Summary
mppx: Gas Draining with padding
Details
When the server acts as the fee_payer, mppx 0.6.27 validates calldata using viem's decodeFunctionData, which is lenient about trailing bytes. The FeePayerPolicy caps gas_limit (≤ 2 M) and max_fee_per_gas (≤ 100 Gwei) but does not check calldata length.
Tempo uses legacy calldata gas pricing: 16 gas per non-zero byte. Appending N bytes of 0x01 padding inflates actual gas used by N × 16 while keeping gas_limit and max_fee_per_gas within policy caps. The server cosigns and broadcasts the padded transaction, draining its fee-payer wallet.
Vulnerable code path: FeePayerPolicy in fee-payer.ts (mppx 0.6.27) enforces maxGas = 2_000_000 and maxFeePerGas = 100 Gwei but has no calldata.length bound. The policy check passes because the inflated gas comes from intrinsic calldata cost, not the declared gas_limit.
Note: In the experiment, a 16 KB max_header_length is enforced, which caps the maximum effective padding at roughly 5,500 bytes. The default padding in this PoC (5,500 bytes) is within that limit.
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-padding .
docker run --rm mppx-padding
There are more details in mppx_typescript/README.md
Impact
A malicious client can force the server to pay ~5x the normal transaction fee. This dramatically increases operational costs and completely destroys the profit margin on low-cost items.
Please provide a way to share the PoC in .zip format to you. Thank you!
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.
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Frequently Asked Questions
- What is CVE-2026-63627? CVE-2026-63627 is a medium-severity improper input validation vulnerability in mppx (npm), affecting versions < 0.8.1. 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-2026-63627? mppx (npm) versions < 0.8.1 is affected.
- Is there a fix for CVE-2026-63627? Yes. CVE-2026-63627 is fixed in 0.8.2. Upgrade to this version or later.
- Is CVE-2026-63627 exploitable, and should I be worried? Whether CVE-2026-63627 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-2026-63627 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-2026-63627? Upgrade
mppxto 0.8.2 or later.