Summary
mppx: Gas Draining with access list
Details
When the server acts as the fee_payer, mppx 0.6.27 copies the client-supplied EIP-2930 access list verbatim into the cosigned fee-payer transaction. The TypeScript SDK's fee-payer cosigning path accepts any access_list the client includes in the 0x78 FeePayerEnvelope without inspecting its length or contents.
Access list gas is charged intrinsically, before any opcode executes, regardless of whether the listed addresses are ever touched.
An attacker submits a valid transferWithMemo alongside fabricated address-only access list entries. The server validates calldata and gas parameters 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.
At the default of 180 entries and 100 Gwei max_fee_per_gas, this inflates the fee-payer cost from the normal ~51,287 gas to ~483,287 gas, a 9.4× multiplier, while staying within the FeePayerPolicy cap of 500K gas, the Node.js 16 KB header limit, and the Moderato RPC's eth_call simulation budget (verified empirically at 180 entries).
PoC
The PoC is provided below. It is configured to reproduce the attack on Tempo Moderate testnet within a Docker environment. Download the PoC and run:
unzip mppx_typescript_PoC.zip
cd mppx_typescript
docker build -t mppx-typescript-access-list .
docker run --rm mppx-typescript-access-list
There are more details in mppx_typescript/README.md
Impact
A malicious client can force the server to pay ~9.4x the normal transaction fee. This dramatically increases operational costs and completely destroys the profit margin on low-cost items.
The application does not adequately validate input before processing it, allowing unexpected values to reach sensitive code paths. Typical impact: varies by context: data corruption, logic bypass, or denial of service.
Affected versions
Security releases
Kodem intelligence
Severity tells you how bad this could be in the worst case. It does not tell you whether you are exposed. Exploitability and impact are functions of runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A vulnerable package can sit in your dependency tree and never run.
Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter. Kodem's runtime-powered SCA identifies whether this CVE is reachable in your applications.
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Frequently Asked Questions
- What is CVE-2026-63628? CVE-2026-63628 is a medium-severity improper input validation vulnerability in mppx (npm), affecting versions < 0.8.2. It is fixed in 0.8.2. The application does not adequately validate input before processing it, allowing unexpected values to reach sensitive code paths.
- Which versions of mppx are affected by CVE-2026-63628? mppx (npm) versions < 0.8.2 is affected.
- Is there a fix for CVE-2026-63628? Yes. CVE-2026-63628 is fixed in 0.8.2. Upgrade to this version or later.
- Is CVE-2026-63628 exploitable, and should I be worried? Whether CVE-2026-63628 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
- What actually determines whether CVE-2026-63628 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
- How do I fix CVE-2026-63628? Upgrade
mppxto 0.8.2 or later.