rk Identity Point Panic in Transaction Verification

Summary

Orchard transactions contain a rk field which is a randomized validating key and also an elliptic curve point. The Zcash specification allows the field to be the identity (a "zero" value), however, the orchard crate which is used to verify Orchard proofs would panic when fed a rk with the identity value. Thus an attacker could send a crafted transaction that would make a Zebra node crash.

Severity

Critical - This is a Denial of Service Vulnerability that could allow an attacker to crash Zebra nodes.

Affected Versions

All Zebra versions prior to version 4.3.1.

Description

The vulnerability exists in the circuits.rs file of the orchard crate; it attempts to get the coordinates of the rk value and calls unwrap() on the results, which causes a panic if rk is the identity. Zebra parses rk as a byte vector; it creates an Orchard "bundle" using the orchard crate and then calls the same crate to verify it, triggering the panic. An attacker could exploit this by:

1. Creating a transaction with a identity rk
2. Submitting it to a Zebra node, making it crash

Impact

Denial of Service

• Attack Vector: Network.
• Effect: Node crash.
• Scope: Any impacted Zebra node.

Fixed Versions

This issue is fixed in Zebra 4.3.1. The fix was agreed with zcashd developers (which has the same issue) to not allow the identity rk anymore and change the specification as such. Zebra now does this when parsing a transaction. This was deemed easier than fixing the issue in orchard, which would make the bug public before the nodes could be patched.

Mitigation

Users should upgrade to Zebra 4.3.1 or later immediately. There are no known workarounds for this issue. Immediate upgrade is the only way to ensure the node remains not vulnerable to denial of service.

Credits

Thanks to Alex “Scalar” Sol for finding and reporting the issue.