OpenVM is a performant and modular zkVM framework built for customization and extensibility. In version 1.0.0, OpenVM is vulnerable to overflow through byte decomposition of pc in AUIPC chip. A typo results in the highest limb of pc being range checked to 8-bits instead of 6-bits. This results in the if statement never being triggered because the enumeration gives i=0,1,2, when instead the enumeration should give i=1,2,3, leaving pc_limbs[3] range checked to 8-bits instead of 6-bits. This leads to a vulnerability where the pc_limbs decomposition differs from the true pc, which means a malicious prover can make the destination register take a different value than the AUIPC instruction dictates, by making the decomposition overflow the BabyBear field. This issue has been patched in version 1.1.0.

Affected: crates.io: openvm

The fix to https://cantina.xyz/code/c486d600-bed0-4fc6-aed1-de759fd29fa2/findings/21 has a typo that still results in the highest limb of `pc` being range checked to 8-bits instead of 6-bits.

In the AIR, we do https://github.com/openvm-org/openvm/blob/0f94c8a3dfa7536c1231465d1bdee5fc607a5993/extensions/rv32im/circuit/src/auipc/core.rs#L135 ``` for (i, limb) in pc_limbs.iter().skip(1).enumerate() { if i == pc_limbs.len() - 1 { ```

It should be ``` for (i, limb) in pc_limbs.iter().enumerate().skip(1) { ```

Right now the if statement is never triggered because the enumeration gives `i=0,1,2` when we instead want `i=1,2,3`. What this means is that `pc_limbs[3]` is range checked to 8-bits instead of 6-bits.

This leads to a vulnerability where the `pc_limbs` decomposition differs from the true `pc`, which means a malicious prover can make the destination register take a different value than the AUIPC instruction dictates, by making the decomposition overflow the BabyBear field.