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A security vulnerability exists in outdated versions of the x402 SDK, specifically within the facilitator component that validates Solana-based payments. The flaw arises from improper verification of transaction proofs submitted by clients. When a resource server relies on a vulnerable facilitator to confirm payments, an attacker can craft a malicious Solana transaction that appears valid but lacks actual fund transfer. The facilitator incorrectly parses the transaction, accepting fake proofs due to missing signature checks or balance validation. This allows unauthorized access to paid resources without incurring costs. The issue does not compromise private keys, smart contracts, or user funds, but directly impacts resource servers accepting payments on Solana. Facilitators running vulnerable SDK versions are susceptible to this bypass. Clients are unaffected and do not need updates. Resource servers that self-facilitate must also upgrade. The vulnerability was addressed in patched SDK releases, which enforce strict transaction verification using Solana’s native libraries. Attackers could exploit this by sending specially crafted transactions with spoofed payment confirmations. The flaw is critical for facilitators as it undermines payment integrity. Patches are available and should be applied immediately. The vulnerability was discovered during internal review and reported via GitHub. No active exploits have been reported, but all facilitators should prioritize updating. The fix includes additional validation steps and updated dependencies.
Platform: x402 SDK
Version: Pre-patch versions
Vulnerability: Payment verification flaw
Severity: High
Date: Mar 7, 2026
Prediction: Patched Mar 6, 2026
What Undercode Say:
Analytics
Check installed version of x402 SDK (example using npm) npm list @coinbase/x402 If using yarn yarn list @coinbase/x402 Expected output: @coinbase/x402@<version> Vulnerable versions are those before the patch (e.g., < 2.1.0) To view latest patched versions npm view @coinbase/x402 versions
Exploit
Hypothetical exploit: Send fake payment proof to vulnerable facilitator
curl -X POST https://facilitator.example.com/verify \
-H "Content-Type: application/json" \
-d '{
"transaction": "fake_solana_tx_without_funds",
"proof": "malicious_signature"
}'
Vulnerable facilitator would accept this, granting resource access.
Protection
Upgrade to patched version immediately npm update @coinbase/x402@latest Or install specific patched version npm install @coinbase/[email protected] Verify upgrade npm list @coinbase/x402
Impact
Facilitators on Solana are at risk of accepting invalid payments, leading to loss of revenue and unauthorized resource access. Client funds and keys remain safe, but service integrity is compromised. Immediate upgrade is required to prevent exploitation.
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Sources:
Reported By: github.com
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