{"id":"CVE-2026-104436","aliases":["GHSA-443g-gwgp-49x4"],"url":"https://o3.security/vulnerability/CVE-2026-104436","summary":"Zebra before 4.5.0 CPU Amplification via Uncapped getblocks/getheaders Locator Length","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.\n\n### Summary\n\nThe `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.\n\nA 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.\n\n### Details\n\nThe `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.\n\nA 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`.\n\nThe 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.\n\n### Patches\n\nPatched 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.\n\n### Workarounds\n\nNo specific workaround is needed. Existing backpressure mechanisms (load shedding, sequential per-peer message processing, connection limits) constrain the practical impact.\n\n### Impact\n\nUnder 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.\n\n### Credit\n\nVulnerability 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`.","published":"2026-10-02T11:38:13.172Z","modified":"2026-10-04T02:46:36.862147690Z","cvss":null,"epss":null,"cisaKev":null,"exploitsKnown":null,"affectedPackages":[{"ecosystem":"crates.io","name":"zebrad","fixedVersion":"4.5.0"},{"ecosystem":"crates.io","name":"zebra-chain","fixedVersion":"8.0.0"}],"fix":{"url":"https://github.com/ZcashFoundation/zebra/pull/10570","label":"ZcashFoundation/zebra#10570"},"references":[{"type":"ADVISORY","url":"https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/104xxx/CVE-2026-104436.json"},{"type":"ADVISORY","url":"https://github.com/ZcashFoundation/zebra/security/advisories/GHSA-443g-gwgp-49x4"},{"type":"ADVISORY","url":"https://nvd.nist.gov/vuln/detail/CVE-2026-104436"},{"type":"ADVISORY","url":"https://www.vulncheck.com/advisories/zebra-before-4.5.0-cpu-amplification-via-uncapped-getblocks-getheaders-locator-length"},{"type":"WEB","url":"https://github.com/ZcashFoundation/zebra/pull/10570"},{"type":"WEB","url":"https://github.com/ZcashFoundation/zebra/commit/8981a1b95d4807cad99e5bb3b94fc8bc723ac033"},{"type":"PACKAGE","url":"https://github.com/ZcashFoundation/zebra"}],"provenance":{"sources":["OSV.dev","FIRST.org (EPSS)"],"lastVerified":"2026-10-04T02:46:36.862147690Z"}}