
Impact
No vulnerable production contracts were found.
The vulnerability can be exploited over the network without needing physical access. It is easy for an attacker to exploit this vulnerability. An attacker does not need any special privileges or access rights. No user interaction is needed for the attacker to exploit this vulnerability. The impact is confined to the system where the vulnerability exists. There is a low impact on the integrity of the data.
Exploitation attempts have been detected. Elevated vigilance and prompt remediation are advised.
The exploit probability is very low. The vulnerability is unlikely to be exploited in the next 30 days.
We did not find any exploit available. Neither in GitHub repositories nor in the Exploit-Database.
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