CVE-2021-20038
SonicWall SMA 100 Appliances Stack-Based Buffer Overflow Vulnerability
Description
CVE-2021-20038 is a CRITICAL unauthenticated stack-based buffer overflow vulnerability in SonicWall SMA 100 series appliances (models SMA 200, 210, 400, 410, and 500v) affecting firmware versions 10.2.0.8-37sv, 10.2.1.1-19sv, and 10.2.1.2-24sv. The flaw allows a remote unauthenticated attacker to execute arbitrary code on the affected appliance by sending specially crafted HTTP requests that trigger a stack-based buffer overflow. Classified under CWE-121 (Stack-based Buffer Overflow) and CWE-787 (Out-of-bounds Write), this vulnerability carries a CVSS 3.1 score of 9.8 (CRITICAL) with a network attack vector requiring no authentication and no user interaction. With an EPSS score of 0.94292 (99.9th percentile), this is among the most actively exploited vulnerabilities in the catalog. CVE-2021-20038 is listed in CISA's KEV catalog with a remediation deadline of February 11, 2022, and is confirmed to be associated with ransomware campaigns.
KEV Information
CVSS Score
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:HOpen in CalculatorCWEs
Affected Products
| Vendor | Product | Version |
|---|---|---|
| sonicwall | sma 200 firmware | 10.2.0.8-37sv; 10.2.1.1-19sv; 10.2.1.2-24sv |
| sonicwall | sma 210 firmware | 10.2.0.8-37sv; 10.2.1.1-19sv; 10.2.1.2-24sv |
| sonicwall | sma 410 firmware | 10.2.0.8-37sv; 10.2.1.1-19sv; 10.2.1.2-24sv |
| sonicwall | sma 400 firmware | 10.2.0.8-37sv; 10.2.1.1-19sv; 10.2.1.2-24sv |
| sonicwall | sma 500v firmware | 10.2.0.8-37sv; 10.2.1.1-19sv; 10.2.1.2-24sv |
Multiple CVSS Assessments
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H
References
- https://github.com/jbaines-r7/badblood(Exploit, Third Party Advisory)
- https://psirt.global.sonicwall.com/vuln-detail/SNWLID-2021-0026(Vendor Advisory)
- https://www.rapid7.com/blog/post/2022/01/11/cve-2021-20038-42-sonicwall-sma-100-multiple-vulnerabilities-fixed-2/(Exploit, Third Party Advisory)
- https://www.cisa.gov/known-exploited-vulnerabilities-catalog?field_cve=CVE-2021-20038(US Government Resource)
Weakness Type
CWE-121: Stack-based Buffer Overflow
Stack-based Buffer Overflow (CWE-121) is a critical vulnerability variant that occurs when a buffer allocated on the program stack is overwritten with data exceeding its allocated size. The stack stores local variables, function parameters, and most critically, return addresses that control program execution flow. When a stack buffer overflows, it can overwrite the saved return address, allowing an attacker to redirect program execution to attacker-controlled code or Return-Oriented Programming (ROP) gadgets. Stack overflows are considered particularly dangerous because the stack's predictable Last-In-First-Out (LIFO) structure makes exploitation more reliable than heap-based attacks.
In CVE-2021-20038, the SonicWall SMA 100 series web interface contains a stack-based buffer overflow in its HTTP request processing code. When the appliance receives a specially crafted HTTP request with oversized parameters, the web server component copies the request data into a fixed-size stack buffer without properly checking the length, causing the buffer to overflow. This overwrites the return address on the stack, enabling the attacker to redirect execution to arbitrary code. Because the SMA 100 appliance is a network edge device that processes unauthenticated HTTP requests from the internet, this vulnerability is directly exploitable by any remote attacker without credentials. Learn more about CWE-121 at turingsecure.com.
Impact Analysis
CVE-2021-20038 carries a CVSS 3.1 score of 9.8 (CRITICAL) with an Unchanged scope, reflecting the maximum severity for an unauthenticated remote code execution vulnerability.
Confidentiality (High): Remote code execution on the SonicWall SMA appliance grants the attacker complete access to all data processed by the device. This includes VPN session data, user credentials (both local and cached LDAP/Active Directory credentials), VPN session tokens, private SSL certificates, and configuration files containing network topology information. As a VPN gateway, the SMA appliance has visibility into all encrypted traffic passing through it, making the confidentiality impact exceptionally severe.
Integrity (High): With code execution on the appliance, an attacker can modify the device firmware, install persistent backdoors, alter VPN routing rules, inject content into VPN sessions, and manipulate access control policies. The attacker can create rogue VPN accounts, modify authentication settings to enable bypasses, and alter logging configurations to conceal their presence. Firmware-level modifications may survive standard updates and factory resets.
Availability (High): An attacker can crash the appliance, rendering VPN services unavailable for all users. They can also selectively disable connectivity for specific users or networks, deploy ransomware on systems accessible through the VPN, or use the compromised appliance as a launching point for destructive attacks against the internal network.
Scope (Unchanged): While the CVSS scope is Unchanged, the practical impact extends far beyond the appliance itself. The SMA 100 is a network perimeter device that provides VPN access to the internal network. Compromising this device gives the attacker a persistent foothold at the network edge with full visibility into VPN traffic and direct access to internal resources. The EPSS score of 0.94292 at the 99.9th percentile confirms near-certain exploitation activity, and the confirmed ransomware association elevates the operational risk significantly.
Exploit Maturity
Active Exploitation: CVE-2021-20038 has been extensively exploited in the wild since its disclosure in December 2021. CISA added it to the KEV catalog on January 28, 2022, confirming active exploitation. SonicWall SMA appliances have been a repeated target for threat actors due to their internet-facing nature and the high value of VPN access as an initial access vector for enterprise networks.
Ransomware Association: CISA's KEV catalog explicitly confirms ransomware association for CVE-2021-20038. Ransomware operators have exploited this vulnerability to gain initial access to enterprise networks through compromised SMA appliances, subsequently deploying ransomware across the internal network. The combination of unauthenticated remote code execution and the appliance's position at the network perimeter makes this an ideal initial access vector for ransomware campaigns.
Public Exploits: A public exploit tool called "badblood" is available on GitHub by jbaines-r7, providing a ready-made exploitation capability. Rapid7 published detailed technical analysis in their blog post documenting the vulnerability and related issues (CVE-2021-20038 through CVE-2021-20042). The public availability of exploit code has enabled widespread exploitation by both sophisticated and commodity threat actors.
EPSS Context: The EPSS score of 0.94292 at the 99.9th percentile places this among the most actively exploited vulnerabilities in the entire CVE catalog. This extreme score reflects the combination of a publicly available exploit, unauthenticated remote exploitability, the internet-facing deployment of SMA appliances, and confirmed use in ransomware operations.
KEV Remediation Deadline: CISA established a remediation deadline of February 11, 2022, just 14 days from the catalog addition, reflecting the critical severity and active ransomware exploitation of this vulnerability.
Remediation
-
Apply the SonicWall firmware update immediately. Install the patched firmware from SonicWall's PSIRT advisory (SNWLID-2021-0026) for all SMA 200, 210, 400, 410, and 500v appliances. Upgrade to firmware version 10.2.1.3-27sv or later, which addresses CVE-2021-20038 along with related vulnerabilities CVE-2021-20039 through CVE-2021-20042.
-
Perform a factory reset after updating if compromise is suspected. Given the confirmed active exploitation and ransomware association, if the appliance was running vulnerable firmware while internet-accessible, treat it as potentially compromised. After applying the firmware update, perform a factory reset and reconfigure from scratch rather than restoring from a backup that may contain attacker-installed persistence mechanisms.
-
Rotate all credentials accessible through the SMA appliance. Change all VPN user passwords, revoke and reissue SSL certificates, rotate any LDAP/Active Directory service account credentials used by the appliance, and invalidate all active VPN session tokens. Assume that any credentials passing through or stored on the compromised appliance may have been captured by an attacker.
-
Implement web application firewall (WAF) rules for the SMA management interface. Deploy WAF or IPS rules to detect and block oversized HTTP request parameters targeting the SMA web interface. While this is not a substitute for patching, it provides an additional detection and blocking layer for exploitation attempts.
-
Restrict management access to the SMA appliance. Ensure that the SMA management interface is not exposed to the internet. Restrict administrative access to a dedicated management VLAN or VPN-connected administrators only. Apply multi-factor authentication for all administrative access to the appliance.
-
Conduct thorough incident response investigation. Given the confirmed ransomware association, investigate the internal network for signs of compromise originating from the SMA appliance. Look for evidence of lateral movement, unauthorized account creation, ransomware staging, data exfiltration, and command-and-control communications. Engage incident response resources if compromise is confirmed or suspected.
Technical Details
CVE-2021-20038 is a stack-based buffer overflow vulnerability in the web management interface of SonicWall SMA 100 series appliances. The vulnerability exists in the HTTP request handling code of the appliance's web server, specifically in the processing of certain HTTP request parameters that are copied into a fixed-size stack buffer without proper length validation.
When the SMA appliance receives an HTTP request with oversized parameter values, the web server copies the parameter data into a local stack buffer using an unsafe string copy operation that does not check whether the input data exceeds the buffer's allocated size. The overflow overwrites adjacent stack data including the saved return address, which the processor uses to determine where to continue execution when the current function returns. By crafting the overflow data to contain a specific return address, the attacker can redirect program execution to arbitrary code.
The CVSS 3.1 vector (AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H) rates this as a network-accessible vulnerability with low attack complexity, no privilege requirements, and no user interaction — the most dangerous combination for a vulnerability in an internet-facing device. The attacker simply needs to send a crafted HTTP request to the SMA appliance's web interface to trigger the overflow and achieve code execution.
The root cause is the use of unsafe C string handling functions in the web server code that do not enforce length limits when copying HTTP request parameters to stack-allocated buffers. The affected firmware versions (10.2.0.8-37sv, 10.2.1.1-19sv, 10.2.1.2-24sv) across all SMA 100 series models share the same vulnerable code base. SonicWall addressed this in firmware version 10.2.1.3-27sv by replacing unsafe string copy operations with bounds-checked alternatives and adding input validation for HTTP request parameters before they are processed by the web server.
Frequently Asked Questions
What is CVE-2021-20038?
CVE-2021-20038 is a critical unauthenticated stack-based buffer overflow vulnerability in SonicWall SMA 100 series VPN appliances (models SMA 200, 210, 400, 410, and 500v). It allows a remote attacker to execute arbitrary code on the appliance by sending a specially crafted HTTP request, without requiring any authentication. The vulnerability carries a CVSS 3.1 score of 9.8 (CRITICAL) and is confirmed by CISA to be associated with ransomware campaigns.
Which SonicWall products are affected by CVE-2021-20038?
CVE-2021-20038 affects all SonicWall SMA 100 series appliances including the SMA 200, SMA 210, SMA 400, SMA 410, and SMA 500v virtual appliance. Vulnerable firmware versions are 10.2.0.8-37sv, 10.2.1.1-19sv, and 10.2.1.2-24sv. Organizations should update to firmware version 10.2.1.3-27sv or later to address this vulnerability along with related issues CVE-2021-20039 through CVE-2021-20042.
Is CVE-2021-20038 used in ransomware attacks?
Yes, CISA's KEV catalog explicitly confirms ransomware association for CVE-2021-20038. Ransomware operators have exploited this vulnerability to gain initial access to enterprise networks through compromised SonicWall SMA appliances. The vulnerability's combination of unauthenticated remote code execution and the appliance's internet-facing position makes it an ideal entry point for ransomware campaigns. Its EPSS score of 0.94292 (99.9th percentile) confirms near-certain exploitation activity.
How do I fix CVE-2021-20038?
Apply the SonicWall firmware update to version 10.2.1.3-27sv or later from SonicWall's PSIRT advisory. If the appliance was exposed while running vulnerable firmware, perform a factory reset after updating and reconfigure from scratch. Rotate all VPN user passwords, SSL certificates, and service account credentials. Investigate the internal network for signs of compromise originating from the SMA appliance, and restrict the management interface to authorized networks only.
Need Help With Vulnerability Management?
Our security experts can help you prioritize and remediate vulnerabilities effectively.