CVE-2026-42577: Netty transport classes epoll Denial of Service Vulnerability

{
  "schema_version" : "1.4.0",
  "id" : "GHSA-rwm7-x88c-3g2p",
  "modified" : "2026-07-24T19:05:52Z",
  "published" : "2026-05-06T23:10:41Z",
  "aliases" : [ "CVE-2026-42577" ],
  "summary" : "Netty epoll transport denial of service via RST on half-closed TCP connection",
  "details" : "## Summary\n\nNetty's epoll transport fails to detect and close TCP connections that receive a RST after being half-closed, leading to stale channels that are never cleaned up and, in some code paths, a 100% CPU busy-loop in the event loop thread.\n\n## Affected versions\n\nAll versions of 4.2.x `netty-transport-classes-epoll` up to and including 4.2.12.Final\n\n## Fixed in\n\n4.2.13.Final (fix merged into the `4.2` branch via [#16689](https://github.com/netty/netty/pull/16689); release not yet cut as of 2026-04-25).\n\n## Severity\n\n**Medium** — Denial of Service (resource exhaustion / CPU spin)\n\n**CVSS:** 3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H - **7.5**\n\n**CWE:** CWE-772: Missing Release of Resource after Effective Lifetime\n\n## Description\n\nWhen a TCP connection using Netty's epoll transport has `ALLOW_HALF_CLOSURE` enabled (or is in a half-closed state via the HTTP codec), and the remote peer:\n\n1. Sends a FIN (half-close), causing the server to mark the input as shutdown, then\n2. Sends a RST (e.g. by closing with `SO_LINGER=0`)\n\nthe server-side channel is never closed. This happens because:\n\n- `epollOutReady()` is a no-op when there is no pending flush.\n- `epollInReady()` short-circuits via `shouldBreakEpollInReady()` because input is already marked as shutdown.\n- The `EPOLLERR`/`EPOLLHUP` error condition is therefore never processed, and `channelInactive` is never fired.\n\nDepending on the Netty version and configuration, this results in:\n\n- **Stale channels**: The connection is never closed or deregistered. An unauthenticated remote attacker can repeat the sequence to accumulate stale connections, exhausting file descriptors, memory, or connection-count limits.\n- **CPU busy-loop**: In code paths where `clearEpollIn0()` is not called during the `ChannelInputShutdownReadComplete` event, `epoll_wait` returns immediately on every iteration for the affected fd, causing 100% CPU utilization on the event loop thread and starving all other connections multiplexed on it.\n\n## Mitigation\n\n- Upgrade to 4.2.13.Final when released (or build from the `4.2` branch at commit [`0ec3d97`](https://github.com/netty/netty/commit/0ec3d97fab376e243d328ac95fbd288ba0f6e22d)).\n- If upgrading is not immediately possible, configure idle timeouts on connections to limit the lifetime of stale channels.\n\n## References\n\n- Issue: https://github.com/netty/netty/issues/16683\n- Fix: https://github.com/netty/netty/pull/16689",
  "severity" : [ {
    "type" : "CVSS_V3",
    "score" : "CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H"
  } ],
  "affected" : [ {
    "package" : {
      "ecosystem" : "Maven",
      "name" : "io.netty:netty-transport-classes-epoll"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "4.2.0.Final"
      }, {
        "fixed" : "4.2.13.Final"
      } ]
    } ]
  } ],
  "references" : [ {
    "type" : "WEB",
    "url" : "https://github.com/netty/netty/security/advisories/GHSA-rwm7-x88c-3g2p"
  }, {
    "type" : "ADVISORY",
    "url" : "https://nvd.nist.gov/vuln/detail/CVE-2026-42577"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/netty/netty/pull/16689"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/netty/netty/commit/0ec3d97fab376e243d328ac95fbd288ba0f6e22d"
  }, {
    "type" : "PACKAGE",
    "url" : "https://github.com/netty/netty"
  } ],
  "database_specific" : {
    "cwe_ids" : [ "CWE-772" ],
    "severity" : "HIGH",
    "github_reviewed" : true,
    "github_reviewed_at" : "2026-05-06T23:10:41Z",
    "nvd_published_at" : "2026-05-13T19:17:23Z"
  }
}