{"id":"CVE-2026-59694","aliases":["EEF-CVE-2026-59694","GHSA-qpxh-ff8m-c62v"],"url":"https://o3.security/vulnerability/CVE-2026-59694","summary":"Unbounded access list in mpp Tempo fee-payer inflates gas cost per payment","details":"### Details\nWhen the server acts as the fee_payer, `mpp` Elixir 0.4.0 copies the client-supplied EIP-2930 access list verbatim into the cosigned fee-payer transaction. In `cosign_fee_payer`, the server re-signs the raw `base_fields` (index 5 of the 0x76 AASigned envelope) without inspecting the access list field, which is part of the signed payload.\n\nAccess list gas is charged **intrinsically** — before any opcode executes — regardless of whether the listed addresses are ever touched.\n\nAn attacker submits a valid `transferWithMemo` alongside fabricated address-only access list entries. The server validates calldata and amount but never inspects `access_list` length. It cosigns and broadcasts a transaction that costs the fee-payer wallet `N × 2,400` extra gas per request with no corresponding work performed on-chain.\n\nAt the default of 137 entries and 100 Gwei `max_fee_per_gas`, this inflates the fee-payer cost from the normal ~51,287 gas to ~380,087 gas — a **7.4× multiplier** — while staying within Bandit's default 10,000-byte per-header-field limit (verified empirically).\n\n### PoC\nThe PoC is provided below. It is configured to reproduce the attack on Tempo Moderate testnet within a Docker environment. Download the PoC and run:\n```bash\nunzip mpp_elixir_PoC.zip\ncd mpp_elixir\ndocker build -t mpp-elixir-access-list .\ndocker run --rm mpp-elixir-access-list\n``` \nThere are more details in `mpp_elixir/README.md`\n\n### Impact\nA malicious client can force the server to pay ~**7.4x** the normal transaction fee. This dramatically increases operational costs and completely destroys the profit margin on low-cost items.","published":"2026-07-17T10:11:49.192Z","modified":"2026-10-21T02:58:49.118411957Z","cvss":null,"epss":null,"cisaKev":null,"exploitsKnown":0,"affectedPackages":[{"ecosystem":"Hex","name":"mpp","fixedVersion":"0.6.0"}],"fix":{"url":"https://github.com/ZenHive/mpp/commit/5d6338e2334084c5f2a78cfcca474830733ed7e8","label":"ZenHive/mpp@5d6338e"},"references":[{"type":"WEB","url":"https://cna.erlef.org/cves/CVE-2026-59694.html"},{"type":"WEB","url":"https://github.com"},{"type":"WEB","url":"https://osv.dev/vulnerability/EEF-CVE-2026-59694"},{"type":"WEB","url":"https://repo.hex.pm"},{"type":"ADVISORY","url":"https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/59xxx/CVE-2026-59694.json"},{"type":"ADVISORY","url":"https://github.com/ZenHive/mpp/security/advisories/GHSA-qpxh-ff8m-c62v"},{"type":"ADVISORY","url":"https://nvd.nist.gov/vuln/detail/CVE-2026-59694"},{"type":"FIX","url":"https://github.com/ZenHive/mpp/commit/5d6338e2334084c5f2a78cfcca474830733ed7e8"},{"type":"PACKAGE","url":"https://github.com/ZenHive/mpp"},{"type":"WEB","url":"https://github.com/ZenHive/mpp/releases/tag/v0.6.0"}],"provenance":{"sources":["OSV.dev","FIRST.org (EPSS)"],"lastVerified":"2026-10-21T02:58:49.118411957Z"}}