CVE-2026-63886

Estado: Pendiente de análisisCrítica (9.8)—

In the Linux kernel, the following vulnerability has been resolved:

scsi: target: iscsi: Validate CHAP_R length before base64 decode

chap_server_compute_hash() allocates client_digest as kzalloc(chap->digest_size) and then, for BASE64-encoded responses, passes chap_r directly to chap_base64_decode() without checking whether the input length could produce more than digest_size bytes of output.

chap_base64_decode() writes to the destination unconditionally as long as there is input to consume.

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With MAX_RESPONSE_LENGTH set to 128 and the "0b" prefix stripped by extract_param(), up to 127 base64 characters can reach the decoder. 127 characters decode to 95 bytes. For SHA-256 (digest_size=32) this overflows client_digest by 63 bytes; for MD5 (digest_size=16) the overflow is 79 bytes.

The length check at line 344 fires after the write has already happened.

The HEX branch in the same switch statement already validates the length up front. Apply the same approach to the BASE64 branch: strip trailing base64 padding characters, then reject any input whose data length exceeds DIV_ROUND_UP(digest_size * 4, 3) before calling the decoder.

Stripping trailing '=' before the comparison handles both padded and unpadded encodings. chap_base64_decode() already returns early on '=', so the full original string is still passed to the decoder unchanged.

The mutual CHAP path decodes CHAP_C into initiatorchg_binhex, which is kzalloc(CHAP_CHALLENGE_STR_LEN). extract_param() caps initiatorchg at CHAP_CHALLENGE_STR_LEN characters, so at most CHAP_CHALLENGE_STR_LEN-1 base64 characters reach the decoder. The maximum decoded size, DIV_ROUND_UP((CHAP_CHALLENGE_STR_LEN-1) * 3, 4), is less than CHAP_CHALLENGE_STR_LEN, so no overflow is possible there. A comment is added at the call site to document this.

CVSS

Versión: 3.1
Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H
Puntuación base: 9.8

Probabilidad de explotación (EPSS)

Probabilidad de explotación en los próximos 30 días: 0.71%
Percentil entre todas las CVEs puntuadas: 52
Fecha de la puntuación: 6/10/2026

EPSS (Exploit Prediction Scoring System, de FIRST) estima la probabilidad de que una vulnerabilidad sea explotada en 30 días. Complementa a CVSS (impacto) y a CISA KEV (explotación confirmada).

🎯 Técnicas ATT&CK

Cómo se explota esta vulnerabilidad y qué consigue el atacante, en el lenguaje de MITRE ATT&CK.

ExplotaciónT1190 Exploit Public-Facing Applicationinitial access92 %
Impacto principalT1068 Exploitation for Privilege Escalationprivilege escalation85 %

Vulnerabilidad de buffer overflow en iSCSI CHAP del kernel Linux (AV:N/AC:L/PR:N). Permite bypass de validación de longitud en base64_decode(), causando corrupción de memoria con acceso remoto no autenticado que escala a RCE/root.

Inferido por nuestro agente de análisis a partir de la descripción oficial, el vector CVSS y la CWE, y comprobado por un supervisor. Puede contener errores.

🛡️ Mitigaciones ATT&CK que cubren estas técnicas

Application Isolation and Sandboxing Exploit Protection Update Software Vulnerability Scanning Threat Intelligence Program Privileged Account Management

Tecnologías afectadas (1)

⚠ Inferidas por IA a partir de la descripción — NVD aún no ha analizado esta CVE; no son CPE verificados.

Linux — Linux Kernel^AI

Referencias

JSON original (NVD)

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{
  "id": "CVE-2026-63886",
  "cveTags": [],
  "metrics": {
    "cvssMetricV31": [
      {
        "type": "Secondary",
        "source": "416baaa9-dc9f-4396-8d5f-8c081fb06d67",
        "cvssData": {
          "scope": "UNCHANGED",
          "version": "3.1",
          "baseScore": 9.8,
          "attackVector": "NETWORK",
          "baseSeverity": "CRITICAL",
          "vectorString": "CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H",
          "integrityImpact": "HIGH",
          "userInteraction": "NONE",
          "attackComplexity": "LOW",
          "availabilityImpact": "HIGH",
          "privilegesRequired": "NONE",
          "confidentialityImpact": "HIGH"
        },
        "impactScore": 5.9,
        "exploitabilityScore": 3.9
      }
    ]
  },
  "affected": [
    {
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      "affectedData": [
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          "vendor": "Linux",
          "product": "Linux",
          "versions": [
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            {
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          ],
          "programFiles": [
            "drivers/target/iscsi/iscsi_target_auth.c"
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          "defaultStatus": "unaffected"
        },
        {
          "repo": "https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git",
          "vendor": "Linux",
          "product": "Linux",
          "versions": [
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            {
              "status": "unaffected",
              "version": "0",
              "lessThan": "6.0",
              "versionType": "semver"
            },
            {
              "status": "unaffected",
              "version": "6.1.176",
              "versionType": "semver",
              "lessThanOrEqual": "6.1.*"
            },
            {
              "status": "unaffected",
              "version": "6.6.143",
              "versionType": "semver",
              "lessThanOrEqual": "6.6.*"
            },
            {
              "status": "unaffected",
              "version": "6.12.93",
              "versionType": "semver",
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            {
              "status": "unaffected",
              "version": "6.18.35",
              "versionType": "semver",
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            },
            {
              "status": "unaffected",
              "version": "7.0.12",
              "versionType": "semver",
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            {
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              "versionType": "original_commit_for_fix",
              "lessThanOrEqual": "*"
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          ],
          "programFiles": [
            "drivers/target/iscsi/iscsi_target_auth.c"
          ],
          "defaultStatus": "affected"
        }
      ]
    }
  ],
  "published": "2026-07-19T16:17:05.723",
  "references": [
    {
      "url": "https://git.kernel.org/stable/c/4a3a19c98a8207ad08bec554703d90f2c34a8cc6",
      "source": "416baaa9-dc9f-4396-8d5f-8c081fb06d67"
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    {
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      "source": "416baaa9-dc9f-4396-8d5f-8c081fb06d67"
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    {
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      "source": "416baaa9-dc9f-4396-8d5f-8c081fb06d67"
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    {
      "url": "https://git.kernel.org/stable/c/c04e85799356120209b351a148ac2db888d5ffd9",
      "source": "416baaa9-dc9f-4396-8d5f-8c081fb06d67"
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    {
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      "source": "416baaa9-dc9f-4396-8d5f-8c081fb06d67"
    }
  ],
  "vulnStatus": "Awaiting Analysis",
  "descriptions": [
    {
      "lang": "en",
      "value": "In the Linux kernel, the following vulnerability has been resolved:\n\nscsi: target: iscsi: Validate CHAP_R length before base64 decode\n\nchap_server_compute_hash() allocates client_digest as\nkzalloc(chap->digest_size) and then, for BASE64-encoded responses,\npasses chap_r directly to chap_base64_decode() without checking whether\nthe input length could produce more than digest_size bytes of output.\n\nchap_base64_decode() writes to the destination unconditionally as long\nas there is input to consume. With MAX_RESPONSE_LENGTH set to 128 and\nthe \"0b\" prefix stripped by extract_param(), up to 127 base64 characters\ncan reach the decoder. 127 characters decode to 95 bytes. For SHA-256\n(digest_size=32) this overflows client_digest by 63 bytes; for MD5\n(digest_size=16) the overflow is 79 bytes.\n\nThe length check at line 344 fires after the write has already happened.\n\nThe HEX branch in the same switch statement already validates the length\nup front. Apply the same approach to the BASE64 branch: strip trailing\nbase64 padding characters, then reject any input whose data length\nexceeds DIV_ROUND_UP(digest_size * 4, 3) before calling the decoder.\n\nStripping trailing '=' before the comparison handles both padded and\nunpadded encodings. chap_base64_decode() already returns early on '=',\nso the full original string is still passed to the decoder unchanged.\n\nThe mutual CHAP path decodes CHAP_C into initiatorchg_binhex, which is\nkzalloc(CHAP_CHALLENGE_STR_LEN). extract_param() caps initiatorchg at\nCHAP_CHALLENGE_STR_LEN characters, so at most CHAP_CHALLENGE_STR_LEN-1\nbase64 characters reach the decoder. The maximum decoded size,\nDIV_ROUND_UP((CHAP_CHALLENGE_STR_LEN-1) * 3, 4), is less than\nCHAP_CHALLENGE_STR_LEN, so no overflow is possible there. A comment is\nadded at the call site to document this."
    }
  ],
  "lastModified": "2026-07-27T17:44:23.777",
  "sourceIdentifier": "416baaa9-dc9f-4396-8d5f-8c081fb06d67"
}