Zebra before 4.5.0 CPU Amplification via Uncapped getblocks/getheaders Locator Length

CISA KEV is observed exploitation evidence. FIRST EPSS is a dated prediction of exploitation in the next 30 days. Missing scores are unknown. This collector requests predictions only for explicit CVE aliases in the public NuGet archive; Microsoft release views retain their existing prediction scope.

No collected risk observation for these identifiers. This does not establish absence of exploitation or a zero probability.

Zebra before 4.5.0 contains an uncontrolled resource consumption vulnerability that allows remote P2P peers to exhaust blocking-pool threads by sending oversized block locator vectors. Attackers can send getblocks or getheaders messages with up to 65,535 locator hashes, triggering per-hash chain lookups that degrade block validation, RPC, and mempool performance.

Source published

2 Oct 2026 11:38:13 UTC

Source modified

29 Oct 2026 21:15:28 UTC

Observed here

29 Oct 2026 22:48:55 UTC

Source terms

CVE Program Terms of Use

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CVSS 4.0 · 6.3 CVSS:4.0/AV:N/AC:H/AT:P/PR:N/UI:N/VC:N/VI:N/VA:L/SC:N/SI:N/SA:N

CVSS 3.1 · 3.7 CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:L

Zebra before 4.5.0 contains an uncontrolled resource consumption vulnerability that allows remote P2P peers to exhaust blocking-pool threads by sending oversized block locator vectors. Attackers can send getblocks or getheaders messages with up to 65,535 locator hashes, triggering per-hash chain lookups that degrade block validation, RPC, and mempool performance.

Source published

Oct 2, 2026, 11:38:13 AM (timezone not stated)

Source modified

Oct 29, 2026, 9:15:28 PM (timezone not stated)

Observed here

5 Nov 2026 03:55:22 UTC

Source terms

ENISA EUVD · source terms and attribution apply

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EUVD CVSS 4.0 · 6.3 CVSS:4.0/AV:N/AC:H/AT:P/PR:N/UI:N/VC:N/VI:N/VA:L/SC:N/SI:N/SA:N

Zebra before 4.5.0 contains an uncontrolled resource consumption vulnerability that allows remote P2P peers to exhaust blocking-pool threads by sending oversized block locator vectors. Attackers can send getblocks or getheaders messages with up to 65,535 locator hashes, triggering per-hash chain lookups that degrade block validation, RPC, and mempool performance.

Source published

2 Oct 2026 12:17:14 UTC

Source modified

29 Oct 2026 21:17:08 UTC

Observed here

31 Oct 2026 06:45:55 UTC

Source terms

NIST NVD usage terms; underlying source attribution retained

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This view projects the publisher's source fields for reading; the original JSON retains additional fields and CVE enrichment containers. Attribution: nvd · NIST NVD usage terms; underlying source attribution retained. Source text is retained; field grouping and change labels are provided by ChangeIntel.

CVSS 4.0 · [email protected] · Secondary · 6.3 CVSS:4.0/AV:N/AC:H/AT:P/PR:N/UI:N/VC:N/VI:N/VA:L/SC:N/SI:N/SA:N/E:X/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/MVI:X/MVA:X/MSC:X/MSI:X/MSA:X/S:X/AU:X/R:X/V:X/RE:X/U:X

CVSS 3.1 · [email protected] · Secondary · 3.7 CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:L

### 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. ### Summary The `read_getblocks` and `read_getheaders` codec paths accepted block locator vectors up to approximately 65,535 entries (the generic `TrustedPreallocate` ceiling derived from `MAX_PROTOCOL_MESSAGE_LEN`), rather than the protocol-specification limit of 101 entries (matching zcashd's `MAX_LOCATOR_SZ`). Each entry in the locator vector triggers a per-hash chain lookup (`HashMap::contains_key` + `RocksDB::contains_hash`) in `find_chain_intersection` on a tokio blocking-pool thread. A single maximally-sized `getblocks` message occupies one blocking-pool thread for approximately 10–65ms. Under sustained load from multiple peers, this can degrade state-read performance for block validation, RPC, and mempool lookups. ### Details The `read_headers` codec path already implements the correct pattern: it reads the CompactSize count, validates against `MAX_HEADERS_PER_MESSAGE = 160` before deserialization, and rejects oversized messages. The `read_getblocks` and `read_getheaders` paths were missing this pre-deserialization count check and instead relied on the generic `block::Hash::max_allocation()` bound, which allows `(MAX_PROTOCOL_MESSAGE_LEN - 1) / 32 = 65,535` hashes. A legitimate block locator is logarithmic in chain length (approximately 30 hashes for the current ~3M-block Zcash chain). Zebra's own send-side cap is `MAX_FIND_BLOCK_HASHES_RESULTS = 500`. The practical impact requires significant attacker bandwidth (approximately 2 MiB per request) and multiple Sybil peers to meaningfully degrade the blocking pool, which limits real-world exploitability. ### Patches Patched in Zebra 4.4.2. The fix caps `block::Hash::max_allocation()` at `MAX_BLOCK_LOCATOR_LENGTH = 101`, matching zcashd's `MAX_LOCATOR_SZ`. This causes the deserializer to reject oversized locators before any allocation or iteration occurs. ### Workarounds No specific workaround is needed. Existing backpressure mechanisms (load shedding, sequential per-peer message processing, connection limits) constrain the practical impact. ### Impact Under sustained load from multiple Sybil peers, oversized locator vectors can occupy blocking-pool threads and degrade state-read performance. The effect is bounded by connection limits and requires significant attacker bandwidth. ### Credit Vulnerability identified by `@dingledropper`, who submitted the fix in [PR #10570](https://github.com/ZcashFoundation/zebra/pull/10570). Downstream CPU/blocking-pool impact analysis contributed by `@ouicate`.

Source published

2 Jul 2026 19:34:21 UTC

Source modified

3 Oct 2026 03:56:03 UTC

Observed here

24 Oct 2026 13:19:54 UTC

Source terms

GitHub Advisory Database · CC-BY-4.0

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CVSS_V3 · CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:L

Publisher rating · LOW