CVE-2021-20038
CVE-2021-20038 is a security flaw in SonicWall SMA 100 Appliances, SonicWall’s firewall and remote-access equipment. Attackers can run their own code on SonicWall SMA 100 Appliances from anywhere, without logging in. The US Cybersecurity and Infrastructure Security Agency (CISA) added it to its catalog of flaws known to be used in attacks on 28 Jan 2022, and says ransomware gangs have used it.[1]
SonicWall SMA 100 Appliances is business software or equipment. Most families won’t have it, but attackers break into it to get inside companies, and from there to what those companies hold about their customers, patients and staff. SonicWall has published a fix or advice.
Description
CISA calls it a stack-based buffer overflow in SonicWall SMA 100 Appliances. In its words: “SonicWall SMA 100 devies are vulnerable to an unauthenticated stack-based buffer overflow vulnerability where exploitation can result in code execution.”[1]
The CVE record adds: “A Stack-based buffer overflow vulnerability in SMA100 Apache httpd server's mod_cgi module environment variables allows a remote unauthenticated attacker to potentially execute code as a 'nobody' user in the appliance. This vulnerability affected SMA 200, 210, 400, 410 and 500v appliances firmware 10.2.0.8-37sv, 10.2.1.1-19sv, 10.2.1.2-24sv and earlier versions.”[2]
The underlying weakness is stack buffer overflow (CWE-121), in MITRE’s list of common software weaknesses.
It scores 9.8 out of 10 on the Common Vulnerability Scoring System, which rates it critical, scored by NVD using CVSS 3.1.[2]
Affected versions
NVD’s record doesn’t list version ranges for it; SonicWall’s advisory has them[3]. CISA names the product as SonicWall SMA 100 Appliances.
Most families won’t have SonicWall SMA 100 Appliances; it’s for whoever looks after IT at work.
Exploitation
CISA added it to its catalog of known exploited vulnerabilities on 28 Jan 2022, 51 days after it was made public. CISA lists a flaw only when it has reliable evidence of it being exploited, and gave US federal agencies until 11 Feb 2022 to fix it.[1] CISA says ransomware gangs have used it.
FIRST’s Exploit Prediction Scoring System, which estimates how likely a flaw is to be used in attacks in the next 30 days from what it sees across the internet, puts it at over 99%. That’s in the top 1% of the flaws it scores.[4]
CISA assesses it as automatable: attackers can run it against many targets at once.
CISA doesn’t say who was attacked, or how.
Fix
SonicWall has published a fix or advice.[3] CISA’s required action: “Apply updates per vendor instructions.”[1]
- Check whether you use it.SonicWall SMA 100 Appliances is business software or equipment, so most families won’t have it. If you run a business, or work from home on company equipment, ask whoever looks after your IT.
- Send this page to your IT person.Ask them to confirm it’s fixed, or not in use. CISA gave US government agencies until 11 Feb 2022 to fix it.
- If it can’t be fixed, take it offline.CISA’s advice when there’s no fix is to stop using the product, or take it off the internet, until there is one.
You can’t fix someone else’s server, but you can watch for what follows a break-in like this: a breach notice, a password reset you didn’t ask for, or a scam message that knows things it shouldn’t. HARD watches for those for you and your family.
Timeline
| Date | Event |
|---|---|
| The flaw is made public in the National Vulnerability Database, with a severity score of 9.8.[2] | |
| CISA adds it to its catalog of flaws known to be used in attacks, noting that ransomware gangs have used it.[1] | |
| CISA’s deadline for US government agencies to fix it.[1] | |
| FIRST puts the odds of it being used in attacks in the next 30 days at over 99%.[4] |
See also
Other SonicWall flaws
- CVE-2026-83549, SonicWall SMA1000 Appliances: OS command injection ·
- CVE-2026-83548, SonicWall SMA1000 Appliances: server-side request forgery ·
- CVE-2026-15410, SonicWall SMA1000 Appliances: code injection ·
- CVE-2026-15409, SonicWall SMA1000 Appliances: server-side request forgery ·
- CVE-2025-40602, SonicWall SMA1000 appliance: sMA1000 missing authorization ·
Other stack buffer overflow flaws
- CVE-2026-7273, Zyxel GS1900 Series Switches: stack-based buffer overflow ·
- CVE-2021-27137, DD-WRT: stack-based buffer overflow ·
- CVE-2025-53521, F5 BIG-IP: stack-based buffer overflow ·
- CVE-2025-20352, Cisco IOS and IOS XE: software SNMP denial of service and remote code execution ·
- CVE-2025-42599, Qualitia Active! Mail: stack-based buffer overflow ·
Added the same day
- CVE-2022-22587, Apple iOS and macOS: memory corruption ·
- CVE-2020-5722, Grandstream UCM6200: networks UCM6200 series SQL injection ·
- CVE-2020-0787, Microsoft Windows: background intelligent transfer service (BITS) improper privilege management ·
- CVE-2017-5689, Intel Active Management Technology (AMT), Small Business Technology (SBT), and Standard Manageability: privilege escalation ·
- CVE-2014-7169, GNU Bourne-Again Shell (Bash): arbitrary code execution ·
Every SonicWall flaw on CISA’s list · Everything added in January 2022
References
- ^ Cybersecurity and Infrastructure Security Agency. “SonicWall SMA 100 Appliances Stack-Based Buffer Overflow Vulnerability”. Known Exploited Vulnerabilities Catalog, added 28 Jan 2022.
- ^ National Institute of Standards and Technology. “CVE-2021-20038 Detail”. National Vulnerability Database, published 8 Dec 2021.
- ^ SonicWall. Advisories and fixes: psirt.global.sonicwall.com.
- ^ FIRST. “Exploit Prediction Scoring System”. EPSS 0.999, percentile 100.0, 31 Oct 2026.
- CVE Program. “CVE-2021-20038”. The official CVE record.
This article is built from CISA’s catalog (version 2026.10.30), NVD’s record and FIRST’s scores, kept in HARD’s database and checked every morning. The plain English is HARD’s.