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CVE-2026-63886 (GCVE-0-2026-63886)

Vulnerability from cvelistv5 – Published: 2026-07-19 14:54 – Updated: 2026-08-05 12:36
VLAI
Title
scsi: target: iscsi: Validate CHAP_R length before base64 decode
Summary
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. 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.
Severity
9.8 (Critical)
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H
Assigner
References
Impacted products
Vendor Product Version CPE status
Linux Linux Affected: 1e5733883421495908f3b90d9d807663038b4136 , < 82454e6f21e56ea9a0a9de7d0ff7e1dfb83e34d6 (git)
Affected: 1e5733883421495908f3b90d9d807663038b4136 , < edd06675a02376ea8347dba7c29ad982ba5b36ee (git)
Affected: 1e5733883421495908f3b90d9d807663038b4136 , < bf154c657828ed05399bca5d98cf1611bb048b12 (git)
Affected: 1e5733883421495908f3b90d9d807663038b4136 , < 4a3a19c98a8207ad08bec554703d90f2c34a8cc6 (git)
Affected: 1e5733883421495908f3b90d9d807663038b4136 , < c04e85799356120209b351a148ac2db888d5ffd9 (git)
Affected: 1e5733883421495908f3b90d9d807663038b4136 , < 85db7391310b1304d2dc8ae3b0b12105a9567147 (git)
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Linux Linux Affected: 6.0
Unaffected: 0 , < 6.0 (semver)
Unaffected: 6.1.176 , ≤ 6.1.* (semver)
Unaffected: 6.6.143 , ≤ 6.6.* (semver)
Unaffected: 6.12.93 , ≤ 6.12.* (semver)
Unaffected: 6.18.35 , ≤ 6.18.* (semver)
Unaffected: 7.0.12 , ≤ 7.0.* (semver)
Unaffected: 7.1 , ≤ * (original_commit_for_fix)
guessed Create a notification for this product.
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          "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-\u003edigest_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 \u0027=\u0027 before the comparison handles both padded and\nunpadded encodings. chap_base64_decode() already returns early on \u0027=\u0027,\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."
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              "value": "AV:N - The bug is triggered by a remote iSCSI initiator sending crafted CHAP login PDUs over TCP to the in-kernel LIO iSCSI target during the network login/authentication handshake.\nAC:L - Once an iSCSI target with CHAP is reachable, an attacker can reliably trigger the overflow by sending CHAP_R with a 0b-prefixed base64 string up to 127 characters; no race or special memory layout is required.\nPR:N - The heap overflow occurs during the CHAP authentication handshake before the password hash comparison succeeds, so exploitation is reachable by an unauthenticated remote network attacker who knows a configured CHAP username.\nUI:N - Exploitation requires only the attacker initiating an iSCSI login connection and sending malicious CHAP PDUs; no action from a victim user is needed.\nS:U - Successful exploitation corrupts kernel heap memory within the same kernel security boundary, enabling denial of service or kernel privilege escalation, not a cross-boundary escape such as VM guest-to-host.\nC:H - The out-of-bounds heap write corrupts adjacent kmalloc objects and can be leveraged for arbitrary kernel memory disclosure through heap grooming, consistent with heap overflow impact guidance.\nI:H - A controlled kernel heap out-of-bounds write of up to 79 bytes past a digest-sized buffer can corrupt adjacent structures and be developed into arbitrary code execution or kernel privilege escalation.\nA:H - The overflow causes slab-out-of-bounds writes confirmed by KASAN and can crash or destabilize the kernel during login processing, constituting high availability impact."
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        "dateUpdated": "2026-08-05T12:36:55.369Z",
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    "assignerShortName": "Linux",
    "cveId": "CVE-2026-63886",
    "datePublished": "2026-07-19T14:54:59.395Z",
    "dateReserved": "2026-07-19T07:54:57.018Z",
    "dateUpdated": "2026-08-05T12:36:55.369Z",
    "state": "PUBLISHED"
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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 \u0027=\u0027 before the comparison handles both padded and\\nunpadded encodings. chap_base64_decode() already returns early on \u0027=\u0027,\\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. 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    "redhat_vex": {
      "aggregate_severity": "Important",
      "current_release_date": "2026-09-04T07:53:10+00:00",
      "cve": "CVE-2026-63886",
      "id": "CVE-2026-63886",
      "initial_release_date": "2026-07-19T00:00:00+00:00",
      "product_status:fixed": "762",
      "product_status:known_affected": "22",
      "product_status:known_not_affected": "43",
      "source": "Red Hat CSAF VEX",
      "status": "final",
      "title": "kernel: scsi: target: iscsi: Validate CHAP_R length before base64 decode",
      "url": "https://security.access.redhat.com/data/csaf/v2/vex/2026/cve-2026-63886.json",
      "version": "3"
    },
    "suse_vex": {
      "aggregate_severity": "important",
      "current_release_date": "2026-08-31T16:32:18Z",
      "cve": "CVE-2026-63886",
      "id": "CVE-2026-63886",
      "initial_release_date": "2026-07-21T13:04:13Z",
      "product_status:first_fixed": "2",
      "product_status:known_not_affected": "352",
      "product_status:recommended": "207",
      "source": "SUSE CSAF VEX",
      "status": "interim",
      "title": "SUSE CVE CVE-2026-63886",
      "url": "https://ftp.suse.com/pub/projects/security/csaf-vex/cve-2026-63886.json",
      "version": "6"
    }
  }
}



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