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EEF-CVE-2026-82751
Summary
Improper Validation of Specified Quantity in Input in ZenHive mpp allows an unauthenticated remote client to inflate the fee-payer's gas cost per sponsored payment by a large multiplier and to have the sponsor pay for provisioning an access key on the client's own account.
When the server sponsors Tempo payments, MPP.Methods.Tempo.FeePayerPolicy.measure/3 in lib/mpp/methods/tempo/fee_payer_policy.ex bounds the gas fields, the fee budget, the validity window and the access list of the client-signed 0x76 envelope, but does not check whether the envelope carries the optional key_authorization field. A client can attach a fully signed key authorization, provisioning a new access key with token spending limits on its own account, alongside the normal payment call. The key and each limit entry are persistent storage writes billed as intrinsic gas to the sponsor, bounded only by the gas_limit ceiling. At the reporter's default of one key with three token limits the sponsored cost rises from about 46,587 gas to about 1,808,700 gas, and the client keeps a valid access key it paid nothing for.
This issue affects mpp: from 0.2.0 before 0.16.1.
Workarounds
Disable Tempo fee sponsorship by leaving "fee_payer" at its default of false and setting no fee_payer_url, so the client pays its own gas. Where sponsorship must stay enabled, lower the max_gas override in fee_payer_policy to just above what a legitimate sponsored payment needs (a TIP-20 transfer is roughly 50,000 gas), so an envelope carrying a key authorization either fails the policy or runs out of gas at a bounded cost. This bounds the inflation without closing it.
Configurations
Only deployments that enable Tempo server-side fee sponsorship are affected, through "fee_payer" => true in method_config with either local co-signing (fee_payer_private_key) or hosted sponsorship (fee_payer_url). Sponsorship is disabled by default.
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The vulnerability can be exploited over the network without needing physical access. It is easy for an attacker to exploit this vulnerability. An attacker does not need any special privileges or access rights. No user interaction is needed for the attacker to exploit this vulnerability.
Exploitation attempts have been detected. Elevated vigilance and prompt remediation are advised.
The exploit probability is very low. The vulnerability is unlikely to be exploited in the next 30 days.
We did not find any exploit available. Neither in GitHub repositories nor in the Exploit-Database.
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