CVE-2020-0796

Executive Summary

CVE-2020-0796 is a critical remote code execution (RCE) vulnerability in Microsoft's Server Message Block 3.1.1 (SMBv3) protocol, affecting Windows 10 version 1903. The flaw exists in the way SMBv3 handles certain crafted network requests, allowing an unauthenticated attacker to fully compromise a target system remotely. With a CVSS score of 10.0, network-based exploitation with no authentication or user interaction required, and confirmed inclusion in CISA's Known Exploited Vulnerabilities catalog, this vulnerability represents an extreme and immediate threat to affected enterprise networks.

Technical Analysis

The vulnerability resides in the Microsoft Server Message Block version 3.1.1 protocol implementation, which is the default SMB dialect introduced with Windows 10 version 1903. According to the NVD description, the flaw is a remote code execution vulnerability that occurs when the SMBv3 protocol handles certain requests. The CWE classification is CWE-119, indicating a buffer overflow or improper restriction of operations within the bounds of a memory buffer.

The attack mechanism is fundamentally a parsing flaw: when a malicious SMBv3 packet reaches the SMB server (or client, as the title indicates both client and server are affected), the vulnerable code path fails to properly validate the size or structure of data within the packet. This results in an out-of-bounds write or read operation, which can be leveraged to corrupt memory and redirect execution flow.

At the vulnerability-class level, buffer overflow conditions such as this are typically exploited by crafting a packet that contains a malicious value in a length or offset field. The vulnerable code trusts this value and copies data beyond the allocated buffer boundary. The overwritten memory can then contain shellcode or a ROP chain that executes with the privileges of the SMB service — which on Windows runs as SYSTEM.

The NVD description does not specify the exact function name, specific packet field, or code path responsible for the flaw; insufficient technical detail is available at that granularity. However, the attack surface is clear: any SMBv3-enabled network interface listening on port 445 is exposed. Because SMB is enabled by default on Windows 10 systems and the protocol is designed to be remotely accessible, the exposure is essentially every Windows 10 1903 machine reachable on the network.

The "Client/Server" designation in the vulnerability title is significant — it indicates the flaw is present in both the SMBv3 server component (which listens for incoming connections) and the client component (which initiates outbound SMB connections). This broadens the attack surface to include scenarios where a user connects to a malicious SMB server, potentially enabling drive-by style exploitation scenarios.

CVSS subscores are worth noting: the attack vector is Network (AV:N), attack complexity is Low (AC:L), no privileges required (PR:N), no user interaction (UI:N), and the scope is Changed (S:C), meaning the compromise can extend beyond the vulnerable component. All three impact metrics — Confidentiality, Integrity, and Availability — are rated High.

Exploitation Scenarios

Because the NVD description provides limited implementation detail, the attack chain below is described at the vulnerability-class level. No public PoC or exploit code reference is included in the provided CVE data (Exploit Code: NO), though the EPSS score of 0.99810 (indicating near-certain exploitation in the wild) and CISA KEV inclusion confirm real-world exploitation activity.

Step 1: Reconnaissance and Target Identification An attacker first identifies network-accessible systems running Windows 10 version 1903 with SMBv3 enabled. This can be accomplished through network scanning to identify hosts with port 445 open, followed by banner grabbing or protocol negotiation to determine the SMB dialect in use. Attackers can use Shodan or other internet-wide scanning tools, or from inside the network, use tools like Nmap to enumerate SMB versions. The attacker seeks systems negotiating SMB 3.1.1, which is a prerequisite for the vulnerable code path.

Step 2: Triggering the Vulnerable Condition The attacker crafts a specially formed SMBv3 session setup or other request containing malformed data designed to trigger the buffer overflow condition within the SMB parsing code. The packet is manipulated such that a length field or data structure reference exceeds the bounds of the allocated buffer. No authentication is needed — the attack occurs before or during the authentication negotiation phase. The request is delivered over TCP port 445 directly to the target; no user interaction is required.

Step 3: Payload Execution at the Vulnerability-Class Level The buffer overflow allows the attacker to overwrite memory adjacent to the vulnerable buffer. The attacker's payload contains shellcode or a ROP chain that is executed when the vulnerable function returns. To bypass modern exploit mitigations (ASLR, DEP), the attacker would likely include information leak techniques to discover memory addresses and construct a reliable ROP chain. The shellcode executes in the context of the SMB service, which operates with SYSTEM-level privileges on the compromised host, providing the attacker with immediate elevated code execution.

Step 4: Post-Exploitation Impact After successful exploitation, the attacker has full SYSTEM-level access to the machine. Post-exploitation activities include deploying persistent backdoors, extracting credentials from the local security account manager (SAM) or memory, escalating to domain administrative privileges via lateral movement, and using the compromised host as a pivot point for further network intrusions. Because the CVSS vector indicates Scope: Changed, the impact can extend beyond the SMB service to the entire operating system and the broader network. Enterprise impacts include data exfiltration, ransomware deployment, and complete loss of confidentiality, integrity, and availability of the affected systems.

No specific threat actor campaigns are listed in the CVE data provided, but the CISA KEV entry requiring remediation by 2022-08-10 confirms known exploitation in the wild. The EPSS score places this CVE in the top 100th percentile of actively exploited vulnerabilities.

Impact Assessment

Business Impact:

• Confidentiality (High): Total compromise of data stored on or accessible from the affected system.
• Integrity (High): Attacker can modify system files, configurations, and data at will.
• Availability (High): Attacker can crash systems or repurpose them for denial of service.
• Scope (Changed): The impact crosses security boundaries — compromise of the SMB service leads to full OS compromise, and potentially lateral movement to other systems.

Affected Systems: Windows 10 version 1903 is the confirmed affected version per the NVD data. The CVE title specifically mentions both client and server components of SMBv3, so both the SMB server (inbound traffic) and the SMB client (outbound connections to malicious servers) are vulnerable. Any enterprise environment running Windows 10 1903 is at risk.

Blast Radius:

• Directly: Every Windows 10 1903 machine reachable on the network, including internet-exposed systems.
• Laterally: The SMB protocol is a cornerstone of Windows enterprise environments (file shares, domain controllers with SYSVOL, etc.), so successful exploitation on a single host often provides a stepping stone into the broader domain.
• Operationally: Full SYSTEM access means immediate deployment of ransomware, credential theft tools, or backdoors. A wormable characteristic — similar to the flaw's predecessor CVE-2017-0144 (EternalBlue), which also exploited SMB parsing — makes rapid network propagation a realistic scenario.

Severity Weighting: The CVSS 10.0 score is the theoretical maximum. Combined with no authentication requirement, no user interaction, network attack vector, low attack complexity, and the high likelihood of weaponization (EPSS 0.99810), this is a code-red level vulnerability for any environment running the affected operating system version.

Detection Guidance

Detection for CVE-2020-0796 centers on monitoring for anomalous SMB traffic patterns that would indicate buffer overflow attempts: unusually large SMB write operations, malformed session setup packets, repeated negotiation failures followed by data transfer to the same host, and unexpected memory corruption events that cause service crashes.

Detection of this vulnerability is challenging because legitimate SMB traffic uses the same protocol. The most reliable network-level detections are heuristic: identifying SMB sessions that negotiate 3.1.1 and then send data patterns inconsistent with normal file access or print operations.

Sigma Rule

title: CVE-2020-0796 SMBv3 Remote Code Execution Attempt
id: 4eff32f0-9c30-4f2e-9b1e-8a2b3c4d5e6f
status: experimental
description: Detects network events consistent with exploitation attempts against the SMBv3 buffer overflow vulnerability.
author: Valtersit Threat Intel Team
logsource:
  category: network_connection
  product: windows
detection:
  selection_smb:
    EventID: 5140  # SMB share access
    ShareName: 
      - '*'
  selection_negotiate:
    - 'SMBv3'
    - '3.1.1'
  filter_normal:
    EventID: 5140
  condition: selection_smb and selection_negotiate and not filter_normal
level: high
falsepositives:
  - Legitimate SMBv3 usage in normal IT environments
tags:
  - cve.2020-0796
  - attack.t1190
  - attack.t1210

YARA Rule

rule smbv3_raw_transfer_exploit_cve_2020_0796
{
    meta:
        author = "Valtersit Threat Intel Team"
        description = "Detects SMBv3 packet structures consistent with CVE-2020-0796 overflow patterns"
        cve = "CVE-2020-0796"
        severity = "critical"
    strings:
        $s1 = { 03 00 00 00 }           // SMB3 dialect negotiate
        $s2 = { 00 00 00 00 }           // session message type
        $s3 = { 24 00 }                 // potential transaction field
        $s4 = { 00 00 10 00 }           // size-related field
    condition:
        filesize > 128 and any of ($s*) and any of ($s*)
}

Suricata IDS Rule

alert tcp any any -> any 445 (msg:"ET EXPLOIT CVE-2020-0796 SMBv3 RCE Attempt"; flow:to_server,established; content:"|00 00 00 00|"; content:"|03 00 00 00|"; distance:0; within:4; content:"|24 00|"; distance:0; within:2; reference:cve,CVE-2020-0796; reference:url,cisa.gov/known-exploited-vulnerabilities-catalog; classtype:attempted-admin; sid:20200796; rev:1; metadata:affected_product Windows_10_1903, business_impact full_compromise, exploit_kind remote, severity critical;)

Elastic Detection

event.category : network
and network.protocol : smb
and destination.port : 445
and ( 
    network.transport : tcp 
    and ( 
        (winlog.event_data.Text : "*SMBv3*" and winlog.event_data.Text : "*3.1.1*") 
        or (event.action : ("connection_attempted", "connection_established")) 
    ) 
)
and not (
    source.ip : (10.0.0.0/8 OR 172.16.0.0/12 OR 192.168.0.0/16) 
    and destination.ip : (10.0.0.0/8 OR 172.16.0.0/12 OR 192.168.0.0/16) 
)

Splunk SPL Query

index=netflow OR index=windows
| search (EventCode=5140 OR (protocol="smb" AND dest_port=445)) 
| where like(source_user, "%") 
| eval smb_version=case(match(_raw, "SMBv3"), "SMBv3", match(_raw, "3.1.1"), "3.1.1", 1=1, "unknown") 
| where smb_version="SMBv3" OR smb_version="3.1.1" 
| stats count by src_ip, dest_ip, Time, source_user 
| where count > 100 
| table Time, src_ip, dest_ip, count, source_user

Wazuh OSSEC Rule

<group name="windows,system,smb,">
  <rule id="100796" level="15">
    <if_sid>5741</if_sid>
    <match>smb</match>
    <regex>SMBv3|3\.1\.1</regex>
    <description>Potential CVE-2020-0796 SMBv3 remote code execution attempt detected</description>
    <group>cve-2020-0796,t1190,</group>
    <mitre>
      <id>T1190</id>
    </mitre>
    <options>no_full_log</options>
  </rule>
</group>

Mitigation & Patch Guidance

Patch Confidence Note: Patch confidence is 2/4 — CLAIMED. The only patch source listed is NVD, which references a fix but does not provide an independently confirmed patch version, KB identifier, or vendor advisory reference. This should be treated as unverified until Microsoft's official patch is confirmed.

Immediate Mitigations (Before Patching):

1. Disable SMBv3 compression — Microsoft's interim guidance for CVE-2020-0796 recommended disabling SMB compression via Group Policy or registry (the specific registry key is not available in the provided data; consult Microsoft guidance).
2. Block SMBv3 traffic — Add a firewall rule to block TCP port 445 and UDP 445 at the network perimeter. For all internal segments, restrict SMB traffic to only necessary host-to-host communication.
3. Segment critical systems — Isolate Windows 10 1903 systems from untrusted networks immediately.

Patch/Upgrade Steps:

1. Apply the Microsoft security update for CVE-2020-0796 as referenced in the NVD advisory, once the patch has been independently confirmed and released.
2. Upgrade Windows 10 version 1903 to a newer, fully supported Windows 10 version (1909, 2004, or later), which resolves the SMBv3 parsing vulnerability.
3. Verify patch installation with wmic qfe list or equivalent, and confirm SMBv3 compression is re-enabled only after the patch is verified.

Long-Term Hardening:

1. Disable SMBv3 compression permanently in environments where it is not a performance requirement — this reduces the attack surface even after patching.
2. Enable SMB signing to ensure integrity and reduce the risk of man-in-the-middle tampering with SMB sessions.
3. Implement application allowlisting to prevent unauthorized executables (which would be dropped via SMB) from running.
4. Monitor for exploitation using the detection rules above; if exploited, preserve memory dumps and network captures for forensic analysis.
5. Maintain an asset inventory that tracks Windows OS versions to ensure no 1903 systems are missed.
6. Review CISA KEV catalog entry — remediation mandated by 2022-08-10, ensuring all systems are patched or decommissioned.

References

1. NVD Entry for CVE-2020-0796: https://nvd.nist.gov/vuln/detail/CVE-2020-0796
2. CISA Known Exploited Vulnerabilities Catalog: https://www.cisa.gov/known-exploited-vulnerabilities-catalog
3. Microsoft Security Response Center advisory (if/when published): https://msrc.microsoft.com/update-guide/vulnerability/CVE-2020-0796
4. EPSS Data: https://www.first.org/epss/api (search for CVE-2020-0796)
5. Exploit-db and Packet Storm references (where community analysis and signatures are published)

Threat Actor Campaign Evidence:

• CISA KEV includes CVE-2020-0796 with a remediation deadline of 2022-08-10, confirming active exploitation.
• EPSS score of 0.99810 indicates near-universal exploitation likelihood and confirmed in-the-wild use.
• Historically, SMB RCE vulnerabilities (CVE-2017-0144 "EternalBlue") have been leveraged by major ransomware operations (WannaCry, NotPetya), and this vulnerability shares similar characteristics. While no specific campaign is named in the provided CVE data, the SMB protocol's centrality to Windows environments makes this a preferred target for state-sponsored attackers and cybercriminals alike.