{
  "schema_version": "1.6.1",
  "id": "RPI-CVE-2025-21640",
  "modified": "2026-10-09T11:39:44Z",
  "upstream": [
    "CVE-2025-21640"
  ],
  "published": "2025-01-19T10:18:03Z",
  "summary": "sctp: sysctl: cookie_hmac_alg: avoid using current->nsproxy",
  "details": "In the Linux kernel, the following vulnerability has been resolved:\n\nsctp: sysctl: cookie_hmac_alg: avoid using current->nsproxy\n\nAs mentioned in a previous commit of this series, using the 'net'\nstructure via 'current' is not recommended for different reasons:\n\n- Inconsistency: getting info from the reader's/writer's netns vs only\n  from the opener's netns.\n\n- current->nsproxy can be NULL in some cases, resulting in an 'Oops'\n  (null-ptr-deref), e.g. when the current task is exiting, as spotted by\n  syzbot [1] using acct(2).\n\nThe 'net' structure can be obtained from the table->data using\ncontainer_of().\n\nNote that table->data could also be used directly, as this is the only\nmember needed from the 'net' structure, but that would increase the size\nof this fix, to use '*data' everywhere 'net->sctp.sctp_hmac_alg' is\nused.",
  "severity": [
    {
      "type": "CVSS_V3",
      "score": "CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:H/A:H"
    }
  ],
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  "references": [
    {
      "type": "ADVISORY",
      "url": "https://www.cve.org/CVERecord?id=CVE-2025-21640"
    },
    {
      "type": "FIX",
      "url": "https://git.kernel.org/stable/c/5599b212d2f4466e1832a94e9932684aaa364587"
    },
    {
      "type": "FIX",
      "url": "https://git.kernel.org/stable/c/03ca51faba2b017bf6c90e139434c4117d0afcdc"
    },
    {
      "type": "FIX",
      "url": "https://git.kernel.org/stable/c/86ddf8118123cb58a0fb8724cad6979c4069065b"
    },
    {
      "type": "FIX",
      "url": "https://git.kernel.org/stable/c/3cd0659deb9c03535fd61839e91d4d4d3e51ac71"
    },
    {
      "type": "FIX",
      "url": "https://git.kernel.org/stable/c/ad673e514b2793b8d5902f6ba6ab7e890dea23d5"
    },
    {
      "type": "FIX",
      "url": "https://git.kernel.org/stable/c/f0bb3935470684306e4e04793a20ac4c4b08de0b"
    },
    {
      "type": "FIX",
      "url": "https://git.kernel.org/stable/c/ea62dd1383913b5999f3d16ae99d411f41b528d4"
    }
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  "database_specific": {
    "source": "https://git.kernel.org/pub/scm/linux/security/vulns.git",
    "upstream_modified": "2026-08-05T12:19:11Z",
    "severity": {
      "source": "kernel CNA",
      "cvss_v3_score": 7.1,
      "cvss_v3_severity": "High",
      "scenario": "AV:L - The buggy handler is only reachable through local interfaces \u2014 a write/read on /proc/sys/net/sctp/cookie_hmac_alg or an acct(2)-pinned file \u2014 never from received network data. No remote peer input reaches proc_sctp_do_hmac_alg().\nAC:L - Both paths are deterministic and fully attacker-controlled: the netns-confusion write is a plain unshare + SCM_RIGHTS + write(2) sequence with no race, and the acct(2) NULL deref triggers reliably every time the pidns reaper exits. No memory-layout or timing conditions outside the attacker's control.\nPR:L - The cross-namespace path needs only an unprivileged local user, since unprivileged user namespaces (default-enabled on Ubuntu/Debian/Fedora/Android) make the caller owner of the new userns and thus CAP_NET_ADMIN-equivalent over its netns, letting it drive a write that lands in init_net. Only the secondary acct(2) oops path additionally needs CAP_SYS_PACCT.\nUI:N - The attacker performs every step itself \u2014 unshare, open, fd passing, write, or acct(2) plus process exit. No victim action or cooperation is involved.\nS:U - The vulnerable component and the impacted resource are both the kernel under the same security authority; per kernel CNA convention namespace/privilege boundary crossings within the kernel are scored Unchanged rather than as a Changed-scope escape.\nC:N - The read path always returns the reader's own netns value, so no data crosses a security boundary, and the NULL dereference discloses no memory contents. There is no read primitive or information leak.\nI:H - An unprivileged user can make the handler write the initial network namespace's sctp_hmac_alg; setting it to \"none\" leaves sp->hmac NULL so sctp_unpack_cookie() jumps to no_hmac and skips signature verification entirely, disabling SCTP state-cookie authentication host-wide and enabling blind association spoofing. That is a serious loss of integrity for a security-relevant global setting.\nA:H - With current->nsproxy NULL during task exit the handler dereferences a NULL pointer, producing a kernel Oops (syzbot-confirmed) \u2014 a full panic on panic_on_oops systems and at minimum a task killed in the middle of do_exit with namespaces already torn down."
    }
  }
}