{"id":"CVE-2026-55831","aliases":["GHSA-6jqx-86gh-f27w"],"url":"https://o3.security/vulnerability/CVE-2026-55831","summary":"Netty SPDY SETTINGS frame count materializes unbounded settings map","details":"### Summary\nNetty's SPDY SETTINGS decoder accepts a peer-declared SETTINGS entry count up to the 24-bit frame-length limit and materializes every unique setting ID in `DefaultSpdySettingsFrame` without an implementation-level count cap. A remote SPDY/3.1 peer can send one syntactically valid roughly 2 MiB SETTINGS frame that creates 262144 map entries, amplifying network input into heap growth and ordered-map insertion work.\n\n### Details\nInbound SPDY bytes enter `SpdyFrameCodec.decode()` and are passed directly to the frame decoder. The decoder reads the peer-controlled flags and 24-bit frame length from the common header, then accepts SETTINGS frames with only `length >= 4`. For SETTINGS payloads, it reads the peer-controlled `numSettings` field and validates only that the remaining payload is divisible into 8-byte entries and exactly matches that count. Each accepted entry then supplies an attacker-controlled 24-bit ID and value, and the normal delegate path forwards it into `spdySettingsFrame.setValue()`. The sink is `DefaultSpdySettingsFrame`: it backs settings with a `TreeMap`, checks only that IDs fit the SPDY 24-bit maximum, and inserts a new `Setting` for each previously unseen ID. There is no count budget between the wire-format count validation and the TreeMap insertion site.\n\n### PoC\n[poc.zip](https://github.com/user-attachments/files/28445737/poc.zip)\n\nrun with\n```bash\nbash ./poc/run.sh\n```\n\nexpected output:\n```text\nNETTY_SPDY_SETTINGS_COUNT_MAP_TRIGGERED settings_count=262144 wire_bytes=2097164 approx_heap_delta=17692272 first_value=1 last_value=262144\n```\n\nThe `NETTY_SPDY_SETTINGS_COUNT_MAP_TRIGGERED` line means the harness decoded the crafted SETTINGS frame and observed all 262144 peer-selected IDs in the resulting settings map. The `wire_bytes=2097164`, `first_value=1`, and `last_value=262144` fields distinguish this from a setup failure: they show the exact oversized frame was accepted and fully materialized.\n\n### Impact\n remote unauthenticated network peer that can speak SPDY/3.1 to a Netty pipeline containing `SpdyFrameCodec` can trigger resource-exhaustion denial of service. The required guards are satisfied by a complete valid SETTINGS frame using the expected SPDY version, a length of `4 + numSettings * 8`, and IDs within the accepted 24-bit range; the verified PoC uses `numSettings=262144` and `wire_bytes=2097164`. On that input, Netty materializes 262144 attacker-controlled entries in a TreeMap-backed `DefaultSpdySettingsFrame`, with local runs observing about 17-18 MiB of heap growth per decoded frame plus CPU work for ordered-map insertion.","published":"2026-07-20T23:00:31.859Z","modified":"2026-08-12T16:09:48.093704Z","cvss":{"score":7.5,"severity":"HIGH","vector":"CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H"},"epss":{"score":0.00439,"percentile":0.37455,"asOf":"2026-09-17"},"cisaKev":null,"exploitsKnown":0,"affectedPackages":[{"ecosystem":"Maven","name":"io.netty:netty-codec-http","fixedVersion":"4.2.16.Final"},{"ecosystem":"Maven","name":"io.netty:netty-codec-http","fixedVersion":"4.1.136.Final"}],"fix":{"url":"https://github.com/netty/netty/commit/5b68c61f37aa4a3045cba624cbea239655c9003b","label":"netty/netty@5b68c61"},"references":[{"type":"WEB","url":"https://github.com/netty/netty/releases/tag/netty-4.1.136.Final"},{"type":"WEB","url":"https://github.com/netty/netty/releases/tag/netty-4.2.16.Final"},{"type":"ADVISORY","url":"https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/55xxx/CVE-2026-55831.json"},{"type":"ADVISORY","url":"https://github.com/netty/netty/security/advisories/GHSA-6jqx-86gh-f27w"},{"type":"ADVISORY","url":"https://nvd.nist.gov/vuln/detail/CVE-2026-55831"},{"type":"FIX","url":"https://github.com/netty/netty/commit/5b68c61f37aa4a3045cba624cbea239655c9003b"},{"type":"FIX","url":"https://github.com/netty/netty/commit/bb2ff68a1fb71cb4b0eb9a9e17b66c52aff680c6"},{"type":"PACKAGE","url":"https://github.com/netty/netty"}],"provenance":{"sources":["OSV.dev","FIRST.org (EPSS)"],"lastVerified":"2026-08-12T16:09:48.093704Z"}}