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CVE-2026-89537 (GCVE-0-2026-89537)

Vulnerability from cvelistv5 – Published: 2026-09-11 19:44 – Updated: 2026-09-13 06:30
VLAI
Title
SUNRPC: Reject short RFC 4121 MIC tokens in gss_krb5_verify_mic_v2
Summary
In the Linux kernel, the following vulnerability has been resolved: SUNRPC: Reject short RFC 4121 MIC tokens in gss_krb5_verify_mic_v2 gss_krb5_verify_mic_v2() reads the token ID at ptr[0..1], the flags byte at ptr[2], and padding at ptr[3..7], then passes ptr + GSS_KRB5_TOK_HDR_LEN and cksum_len to gss_krb5_mic_build_sg(). None of these accesses check read_token->len first. The minimum safe token size is GSS_KRB5_TOK_HDR_LEN (16) plus ctx->krb5e->cksum_len (12-24, depending on the enctype). All callers accept shorter tokens from the wire: - gss_unwrap_resp_integ() enforces only an upper bound (offset + len <= rcv_buf->len) before allocating mic.data = kmalloc(len) and passing it to gss_verify_mic(). A malicious NFS server can therefore supply a short checksum opaque, producing a small slab allocation that the Kerberos MIC verifier reads past. - gss_validate() enforces only len <= RPC_MAX_AUTH_SIZE (400) before passing the wire-supplied length to gss_validate_seqno_mic(), which constructs a mic xdr_netobj and calls gss_verify_mic(). - svcauth_gss_verify_header() enforces only checksum.len >= XDR_UNIT (4 bytes) before dispatching to gss_verify_mic(). - svcauth_gss_unwrap_integ() checks only that the checksum fits in gsd->gsd_scratch. Add a length guard at the top of gss_krb5_verify_mic_v2(), before any ptr[] access or scatterlist construction. Well-formed MIC tokens from gss_krb5_get_mic_v2() already have exactly GSS_KRB5_TOK_HDR_LEN + cksum_len bytes, so valid traffic is unaffected.
Severity
9.1 (Critical)
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:N/A:H
Assigner
References
Impacted products
Vendor Product Version CPE status
Linux Linux Affected: de9c17eb4a912c9028f7b470eb80815144883b26 , < 7a946b2e7207f968902f2147ab9b30726f82f7ab (git)
Affected: de9c17eb4a912c9028f7b470eb80815144883b26 , < b94f6719dcd9f7a609bc5f459f85795900e77d25 (git)
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Linux Linux Affected: 2.6.35
Unaffected: 0 , < 2.6.35 (semver)
Unaffected: 7.2.4 , ≤ 7.2.* (semver)
Unaffected: 7.3-rc1 , ≤ * (original_commit_for_fix)
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{
  "containers": {
    "cna": {
      "affected": [
        {
          "defaultStatus": "unaffected",
          "product": "Linux",
          "programFiles": [
            "net/sunrpc/auth_gss/gss_krb5_unseal.c"
          ],
          "repo": "https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git",
          "vendor": "Linux",
          "versions": [
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              "lessThan": "7a946b2e7207f968902f2147ab9b30726f82f7ab",
              "status": "affected",
              "version": "de9c17eb4a912c9028f7b470eb80815144883b26",
              "versionType": "git"
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              "status": "affected",
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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": [
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              "version": "2.6.35"
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              "status": "unaffected",
              "version": "0",
              "versionType": "semver"
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              "status": "unaffected",
              "version": "7.2.4",
              "versionType": "semver"
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            {
              "lessThanOrEqual": "*",
              "status": "unaffected",
              "version": "7.3-rc1",
              "versionType": "original_commit_for_fix"
            }
          ]
        }
      ],
      "cpeApplicability": [
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          "nodes": [
            {
              "cpeMatch": [
                {
                  "criteria": "cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*",
                  "versionEndExcluding": "7.2.4",
                  "versionStartIncluding": "2.6.35",
                  "vulnerable": true
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                  "criteria": "cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*",
                  "versionEndExcluding": "7.3-rc1",
                  "versionStartIncluding": "2.6.35",
                  "vulnerable": true
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              "negate": false,
              "operator": "OR"
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          ]
        }
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      "descriptions": [
        {
          "lang": "en",
          "value": "In the Linux kernel, the following vulnerability has been resolved:\n\nSUNRPC: Reject short RFC 4121 MIC tokens in gss_krb5_verify_mic_v2\n\ngss_krb5_verify_mic_v2() reads the token ID at ptr[0..1], the flags\nbyte at ptr[2], and padding at ptr[3..7], then passes\nptr + GSS_KRB5_TOK_HDR_LEN and cksum_len to gss_krb5_mic_build_sg().\nNone of these accesses check read_token-\u003elen first.\n\nThe minimum safe token size is GSS_KRB5_TOK_HDR_LEN (16) plus\nctx-\u003ekrb5e-\u003ecksum_len (12-24, depending on the enctype).  All callers\naccept shorter tokens from the wire:\n\n - gss_unwrap_resp_integ() enforces only an upper bound\n   (offset + len \u003c= rcv_buf-\u003elen) before allocating\n   mic.data = kmalloc(len) and passing it to gss_verify_mic().\n   A malicious NFS server can therefore supply a short checksum\n   opaque, producing a small slab allocation that the Kerberos MIC\n   verifier reads past.\n\n - gss_validate() enforces only len \u003c= RPC_MAX_AUTH_SIZE (400)\n   before passing the wire-supplied length to\n   gss_validate_seqno_mic(), which constructs a mic xdr_netobj\n   and calls gss_verify_mic().\n\n - svcauth_gss_verify_header() enforces only\n   checksum.len \u003e= XDR_UNIT (4 bytes) before dispatching to\n   gss_verify_mic().\n\n - svcauth_gss_unwrap_integ() checks only that the checksum fits\n   in gsd-\u003egsd_scratch.\n\nAdd a length guard at the top of gss_krb5_verify_mic_v2(), before any\nptr[] access or scatterlist construction.  Well-formed MIC tokens from\ngss_krb5_get_mic_v2() already have exactly GSS_KRB5_TOK_HDR_LEN +\ncksum_len bytes, so valid traffic is unaffected."
        }
      ],
      "metrics": [
        {
          "cvssV3_1": {
            "baseScore": 9.1,
            "baseSeverity": "CRITICAL",
            "vectorString": "CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:N/A:H",
            "version": "3.1"
          },
          "scenarios": [
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              "lang": "en",
              "value": "AV:N - gss_krb5_verify_mic_v2() is reached from nfsd RPCSEC_GSS on TCP/2049 (svcauth_gss_accept \u2192 svcauth_gss_verify_header/unwrap_integ) and from the NFS client processing a remote server\u0027s RPCSEC_GSS verifier or krb5i checksum, so a remote peer triggers it with crafted SUNRPC traffic.\nAC:L - A single RPCSEC_GSS Call or Reply with a MIC/checksum shorter than GSS_KRB5_TOK_HDR_LEN plus cksum_len (callers accept as little as 4 bytes) deterministically over-reads; no race or attacker-uncontrollable layout is required. Kerberized NFS is a standard enterprise deployment.\nPR:N - svcauth_gss_verify_header() is the MIC check itself and runs before credentials are verified; the only gate is an rsc-cache lookup on a cleartext, monotonically increasing 64-bit GSS handle that an unauthenticated client can sniff or enumerate. A malicious NFS server needs no privileges on the client.\nUI:N - On nfsd the over-read is triggered solely by an inbound RPCSEC_GSS message; on the client it fires from the RPC receive path once an NFS/RPCSEC_GSS mount exists. No separate victim action is required at attack time.\nS:U - The slab over-read and any resulting oops stay inside the host kernel SUNRPC/GSS code. This is ordinary same-host kernel impact, not a VM escape, IOMMU bypass, or other changed-scope boundary.\nC:H - gss_unwrap_resp_integ() kmallocs only the attacker-chosen checksum length, then gss_krb5_verify_mic_v2() reads GSS_KRB5_TOK_HDR_LEN+cksum_len (28-40) bytes via gss_krb5_mic_build_sg()/sg_pcopy_to_buffer, an out-of-bounds read of adjacent slab well beyond a few bytes.\nI:N - The defect is a read-only over-read consumed as a MIC comparison; rfc3961_verify_mic copies those bytes into a local buffer and memcmp()s them. There is no out-of-bounds write, use-after-free, or control-flow hijack primitive.\nA:H - Reading 28-40 bytes past a small kmalloc object or past an RPC receive buffer near a page boundary can oops or panic on KASAN, redzone, or unmapped next-page kernels, and the attacker can repeat the crafted RPC to deny NFS service."
            }
          ]
        }
      ],
      "providerMetadata": {
        "dateUpdated": "2026-09-13T06:30:34.939Z",
        "orgId": "416baaa9-dc9f-4396-8d5f-8c081fb06d67",
        "shortName": "Linux"
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      "references": [
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          "url": "https://git.kernel.org/stable/c/7a946b2e7207f968902f2147ab9b30726f82f7ab"
        },
        {
          "url": "https://git.kernel.org/stable/c/b94f6719dcd9f7a609bc5f459f85795900e77d25"
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      "title": "SUNRPC: Reject short RFC 4121 MIC tokens in gss_krb5_verify_mic_v2",
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    "assignerShortName": "Linux",
    "cveId": "CVE-2026-89537",
    "datePublished": "2026-09-11T19:44:15.216Z",
    "dateReserved": "2026-09-11T19:38:34.721Z",
    "dateUpdated": "2026-09-13T06:30:34.939Z",
    "state": "PUBLISHED"
  },
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  "vulnerability-lookup:meta": {
    "epss": {
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      "date": "2026-09-16",
      "epss": "0.00516",
      "percentile": "0.42645"
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      "cve": "CVE-2026-89537",
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      "product_status:known_affected": "1",
      "source": "Microsoft CSAF VEX",
      "status": "final",
      "title": "SUNRPC: Reject short RFC 4121 MIC tokens in gss_krb5_verify_mic_v2",
      "url": "https://msrc.microsoft.com/csaf/vex/2026/msrc_cve-2026-89537.json",
      "version": "2"
    },
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    "redhat_vex": {
      "aggregate_severity": "Moderate",
      "current_release_date": "2026-09-13T06:57:10+00:00",
      "cve": "CVE-2026-89537",
      "id": "CVE-2026-89537",
      "initial_release_date": "2026-09-11T19:44:15.216000+00:00",
      "product_status:known_affected": "2",
      "source": "Red Hat CSAF VEX",
      "status": "final",
      "title": "kernel: Linux kernel: Memory corruption in SUNRPC due to short Kerberos MIC tokens",
      "url": "https://security.access.redhat.com/data/csaf/v2/vex/2026/cve-2026-89537.json",
      "version": "3"
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    "suse_vex": {
      "aggregate_severity": "moderate",
      "current_release_date": "2026-09-16T15:37:12Z",
      "cve": "CVE-2026-89537",
      "id": "CVE-2026-89537",
      "initial_release_date": "2026-09-12T16:26:31Z",
      "product_status:known_affected": "353",
      "source": "SUSE CSAF VEX",
      "status": "interim",
      "title": "SUSE CVE CVE-2026-89537",
      "url": "https://ftp.suse.com/pub/projects/security/csaf-vex/cve-2026-89537.json",
      "version": "4"
    }
  }
}



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