CVE-2026-52737

MEDIUMPre-NVD 5.35.3
EchelonGraph scoreLOW confidence

This medium-severity CVE scores 5.3 under the CNA's CVSS (NVD's own analysis pending). EPSS exploit-prediction score not yet available (the EPSS model rescores nightly; freshly-published CVEs typically appear within 48 hours). GitHub Security Advisory data not yet ingested — confidence will rise once GHSA publishes (typical lag: hours to days for open-source ecosystem CVEs; never for infrastructure-only CVEs).

Triggered by: NVD CVSS baseline
Sources: cna:github_m
5.3
EchelonGraph verdictMonitorLow exploitation likelihood right now — keep watching.
  • Lower severity and no public exploit yet
CISA-KEV: Not listedEPSS: —CVSS: 5.3Exploit: NoneExposed: 0

No vendor fix yet — apply a workaround or compensating control (WAF / firewall / segmentation) and watch for a patch.

Zebra has sync restart poisoning from single unauthenticated peer via above-lookahead block

Am I affected

You are affected if:

  • You run zebrad up to and including v4.4.1.
  • Your node accepts inbound P2P connections and is syncing or catching up to the chain tip.

Summary

A malicious peer can answer Zebra's outbound getblocks/FindBlocks request with a small two-hash inventory, then serve a syntactically valid block whose coinbase height is far above the victim's local tip. The AboveLookaheadHeightLimit error in the sync download pipeline triggers a global sync restart rather than being scoped to the offending peer. The peer is never scored or disconnected because the error type does not carry the advertiser address.

On mainnet, each successful cycle imposes a 67-second sync restart delay. All in-flight downloads from honest peers are cancelled on each restart.

Details

The bug is the interaction of three layers:

  • The syncer promotes unvalidated FindBlocks peer responses into concrete download schedules without checking that the advertised hashes are plausible chain extensions.
  • When a downloaded block's coinbase height exceeds tip + VERIFICATION_PIPELINE_DROP_LIMIT, the sync downloader returns BlockDownloadVerifyError::AboveLookaheadHeightLimit. This error variant carries only the block height and hash, not the advertiser peer address.
  • The sync error handler in handle_block_response only sends misbehaviour scores for BlockDownloadVerifyError::Invalid errors that carry an advertiser_addr and have a nonzero misbehavior_score(). AboveLookaheadHeightLimit falls through to the default restart-worthy path, cancelling all in-flight downloads and waiting 67 seconds before restarting sync.

The attacker needs only an unauthenticated P2P connection (post-handshake), a tiny payload (one two-hash inv message plus one small block per cycle), and no mining capability, funds, or valid chain data. The peer is never penalised, so the attack is repeatable indefinitely.

Additionally, several other pre-consensus sync-layer errors had zero misbehaviour scores even when peer-attributed. Contextual validation failures (InvalidDifficultyThreshold, TimeTooEarly, TimeTooLate, NonSequentialBlock) and locktime failures from block-serving peers all scored zero, allowing repeated abuse without penalty.

Patches

Patched in Zebra 4.4.2. The fix:

  • Carries advertiser_addr through AboveLookaheadHeightLimit and InvalidHeight error variants.
  • Makes above-lookahead and invalid-height failures peer-local (the block is dropped and the peer is banned with score 100) rather than triggering a global sync restart.
  • Expands misbehavior_score() across BlockError, VerifyBlockError, and CommitBlockError to cover contextual validation failures that previously scored zero.

Workarounds

No configuration-level workaround is available. The attack is mitigated by having a diverse honest peer set, but cannot be prevented while the vulnerable code is running.

Impact

A single unauthenticated peer can repeatedly stall a syncing Zebra node by forcing 67-second global sync restart cycles. All unrelated in-flight downloads are cancelled on each restart. The node does not crash and no state is corrupted, but sync progress is significantly degraded for the duration of the attack.

Credit

Reported by @ipwning.

CVSS v3
5.3
EG Score
5.3(low)
EPSS
—
KEV
Not listed

Published

July 2, 2026

Last Modified

July 2, 2026

Vendor Advisories for CVE-2026-52737(1)

These vendors published their own advisory mentioning this CVE — often with vendor-specific remediation steps + affected product lists not in NVD.

Data Freshness Timeline

(refreshed 5× in last 7d / 5× in last 30d)

Each row is a source pipeline that fetched or updated this CVE on that date, with what changed. For example, "NVD update" means NVD published or revised its analysis for this CVE; "MITRE cvelistV5" means we ingested or refreshed it from the CNA feed. Most recent first.

  1. 2026-07-07 03:25 UTCEG score recompute
  2. 2026-07-06 01:35 UTCEG score recompute
  3. 2026-07-04 23:46 UTCEG score recompute
  4. 2026-07-03 21:56 UTCEG score recompute
  5. 2026-07-02 20:07 UTCEG score recompute

Frequently asked(4)

What is CVE-2026-52737?
CVE-2026-52737 is a medium vulnerability published on July 2, 2026. Zebra has sync restart poisoning from single unauthenticated peer via above-lookahead block Am I affected You are affected if: 1. You run zebrad up to and including v4.4.1. 2. Your node accepts inbound P2P connections and is syncing or catching up to the chain tip. Summary A malicious peer can…
When was CVE-2026-52737 disclosed?
CVE-2026-52737 was first published in the National Vulnerability Database on July 2, 2026. EchelonGraph re-ingests CVE updates from NVD on a 2-hour cycle, so this page reflects the latest published state.
What is the CVSS score of CVE-2026-52737?
CVE-2026-52737 has a CVSS v4.0 base score of 5.3 (CNA self-assessment; NVD's own analysis pending). The EG score is currently aggregating — additional source signals are being incorporated as they become available..
How do I remediate CVE-2026-52737?
Patch to the fixed version published by the affected vendor. Where vendor advisories exist for CVE-2026-52737, EchelonGraph cross-links them in the Vendor Advisories panel below — those typically contain the canonical remediation steps, fixed version numbers, and any vendor-specific mitigations.

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