Vulnerability-Lookup

CVE-2026-52737 (GCVE-0-2026-52737)

Vulnerability from cvelistv5 – Published: 2026-08-18 19:19 – Updated: 2026-08-18 19:33

Title

ZEBRA: Sync restart poisoning from single unauthenticated peer via above-lookahead block

Summary

ZEBRA is a Zcash node written entirely in Rust. Prior to 4.5.0, a malicious unauthenticated P2P peer can answer Zebra's outbound getblocks or FindBlocks request with a small two-hash inventory and then serve a syntactically valid block whose coinbase height is far above the local chain tip. In zebrad/src/components/sync/downloads.rs, BlockDownloadVerifyError::AboveLookaheadHeightLimit originally carried only the block height and hash, so handle_block_response could not attribute the failure to the advertising peer. The error then reached the restart-worthy default path in zebrad/src/components/sync.rs, cancelling all in-flight downloads from honest peers and imposing a 67-second sync restart delay on mainnet. Because the peer was neither scored nor disconnected, the peer could repeat the cycle indefinitely with minimal bandwidth and significantly degrade synchronization without corrupting state. This issue is fixed in version 4.5.0.

Severity

5.3 (Medium)


                        
                          CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L

SSVC

Exploitation: none Automatable: yes Technical Impact: partial

CISA Coordinator · CISA-ADP (v2.0.3)

Decision recorded 2026-08-18 19:33 UTC

CWE

CWE-345 - Insufficient Verification of Data Authenticity

Assigner

GitHub_M mitre root

References

5 references

URL	Tags
https://github.com/ZcashFoundation/zebra/security…	x_refsource_CONFIRM
https://github.com/ZcashFoundation/zebra/pull/10647	x_refsource_MISC
https://github.com/ZcashFoundation/zebra/commit/1…	x_refsource_MISC
https://github.com/ZcashFoundation/zebra/commit/b…	x_refsource_MISC
https://github.com/ZcashFoundation/zebra/releases…	x_refsource_MISC

Impacted products

2 products

Vendor	Product	Version	CPE status
ZcashFoundation	zebra	Affected: < 4.5.0	guessed
ZcashFoundation	zebra-consensus	Affected: < 7.0.0	guessed

Show details on NVD website

JSON

To clipboard

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FKIE_CVE-2026-52737

Vulnerability from fkie_nvd - Published: 2026-08-18 20:17 - Updated: 2026-08-18 20:17

Severity

5.3 (Medium) - CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L

Summary

References

	URL	Tags
	security-advisories@github.com	https://github.com/ZcashFoundation/zebra/commit/1440b43ca7df59aca948090d45117557b217a6cd
	security-advisories@github.com	https://github.com/ZcashFoundation/zebra/commit/bc64dfe45746a86d14546a0a5a766387dd5a2942
	security-advisories@github.com	https://github.com/ZcashFoundation/zebra/pull/10647
	security-advisories@github.com	https://github.com/ZcashFoundation/zebra/releases/tag/v4.5.0
	security-advisories@github.com	https://github.com/ZcashFoundation/zebra/security/advisories/GHSA-gvjc-3w7c-92jx

Impacted products

	Vendor	Product	Version

JSON

To clipboard

{
  "affected": [
    {
      "affectedData": [
        {
          "product": "zebra",
          "vendor": "ZcashFoundation",
          "versions": [
            {
              "status": "affected",
              "version": "\u003c 4.5.0"
            }
          ]
        },
        {
          "product": "zebra-consensus",
          "vendor": "ZcashFoundation",
          "versions": [
            {
              "status": "affected",
              "version": "\u003c 7.0.0"
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    }
  ],
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      "lang": "en",
      "value": "ZEBRA is a Zcash node written entirely in Rust. Prior to 4.5.0, a malicious unauthenticated P2P peer can answer Zebra\u0027s outbound getblocks or FindBlocks request with a small two-hash inventory and then serve a syntactically valid block whose coinbase height is far above the local chain tip. In zebrad/src/components/sync/downloads.rs, BlockDownloadVerifyError::AboveLookaheadHeightLimit originally carried only the block height and hash, so handle_block_response could not attribute the failure to the advertising peer. The error then reached the restart-worthy default path in zebrad/src/components/sync.rs, cancelling all in-flight downloads from honest peers and imposing a 67-second sync restart delay on mainnet. Because the peer was neither scored nor disconnected, the peer could repeat the cycle indefinitely with minimal bandwidth and significantly degrade synchronization without corrupting state. This issue is fixed in version 4.5.0."
    }
  ],
  "id": "CVE-2026-52737",
  "lastModified": "2026-08-18T20:17:17.750",
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GHSA-GVJC-3W7C-92JX

Vulnerability from github – Published: 2026-07-02 19:44 – Updated: 2026-07-02 19:44

Summary

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

Details

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.

Severity

5.3 (Medium)


                  
                    CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L

Show details on source website

JSON

To clipboard

{
  "affected": [
    {
      "database_specific": {
        "last_known_affected_version_range": "\u003c= 6.0.0"
      },
      "package": {
        "ecosystem": "crates.io",
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      },
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            {
              "introduced": "0"
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            }
          ],
          "type": "ECOSYSTEM"
        }
      ]
    },
    {
      "database_specific": {
        "last_known_affected_version_range": "\u003c= 4.4.1"
      },
      "package": {
        "ecosystem": "crates.io",
        "name": "zebrad"
      },
      "ranges": [
        {
          "events": [
            {
              "introduced": "0"
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            }
          ],
          "type": "ECOSYSTEM"
        }
      ]
    }
  ],
  "aliases": [
    "CVE-2026-52737"
  ],
  "database_specific": {
    "cwe_ids": [
      "CWE-345"
    ],
    "github_reviewed": true,
    "github_reviewed_at": "2026-07-02T19:44:24Z",
    "nvd_published_at": null,
    "severity": "MODERATE"
  },
  "details": "### Am I affected\n\nYou are affected if:\n\n1. You run `zebrad` up to and including `v4.4.1`.\n2. Your node accepts inbound P2P connections and is syncing or catching up to the chain tip.\n\n### Summary\n\nA malicious peer can answer Zebra\u0027s outbound `getblocks`/`FindBlocks` request with a small two-hash inventory, then serve a syntactically valid block whose coinbase height is far above the victim\u0027s 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.\n\nOn mainnet, each successful cycle imposes a 67-second sync restart delay. All in-flight downloads from honest peers are cancelled on each restart.\n\n### Details\n\nThe bug is the interaction of three layers:\n\n1. The syncer promotes unvalidated `FindBlocks` peer responses into concrete download schedules without checking that the advertised hashes are plausible chain extensions.\n\n2. When a downloaded block\u0027s 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.\n\n3. 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.\n\nThe 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.\n\nAdditionally, 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.\n\n### Patches\n\nPatched in Zebra 4.4.2. The fix:\n\n- Carries `advertiser_addr` through `AboveLookaheadHeightLimit` and `InvalidHeight` error variants.\n- 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.\n- Expands `misbehavior_score()` across `BlockError`, `VerifyBlockError`, and `CommitBlockError` to cover contextual validation failures that previously scored zero.\n\n### Workarounds\n\nNo 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.\n\n### Impact\n\nA 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.\n\n### Credit\n\nReported by `@ipwning`.",
  "id": "GHSA-gvjc-3w7c-92jx",
  "modified": "2026-07-02T19:44:25Z",
  "published": "2026-07-02T19:44:24Z",
  "references": [
    {
      "type": "WEB",
      "url": "https://github.com/ZcashFoundation/zebra/security/advisories/GHSA-gvjc-3w7c-92jx"
    },
    {
      "type": "PACKAGE",
      "url": "https://github.com/ZcashFoundation/zebra"
    }
  ],
  "schema_version": "1.4.0",
  "severity": [
    {
      "score": "CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L",
      "type": "CVSS_V3"
    }
  ],
  "summary": "Zebra has sync restart poisoning from single unauthenticated peer via above-lookahead block"
}

Sightings

Author	Source	Type	Date	Other

Nomenclature

Seen: The vulnerability was mentioned, discussed, or observed by the user.
Confirmed: The vulnerability has been validated from an analyst's perspective.
Published Proof of Concept: A public proof of concept is available for this vulnerability.
Exploited: The vulnerability was observed as exploited by the user who reported the sighting.
Patched: The vulnerability was observed as successfully patched by the user who reported the sighting.
Not exploited: The vulnerability was not observed as exploited by the user who reported the sighting.
Not confirmed: The user expressed doubt about the validity of the vulnerability.
Not patched: The vulnerability was not observed as successfully patched by the user who reported the sighting.

Detection rules are retrieved from Rulezet.

The MITRE ATT&CK techniques below are AI-generated suggestions, inferred from the description of the vulnerability by the CIRCL/vulnerability-attack-technique-classification-roberta-base model, served locally by ML-Gateway. They have not been verified by an analyst and are provided for guidance only.
The approach is described in our paper Mapping CVEs to MITRE ATT&CK Techniques: A Curated Gold-Set Classifier and the Limits of LLM-Assisted Label Expansion.

Action not permitted

CVE-2026-52737 (GCVE-0-2026-52737)

FKIE_CVE-2026-52737

GHSA-GVJC-3W7C-92JX

Am I affected

Summary

Details

Patches

Workarounds

Impact

Credit

Tags

Sightings

Nomenclature