CVE-2026-63808 (GCVE-0-2026-63808)
Vulnerability from cvelistv5
Published
2026-07-19 12:02
Modified
2026-08-17 04:51
Severity ?
9.8 (Critical) - CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H
Summary
In the Linux kernel, the following vulnerability has been resolved: exfat: fix potential use-after-free in exfat_find_dir_entry() In exfat_find_dir_entry(), the buffer_head obtained from exfat_get_dentry() is released with brelse(bh) before the fall-through TYPE_EXTEND branch reads the directory entry through ep (which points into bh->b_data): brelse(bh); if (entry_type == TYPE_EXTEND) { ... len = exfat_extract_uni_name(ep, entry_uniname); ... } After brelse() drops our reference, nothing guarantees that the underlying page backing bh->b_data remains valid for the subsequent exfat_extract_uni_name() read. This is the same pattern fixed in commit fc961522ddbd ("exfat: Fix potential use after free in exfat_load_upcase_table()"). Move brelse(bh) so it runs after ep is no longer dereferenced on each branch. Confirmed on QEMU x86_64 with CONFIG_KASAN=y + CONFIG_DEBUG_PAGEALLOC=y + CONFIG_PAGE_POISONING=y on linux-next, using a crafted exFAT image (long filename with same-hash collisions forcing the TYPE_EXTEND path). With a debug-only invalidate_bdev() inserted between brelse(bh) and the ep read to make the stale-deref window deterministic, the unpatched kernel faults: BUG: KASAN: use-after-free in exfat_find_dir_entry+0x133b/0x15a0 BUG: unable to handle page fault for address: ffff88801a5fa0c2 Oops: 0000 [#1] SMP DEBUG_PAGEALLOC KASAN NOPTI RIP: 0010:exfat_find_dir_entry+0x1188/0x15a0 With this patch applied, the same instrumented harness completes cleanly under the same sanitizer stack. I have not reproduced a crash on an uninstrumented kernel under ordinary reclaim; the instrumented A/B establishes the lifetime violation and that the patch closes it, not an unaided triggerability claim.
References
Impacted products
Vendor Product Version
Linux Linux Version: ca06197382bde0a3bc20215595d1c9ce20c6e341
Version: ca06197382bde0a3bc20215595d1c9ce20c6e341
Version: ca06197382bde0a3bc20215595d1c9ce20c6e341
Version: ca06197382bde0a3bc20215595d1c9ce20c6e341
Version: ca06197382bde0a3bc20215595d1c9ce20c6e341
Version: ca06197382bde0a3bc20215595d1c9ce20c6e341
Version: ca06197382bde0a3bc20215595d1c9ce20c6e341
Version: ca06197382bde0a3bc20215595d1c9ce20c6e341
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{
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          "product": "Linux",
          "programFiles": [
            "fs/exfat/dir.c"
          ],
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            }
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      "cpeApplicability": [
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              "negate": false,
              "operator": "OR"
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      ],
      "descriptions": [
        {
          "lang": "en",
          "value": "In the Linux kernel, the following vulnerability has been resolved:\n\nexfat: fix potential use-after-free in exfat_find_dir_entry()\n\nIn exfat_find_dir_entry(), the buffer_head obtained from\nexfat_get_dentry() is released with brelse(bh) before the fall-through\nTYPE_EXTEND branch reads the directory entry through ep (which points\ninto bh-\u003eb_data):\n\n\tbrelse(bh);\n\tif (entry_type == TYPE_EXTEND) {\n\t\t...\n\t\tlen = exfat_extract_uni_name(ep, entry_uniname);\n\t\t...\n\t}\n\nAfter brelse() drops our reference, nothing guarantees that the\nunderlying page backing bh-\u003eb_data remains valid for the subsequent\nexfat_extract_uni_name() read. This is the same pattern fixed in\ncommit fc961522ddbd (\"exfat: Fix potential use after free in\nexfat_load_upcase_table()\").\n\nMove brelse(bh) so it runs after ep is no longer dereferenced on\neach branch.\n\nConfirmed on QEMU x86_64 with CONFIG_KASAN=y + CONFIG_DEBUG_PAGEALLOC=y\n+ CONFIG_PAGE_POISONING=y on linux-next, using a crafted exFAT image\n(long filename with same-hash collisions forcing the TYPE_EXTEND path).\nWith a debug-only invalidate_bdev() inserted between brelse(bh) and\nthe ep read to make the stale-deref window deterministic, the\nunpatched kernel faults:\n\n  BUG: KASAN: use-after-free in exfat_find_dir_entry+0x133b/0x15a0\n  BUG: unable to handle page fault for address: ffff88801a5fa0c2\n  Oops: 0000 [#1] SMP DEBUG_PAGEALLOC KASAN NOPTI\n  RIP: 0010:exfat_find_dir_entry+0x1188/0x15a0\n\nWith this patch applied, the same instrumented harness completes\ncleanly under the same sanitizer stack. I have not reproduced a\ncrash on an uninstrumented kernel under ordinary reclaim; the\ninstrumented A/B establishes the lifetime violation and that the\npatch closes it, not an unaided triggerability claim."
        }
      ],
      "metrics": [
        {
          "cvssV3_1": {
            "baseScore": 9.8,
            "baseSeverity": "CRITICAL",
            "vectorString": "CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H",
            "version": "3.1"
          },
          "scenarios": [
            {
              "lang": "en",
              "value": "AV:N - exfat_find_dir_entry() is invoked during VFS directory lookups on mounted exFAT volumes, reachable remotely when that volume is exported via ksmbd or nfsd (common on NAS/router USB shares). A network client resolving paths on the share triggers server-side exfat_lookup without local shell access.\nAC:L - The attacker fully controls the crafted exFAT image to force the TYPE_EXTEND code path and can repeatedly trigger lookups (including concurrent SMB/NFS requests and memory pressure) to win the post-brelse buffer reclaim window. UAF lifetime violations are treated as low complexity per kernel guidance.\nPR:N - Exploitation requires only access to the mounted exFAT filesystem, not real root; guest/anonymous SMB shares on consumer NAS devices and auto-mounted removable media grant unprivileged attackers filesystem access without elevated kernel credentials.\nUI:N - Once a crafted exFAT image is present on a shared or auto-mounted volume, triggering the bug requires only automated path lookups (SMB OPEN/LOOKUP, openat, statx) with no additional victim interaction beyond normal filesystem access.\nS:U - Successful exploitation compromises kernel memory within the same security authority; this is a standard kernel memory corruption issue, not a cross-boundary escape such as VM guest-to-host breakout.\nC:H - The UAF reads freed buffer_head page cache memory through exfat_extract_uni_name(), enabling out-of-bounds kernel heap reads and information disclosure from attacker-influenced reclaimed data.\nI:H - Use-after-free on buffer_head data provides a foundation for heap grooming and arbitrary memory corruption primitives that can be leveraged for kernel code execution and integrity compromise.\nA:H - The bug is a confirmed kernel use-after-free that produced a KASAN fault and page fault oops; even without full exploitation it can cause kernel crashes and denial of service."
            }
          ]
        }
      ],
      "providerMetadata": {
        "dateUpdated": "2026-08-17T04:51:14.402Z",
        "orgId": "416baaa9-dc9f-4396-8d5f-8c081fb06d67",
        "shortName": "Linux"
      },
      "references": [
        {
          "url": "https://git.kernel.org/stable/c/e6f1a11cfb808441a43ffae9b476cc135732cd27"
        },
        {
          "url": "https://git.kernel.org/stable/c/e48f413c2815787b8cade2795e194e3c4cd782ef"
        },
        {
          "url": "https://git.kernel.org/stable/c/06c4e1e9967d332ac33ba38b7819851089ff9359"
        },
        {
          "url": "https://git.kernel.org/stable/c/8e0abc17fbd7e305802e84fe98b4950d50f9c433"
        },
        {
          "url": "https://git.kernel.org/stable/c/4d101016d5e587f820b3ae2d5bb6770d86342649"
        },
        {
          "url": "https://git.kernel.org/stable/c/adfacfbaeae2cb760f492357cc36b41f84ef7f86"
        },
        {
          "url": "https://git.kernel.org/stable/c/708b97e792945d3e4653939fd3405d71a61ad065"
        },
        {
          "url": "https://git.kernel.org/stable/c/3f5f8ee9917cc2b9076ac533492d8a200edcabb8"
        }
      ],
      "title": "exfat: fix potential use-after-free in exfat_find_dir_entry()",
      "x_generator": {
        "engine": "bippy-1.2.0"
      }
    }
  },
  "cveMetadata": {
    "assignerOrgId": "416baaa9-dc9f-4396-8d5f-8c081fb06d67",
    "assignerShortName": "Linux",
    "cveId": "CVE-2026-63808",
    "datePublished": "2026-07-19T12:02:11.323Z",
    "dateReserved": "2026-07-19T07:54:57.013Z",
    "dateUpdated": "2026-08-17T04:51:14.402Z",
    "state": "PUBLISHED"
  },
  "dataType": "CVE_RECORD",
  "dataVersion": "5.2"
}


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