CVE-2026-104419

Executive Summary

CVE-2026-104419 is a peer-identification flaw in the Zebra cryptocurrency node (zebrad) versions 4.5.0 through 6.3.0. During block synchronization, the node discards the identity of the peer that supplied block hashes in FindBlocks responses, then attributes 100 misbehavior points (the ban threshold) to whichever peer serves a requested block more than 50,000 heights above the current tip. This allows a remote peer to return legitimate far-ahead block hashes to a syncing node, causing honest peers to be banned and degrading the victim's peer set, thereby raising eclipse attack risk.

Technical Analysis

Zebra is a full-node implementation of the Zcash protocol. Its block synchronization logic issues FindBlocks requests to peers to discover block hashes and then proceeds to fetch the corresponding blocks. The vulnerability centers on the bookkeeping that links received data back to the peer that provided it.

In affected versions (4.5.0 before 6.3.0), the node discards which peer supplied the block hashes in a FindBlocks response. The hash list and the peer identity are not jointly tracked. Consequently, when a requested block turns out to be more than 50,000 heights above the node's current tip, the node applies 100 misbehavior points — the ban threshold — to whichever peer ultimately serves that block, regardless of whether that peer was responsible for advertising the far-ahead hash. A peer reaching 100 misbehavior points is banned and disconnected.

Because the block hashes being returned are real, any correctness checks against the hashes themselves pass. The flaw is purely in attribution: the node cannot distinguish the advertising peer from the serving peer, so the penalty lands on an arbitrary honest peer. This is a peer-set integrity problem (CWE-345, insufficient verification of data authenticity), not a memory-safety or consensus issue — no funds, keys, or chain state are directly at risk. The CVSS vector (AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:L/A:L) reflects network reachability, high attack complexity due to the synchronization-state precondition, a low integrity impact on node peer set state, and a low availability impact from peer disconnects.

Exploitation Scenarios

No public proof-of-concept, Nuclei template, or exploit code is available; the chain below is inferred from the vulnerability class and the NVD description. Specific function, endpoint, and parameter names are not provided in the source data and are therefore not fabricated here.

1. Reconnaissance / target identification. The attacker identifies a Zebra node running an affected version (4.5.0 through 6.3.0) that is in a syncing state or can be induced into one. Visibility of zebrad nodes can be obtained via network scanning for the Zcash peer-to-peer service and, in some deployments, the node's RPC/status surface.

2. Triggering the flaw class. The attacker operates one or more malicious peers and connects (or is connected) to the target. The attacker responds to the target's FindBlocks requests with real block hashes that are more than 50,000 heights above the target's current tip. Because the hashes are genuine, the target's validity checks are satisfied and the far-ahead condition is reached.

3. Execution / misbehavior attribution. When the target subsequently requests the corresponding block and receives it from a peer, the target — having discarded which peer supplied the far-ahead hashes — assigns 100 misbehavior points (the ban threshold) to the serving peer. Where the serving peer is an honest node, that honest peer is banned and disconnected.

4. Post-exploitation impact. Repeated across sync cycles and against multiple competing honest peers, the attacker causes the victim node to progressively ban and drop honest peers, shrinking its outbound/inbound peer set. A reduced, attacker-influenced peer set materially increases eclipse risk, since surrounding the node with attacker-controlled peers becomes easier — a prerequisite for withholding or biasing block data. The direct impact is slow, incremental peer-set erosion rather than single-shot compromise.

Impact Assessment

The primary impact is on node availability and peer-set integrity, not confidentiality or direct integrity of balances or keys (CVSS C:N/I:L/A:L, base 4.8). Affected systems are Zcash full nodes running Zebra 4.5.0 through 6.3.0, particularly nodes actively syncing and those with limited peer diversity.

Business impact includes degraded synchronization reliability, additional bandwidth/compute consumed re-fetching and re-peering, and an elevated risk of eclipse for operators whose nodes underpin wallets, exchanges, or mining/validation infrastructure. Blast radius is per-node but cumulative: once an operator's peer set is contaminated, the node becomes a weaker source of chain truth, which can indirectly affect downstream services relying on that node. High attack complexity (AC:H) stems from the need to interleave requests with a node in the correct sync state and to win block-serving races against honest peers.

Detection Guidance

Detection should focus on zebrad logs and peer-connection telemetry. Operators should enable verbose sync/peer logging and alert on the signature events below. As with all network-facing node detections, tune thresholds to local baselines.

Sigma Rule

title: Zebra Node Peer Banning via Far-Ahead Block Hash Attribution Flaw
id: 8f2c1a94-3b7e-4d21-9c5a-zebra104419
status: experimental
description: Detects Zebra (zebrad) banning peers after serving blocks far above the chain tip, consistent with CVE-2026-104419
references:
  - https://nvd.nist.gov/vuln/detail/CVE-2026-104419
author: valtersit.com
date: 2026/01/01
logsource:
  product: zebra
  service: zebrad
detection:
  selection_ban:
    message|contains:
      - 'misbehavior'
      - 'ban'
  selection_far:
    message|contains:
      - 'FindBlocks'
      - '50000'
      - 'far'
  condition: selection_ban and selection_far
fields:
  - peer_addr
  - height
  - tip_height
falsepositives:
  - Legitimate deep reorg handling or normal ban of genuinely misbehaving peers
level: medium

YARA Rule

rule CVE_2026_104419_Zebra_FindBlocks_PeerAttribution
{
    meta:
        description = "Identifies Zebra source/version markers associated with the FindBlocks peer attribution flaw (CVE-2026-104419)"
        author = "valtersit.com"
        reference = "https://nvd.nist.gov/vuln/detail/CVE-2026-104419"
        date = "2026-01-01"
    strings:
        $s1 = "FindBlocks" ascii
        $s2 = "misbehavior" ascii nocase
        $s3 = "ban" ascii nocase
        $s4 = "50000" ascii
    condition:
        $s1 and 2 of ($s2, $s3, $s4)
}

Suricata IDS Rule

alert tcp any any -> any 8233 (msg:"CVE-2026-104419 Zebra FindBlocks far-ahead block hash abuse"; flow:established,to_server; content:"FindBlocks"; nocase; sid:2026104419; rev:1;)

Elastic Detection

sequence by process.entity_id with maxspan=5m
  [network where process.name == "zebrad" and network.protocol == "zcash"]
  [process where process.name == "zebrad" and message like~ "*misbehavior*" and message like~ "*ban*"]

Splunk SPL Query

index=zebra sourcetype=zebrad:log
| search ("FindBlocks" OR "misbehavior" OR "ban")
| stats count values(message) as events by peer_addr, host
| where count >= 1
| sort - count

Wazuh OSSEC Rule

<group name="zebra,cve-2026-104419,">
  <rule id="100419" level="7">
    <decoded_as>json</decoded_as>
    <field name="message">FindBlocks|misbehavior|ban</field>
    <description>CVE-2026-104419: Possible Zebra peer banning via far-ahead FindBlocks hashes</description>
  </rule>
</group>

Mitigation & Patch Guidance

Patch confidence: 0/4 — UNPATCHED. No vendor patch, advisory, or workaround is evidenced in any tracked source.

1. Immediate mitigations. Isolate or harden node peer ties: whitelist a curated set of trusted peers, cap the number of connections accepted from unknown sources, and monitor for repeated ban events tied to sync. Where available, lower ban sensitivity or extend ban review for far-ahead hash cases through configuration; verify any such option exists in the deployed version before relying on it. Log and alert on misbehavior/ban events correlated with FindBlocks activity to detect an active attempt.

2. Patch / upgrade steps. No fixed version is published for CVE-2026-104419. Monitor the Zebra project's release channel and advisory feed for versions after 6.3.0; treat any such release as the remediation target and schedule upgrades accordingly. Do not assume version 6.3.0 resolves the issue — it is the last known-affected version boundary.

3. Long-term hardening. Track peer-set health as a security-relevant metric, alert on abnormal peer churn, maintain geographically and operator-diversified peer lists, and prefer authenticated/trusted peer connections for critical infrastructure nodes. Where feasible, enable and retain detailed sync and peer-attribution logs to preserve forensic visibility should a fix require retroactive analysis.

References

• NVD — https://nvd.nist.gov/vuln/detail/CVE-2026-104419
• Zebra project repository and release advisories (monitor for versions after 6.3.0) — https://github.com/ZcashFoundation/zebra
• CWE-345: Insufficient Verification of Data Authenticity — https://cwe.mitre.org/data/definitions/345.html
• CVSS v3.1 Specification — https://www.first.org/cvss/v3.1/specification-document
• CISA Known Exploited Vulnerabilities Catalog (CVE-2026-104419 not listed at time of writing) — https://www.cisa.gov/known-exploited-vulnerabilities-catalog

Note: No public proof-of-concept, exploit code, or in-the-wild campaign evidence is currently associated with CVE-2026-104419. No EPSS score has been assigned by FIRST.org.