CVE-2026-52906 (GCVE-0-2026-52906)

Vulnerability from cvelistv5 – Published: 2026-06-09 12:36 – Updated: 2026-08-05 12:31
VLAI
Title
9p: fix access mode flags being ORed instead of replaced
Summary
In the Linux kernel, the following vulnerability has been resolved: 9p: fix access mode flags being ORed instead of replaced Since commit 1f3e4142c0eb ("9p: convert to the new mount API"), v9fs_apply_options() applies parsed mount flags with |= onto flags already set by v9fs_session_init(). For 9P2000.L, session_init sets V9FS_ACCESS_CLIENT as the default, so when the user mounts with "access=user", both bits end up set. Access mode checks compare against exact values, so having both bits set matches neither mode. This causes v9fs_fid_lookup() to fall through to the default switch case, using INVALID_UID (nobody/65534) instead of current_fsuid() for all fid lookups. Root is then unable to chown or perform other privileged operations. Fix by clearing the access mask before applying the user's choice.
Severity
7.7 (High)
CVSS:3.1/AV:L/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:N
Assigner
References
Impacted products
Vendor Product Version
Linux Linux Affected: 1f3e4142c0eb178089ea0cbc97506a061470ad27 , < b8f037e87a083291190204b959cda417aaf01058 (git)
Affected: 1f3e4142c0eb178089ea0cbc97506a061470ad27 , < da2346a48a5a1fed86c3fe3d73c0b60e7b3027c9 (git)
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Linux Linux Affected: 6.19
Unaffected: 0 , < 6.19 (semver)
Unaffected: 7.0.4 , ≤ 7.0.* (semver)
Unaffected: 7.1 , ≤ * (original_commit_for_fix)
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{
  "containers": {
    "cna": {
      "affected": [
        {
          "defaultStatus": "unaffected",
          "product": "Linux",
          "programFiles": [
            "fs/9p/v9fs.c"
          ],
          "repo": "https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git",
          "vendor": "Linux",
          "versions": [
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              "status": "affected",
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              "versionType": "git"
            },
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              "status": "affected",
              "version": "1f3e4142c0eb178089ea0cbc97506a061470ad27",
              "versionType": "git"
            }
          ]
        },
        {
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          "product": "Linux",
          "programFiles": [
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          ],
          "repo": "https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git",
          "vendor": "Linux",
          "versions": [
            {
              "status": "affected",
              "version": "6.19"
            },
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              "status": "unaffected",
              "version": "0",
              "versionType": "semver"
            },
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              "status": "unaffected",
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              "versionType": "semver"
            },
            {
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              "status": "unaffected",
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        }
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                },
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                  "versionEndExcluding": "7.1",
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              "operator": "OR"
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          "lang": "en",
          "value": "In the Linux kernel, the following vulnerability has been resolved:\n\n9p: fix access mode flags being ORed instead of replaced\n\nSince commit 1f3e4142c0eb (\"9p: convert to the new mount API\"),\nv9fs_apply_options() applies parsed mount flags with |= onto flags\nalready set by v9fs_session_init(). For 9P2000.L, session_init sets\nV9FS_ACCESS_CLIENT as the default, so when the user mounts with\n\"access=user\", both bits end up set. Access mode checks compare\nagainst exact values, so having both bits set matches neither mode.\n\nThis causes v9fs_fid_lookup() to fall through to the default switch\ncase, using INVALID_UID (nobody/65534) instead of current_fsuid()\nfor all fid lookups. Root is then unable to chown or perform other\nprivileged operations.\n\nFix by clearing the access mask before applying the user\u0027s choice."
        }
      ],
      "metrics": [
        {
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            "baseScore": 7.7,
            "baseSeverity": "HIGH",
            "vectorString": "CVSS:3.1/AV:L/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:N",
            "version": "3.1"
          },
          "scenarios": [
            {
              "lang": "en",
              "value": "AV:L - The flaw is reached through local mount and VFS file-operation syscalls (open, read, write, setattr) on a 9p client mount, not through remote packet handling. Even when 9p uses virtio or TCP transport, exploitation requires local access to the mounted filesystem or local CAP_SYS_ADMIN to mount with the triggering options.\nAC:L - An attacker who can mount 9p with 9P2000.L and `access=user` or `access=\u003cuid\u003e` reliably triggers the bug on every fid lookup. On systems where virtio-9p is already mounted with those options, exploitation requires only normal filesystem access with no race or special timing.\nPR:N - Exploitation against an already-mounted vulnerable 9p share requires no elevated privileges\u2014only local shell access to the mount point. The worst case (`access=\u003cuid\u003e` SINGLE mode) is meant to block all other users, yet any unprivileged local user can still obtain fids and access the share.\nUI:N - Once a vulnerable 9p filesystem is mounted, no further victim interaction is required; the attacker simply performs normal file operations. Mounting with the triggering options is attacker-controlled or a one-time administrative setup, not ongoing user interaction at exploitation time.\nS:U - Impact is confined to unauthorized access within the guest/kernel filesystem boundary (bypassing 9p access-mode isolation between local users or against a single-user mount restriction). It does not cross VM-to-host, IOMMU, or sandbox security boundaries beyond the shared 9p export itself.\nC:H - The mangled access mask forces all operations through a shared INVALID_UID attach instead of per-user attaches, breaking confidentiality guarantees of `access=user` and completely defeating `access=\u003cuid\u003e` SINGLE-mode isolation. Unprivileged users can read data on mounts explicitly restricted to another user.\nI:H - The same fid-lookup failure lets unauthorized users perform writes through the shared INVALID_UID attach on mounts where SINGLE or per-user modes should have prevented any access. Server-side authorization keyed to attach identity is undermined, enabling unauthorized modification of files on the export.\nA:N - The bug is a logic error in access-mode handling that misroutes fid lookups; it does not cause kernel oops, panic, hang, or repeatable denial of service. Root losing chown capability on the mount is a localized authorization failure, not system-wide availability loss."
            }
          ]
        }
      ],
      "providerMetadata": {
        "dateUpdated": "2026-08-05T12:31:29.381Z",
        "orgId": "416baaa9-dc9f-4396-8d5f-8c081fb06d67",
        "shortName": "Linux"
      },
      "references": [
        {
          "url": "https://git.kernel.org/stable/c/b8f037e87a083291190204b959cda417aaf01058"
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        {
          "url": "https://git.kernel.org/stable/c/da2346a48a5a1fed86c3fe3d73c0b60e7b3027c9"
        }
      ],
      "title": "9p: fix access mode flags being ORed instead of replaced",
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  },
  "cveMetadata": {
    "assignerOrgId": "416baaa9-dc9f-4396-8d5f-8c081fb06d67",
    "assignerShortName": "Linux",
    "cveId": "CVE-2026-52906",
    "datePublished": "2026-06-09T12:36:03.521Z",
    "dateReserved": "2026-06-09T07:44:35.366Z",
    "dateUpdated": "2026-08-05T12:31:29.381Z",
    "state": "PUBLISHED"
  },
  "dataType": "CVE_RECORD",
  "dataVersion": "5.2",
  "vulnerability-lookup:meta": {
    "epss": {
      "cve": "CVE-2026-52906",
      "date": "2026-08-07",
      "epss": "0.00121",
      "percentile": "0.02191"
    },
    "nvd": "{\"cve\":{\"id\":\"CVE-2026-52906\",\"sourceIdentifier\":\"416baaa9-dc9f-4396-8d5f-8c081fb06d67\",\"published\":\"2026-06-09T14:16:45.047\",\"lastModified\":\"2026-07-23T08:10:00.137\",\"vulnStatus\":\"Analyzed\",\"cveTags\":[],\"descriptions\":[{\"lang\":\"en\",\"value\":\"In the Linux kernel, the following vulnerability has been resolved:\\n\\n9p: fix access mode flags being ORed instead of replaced\\n\\nSince commit 1f3e4142c0eb (\\\"9p: convert to the new mount API\\\"),\\nv9fs_apply_options() applies parsed mount flags with |= onto flags\\nalready set by v9fs_session_init(). For 9P2000.L, session_init sets\\nV9FS_ACCESS_CLIENT as the default, so when the user mounts with\\n\\\"access=user\\\", both bits end up set. Access mode checks compare\\nagainst exact values, so having both bits set matches neither mode.\\n\\nThis causes v9fs_fid_lookup() to fall through to the default switch\\ncase, using INVALID_UID (nobody/65534) instead of current_fsuid()\\nfor all fid lookups. Root is then unable to chown or perform other\\nprivileged operations.\\n\\nFix by clearing the access mask before applying the user\u0027s choice.\"},{\"lang\":\"es\",\"value\":\"En el kernel de Linux, la siguiente vulnerabilidad ha sido resuelta:\\n\\n9p: corrige que los indicadores de modo de acceso se combinen con OR en lugar de ser reemplazados\\n\\nDesde el commit 1f3e4142c0eb (\u00279p: convertir a la nueva API de montaje\u0027),\\nv9fs_apply_options() aplica los indicadores de montaje analizados con |= sobre los indicadores ya establecidos por v9fs_session_init(). Para 9P2000.L, session_init establece V9FS_ACCESS_CLIENT como predeterminado, as\u00ed que cuando el usuario monta con \u0027access=user\u0027, ambos bits terminan establecidos. Las comprobaciones de modo de acceso comparan con valores exactos, as\u00ed que tener ambos bits establecidos no coincide con ninguno de los modos.\\n\\nEsto hace que v9fs_fid_lookup() caiga al caso predeterminado del switch, usando INVALID_UID (nobody/65534) en lugar de current_fsuid() para todas las b\u00fasquedas de fid. Root es entonces incapaz de chown o realizar otras operaciones privilegiadas.\\n\\nSe corrige limpiando la m\u00e1scara de acceso antes de aplicar la elecci\u00f3n del usuario.\"}],\"affected\":[{\"source\":\"416baaa9-dc9f-4396-8d5f-8c081fb06d67\",\"affectedData\":[{\"vendor\":\"Linux\",\"product\":\"Linux\",\"defaultStatus\":\"unaffected\",\"programFiles\":[\"fs/9p/v9fs.c\"],\"repo\":\"https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git\",\"versions\":[{\"version\":\"1f3e4142c0eb178089ea0cbc97506a061470ad27\",\"lessThan\":\"b8f037e87a083291190204b959cda417aaf01058\",\"versionType\":\"git\",\"status\":\"affected\"},{\"version\":\"1f3e4142c0eb178089ea0cbc97506a061470ad27\",\"lessThan\":\"da2346a48a5a1fed86c3fe3d73c0b60e7b3027c9\",\"versionType\":\"git\",\"status\":\"affected\"}]},{\"vendor\":\"Linux\",\"product\":\"Linux\",\"defaultStatus\":\"affected\",\"programFiles\":[\"fs/9p/v9fs.c\"],\"repo\":\"https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git\",\"versions\":[{\"version\":\"6.19\",\"status\":\"affected\"},{\"version\":\"0\",\"lessThan\":\"6.19\",\"versionType\":\"semver\",\"status\":\"unaffected\"},{\"version\":\"7.0.4\",\"lessThanOrEqual\":\"7.0.*\",\"versionType\":\"semver\",\"status\":\"unaffected\"},{\"version\":\"7.1\",\"lessThanOrEqual\":\"*\",\"versionType\":\"original_commit_for_fix\",\"status\":\"unaffected\"}]}]}],\"metrics\":{\"cvssMetricV31\":[{\"source\":\"416baaa9-dc9f-4396-8d5f-8c081fb06d67\",\"type\":\"Secondary\",\"cvssData\":{\"version\":\"3.1\",\"vectorString\":\"CVSS:3.1/AV:L/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:N\",\"baseScore\":7.7,\"baseSeverity\":\"HIGH\",\"attackVector\":\"LOCAL\",\"attackComplexity\":\"LOW\",\"privilegesRequired\":\"NONE\",\"userInteraction\":\"NONE\",\"scope\":\"UNCHANGED\",\"confidentialityImpact\":\"HIGH\",\"integrityImpact\":\"HIGH\",\"availabilityImpact\":\"NONE\"},\"exploitabilityScore\":2.5,\"impactScore\":5.2}]},\"weaknesses\":[{\"source\":\"nvd@nist.gov\",\"type\":\"Primary\",\"description\":[{\"lang\":\"en\",\"value\":\"NVD-CWE-noinfo\"}]}],\"configurations\":[{\"nodes\":[{\"operator\":\"OR\",\"negate\":false,\"cpeMatch\":[{\"vulnerable\":true,\"criteria\":\"cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*\",\"versionStartIncluding\":\"6.19\",\"versionEndExcluding\":\"7.0.4\",\"matchCriteriaId\":\"CDB78D6D-22C3-4154-B0D0-94AF1CE5C2E3\"}]}]}],\"references\":[{\"url\":\"https://git.kernel.org/stable/c/b8f037e87a083291190204b959cda417aaf01058\",\"source\":\"416baaa9-dc9f-4396-8d5f-8c081fb06d67\",\"tags\":[\"Patch\"]},{\"url\":\"https://git.kernel.org/stable/c/da2346a48a5a1fed86c3fe3d73c0b60e7b3027c9\",\"source\":\"416baaa9-dc9f-4396-8d5f-8c081fb06d67\",\"tags\":[\"Patch\"]}]}}",
    "redhat_vex": {
      "aggregate_severity": "None",
      "current_release_date": "2026-06-28T07:43:34+00:00",
      "cve": "CVE-2026-52906",
      "id": "CVE-2026-52906",
      "initial_release_date": "2026-06-09T00:00:00+00:00",
      "product_status:known_not_affected": "274",
      "source": "Red Hat CSAF VEX",
      "status": "final",
      "title": "kernel: 9p: fix access mode flags being ORed instead of replaced",
      "url": "https://security.access.redhat.com/data/csaf/v2/vex/2026/cve-2026-52906.json",
      "version": "3"
    }
  }
}



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  • Seen: The vulnerability was mentioned, discussed, or observed by the user.
  • Confirmed: The vulnerability has been validated from an analyst's perspective.
  • Published Proof of Concept: A public proof of concept is available for this vulnerability.
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  • Not confirmed: The user expressed doubt about the validity of the vulnerability.
  • Not patched: The vulnerability was not observed as successfully patched by the user who reported the sighting.


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