The block sync download path in Zebra (zebrad) before 6.3.0 reads a block's height from its unvalidated coinbase scriptSig and drops blocks that appear too far behind the tip before consensus validation, without penalizing the supplying peer. Because V5 transaction IDs exclude the scriptSig, a malicious peer can repeatedly serve a canonical block whose coinbase claims height 1 while keeping the requested hash, delaying the node's discovery of the newest block.

Zebra (zebrad) before 6.2.1 contains an asymmetric resource consumption vulnerability that allows unauthenticated peers to stall block verification by pushing V6 mempool transactions with invalid Halo2 proofs. Attackers can flood the shared unprioritized Halo2 verification queue with zero-fee transactions carrying zero-filled Orchard and Ironwood proofs, causing nodes to fall behind the chain tip.

Zebra zebrad 4.5.0 and zebra-script 7.0.0 count P2SH redeem script signature operations in legacy mode rather than zcashd's accurate P2SH mode, overcounting CHECKMULTISIG preceded by OP1 through OP16 as 20 sigops and causing a consensus divergence. Remote attackers can broadcast P2SH spends using low-threshold multisig redeem scripts so that a block zcashd accepts exceeds Zebra's inflated MAXBLOCKSIGOPS count, causing Zebra nodes to reject it and stall off the chain.

Zebra (zebrad) 4.5.0 before 6.3.0 discards which peer supplied the block hashes in FindBlocks responses, then assigns 100 misbehavior points, the ban threshold, to whichever peer serves a requested block more than 50,000 heights above the tip. A remote peer can return real far-ahead hashes to a syncing node so that honest peers get banned, eroding its peer set and raising eclipse risk.

Zebra zebrad 4.4.0 and zebra-script 6.0.0 fail to enforce a ZIP-244 consensus rule, accepting V5 transparent inputs signed with SIGHASHSINGLE that lack a corresponding output. Attackers can broadcast crafted V5 transactions with more inputs than outputs that Zebra accepts but zcashd rejects, causing a network consensus split.

Zebra (zebrad) 5.0.0 before 6.0.0-rc.0 does not apply its per-peer mempool admission cap to transactions received as direct P2P tx messages, because these are queued without the sending peer recorded as their source. A remote inbound peer can push many unique transactions to occupy a disproportionate share of mempool admission slots, crowding out honest peers' transaction relay.

Summary

A composite denial-of-service vulnerability in Zebra's block discovery pipeline allows an unauthenticated remote attacker to permanently halt all new block discovery on a targeted node. The attack exploits three independent weaknesses in the gossip, syncer, and download subsystems — all exercisable from a single TCP connection — to create a monotonically growing block deficit that never self-heals.

Severity

Critical — This is a Denial of Service vulnerability that requires no authentication, no special privileges, and only a single peer connection. The halt is permanent: the node will never recover without operator intervention.

Affected Versions

All Zebra versions prior to 4.4.0.

Description

Zebra discovers new blocks through two complementary paths: a gossip path (peers announce blocks via inv messages, triggering individual block downloads) and a syncer path (Zebra periodically queries peers with FindBlocks/FindHeaders to discover chains of missing blocks). Both paths must function for normal operation.

The gossip path was vulnerable because there was no per-connection rate limit on inv messages. A single connection could send enough sequential inv messages with fake block hashes to fill the entire gossip download queue in under a millisecond. The FullQueue return value was silently ignored, so legitimate block announcements from honest peers were dropped with no warning.

The syncer backup path could be degraded by responding with empty inv to FindBlocks requests and with NotFound to block download requests. Both are valid protocol responses that carried zero misbehavior penalty. The attacker's connection was never banned and never disconnected, allowing the degradation to persist indefinitely.

Combining these two vectors, an attacker could suppress both block discovery paths simultaneously from a single connection, causing the node to fall permanently behind the chain tip.

Impact

Denial of Service

Attack Vector: Network, unauthenticated. Requires only a single TCP peer connection. Effect: Permanent halt of block discovery. The targeted node falls behind the chain tip and never recovers without operator intervention. Scope: Any Zebra node reachable by the attacker over the peer-to-peer network.

Fixed Versions

This issue is fixed in Zebra 4.4.0.

The fix drops connections that send empty responses to FindBlocks and FindHeaders messages, preventing attackers from degrading the syncer path without consequence.

Mitigation

Users should upgrade to Zebra 4.4.0 or later immediately.

There are no known workarounds for this issue. Immediate upgrade is the only way to protect against this attack.

Credits

Zebra the researcher who reported this issue through the coordinated disclosure process.

ZEBRA is a Zcash node written entirely in Rust. Prior to zebrad version 4.3.1 and prior to zebra-script version 5.0.2, after a refactoring, Zebra failed to validate a consensus rule that restricted the possible values of sighash hash types for V5 transactions which were enabled in the NU5 network upgrade. Zebra nodes could thus accept and eventually mine a block that would be considered invalid by zcashd nodes, creating a consensus split between Zebra and zcashd nodes. In a similar vein, for V4 transactions, Zebra mistakenly used the "canonical" hash type when computing the sighash while zcashd (correctly per the spec) uses the raw value, which could also crate a consensus split. This issue has been patched in zebrad version 4.3.1 and zebra-script version 5.0.2.

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Zebra Industrial Printers All Versions, Zebra printers are shipped with unrestricted end-user access to front panel options. If the option to use a passcode to limit the functionality of the front panel is applied, specially crafted packets could be sent over the same network to a port on the printer and the printer will respond with an array of information that includes the front panel passcode for the printer. Once the passcode is retrieved, an attacker must have physical access to the front panel of the printer to enter the passcode to access the full functionality of the front panel.

Motorola FX9500 devices allow remote attackers to conduct absolute path traversal attacks, as demonstrated by PL/SQL Server Pages files such as /include/viewtagdb.psp.

UNSUPPORTED WHEN ASSIGNED An issue was discovered on Zebra (formerly Motorola Solutions) Fixed RFID Reader FX9500 devices. An unauthenticated attacker can upload arbitrary files to the filesystem that can then be accessed through the web interface. This can lead to information disclosure and code execution. NOTE: This vulnerability only affects products that are no longer supported by the maintainer.