CVE-2021-35211
CVE-2021-35211 is a security flaw in SolarWinds Serv-U, SolarWinds’ IT management software. Attackers can run their own code on SolarWinds Serv-U. The US Cybersecurity and Infrastructure Security Agency (CISA) added it to its catalog of flaws known to be used in attacks on 3 Nov 2021, and says ransomware gangs have used it.[1]
SolarWinds Serv-U 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. Fixed in version 15.2.3 HF1 of Serv-U Managed File Transfer Server and Serv-U Secured FTP.
Description
CISA calls it a remote code execution in SolarWinds Serv-U. In its words: “SolarWinds Serv-U contains an unspecified memory escape vulnerability which can allow for remote code execution.”[1]
The CVE record adds: “Microsoft discovered a remote code execution (RCE) vulnerability in the SolarWinds Serv-U product utilizing a Remote Memory Escape Vulnerability. If exploited, a threat actor may be able to gain privileged access to the machine hosting Serv-U Only. SolarWinds Serv-U Managed File Transfer and Serv-U Secure FTP for Windows before 15.2.3 HF2 are affected by this vulnerability.”[2]
The underlying weakness is out-of-bounds write (CWE-787), in MITRE’s list of common software weaknesses.
It scores 10.0 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 lists one fixed version: everything before it is affected.[2]
| Product | Fixed in |
|---|---|
| Serv-U Managed File Transfer Server and Serv-U Secured FTP | 15.2.3 HF1 |
Most families won’t have SolarWinds Serv-U; it’s for whoever looks after IT at work.
Exploitation
CISA added it to its catalog of known exploited vulnerabilities on 3 Nov 2021, 112 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 17 Nov 2021 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 91%. That’s in the top 1% of the flaws it scores.[4]
CISA assesses it as not automatable: each attack takes some work.
CISA doesn’t say who was attacked, or how.
Fix
SolarWinds has published a fix or advice.[3] CISA’s required action: “Apply updates per vendor instructions.”[1]
- Check whether you use it.SolarWinds Serv-U 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 (fixed in version 15.2.3 HF1 of Serv-U Managed File Transfer Server and Serv-U Secured FTP), or not in use. CISA gave US government agencies until 17 Nov 2021 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 10.0.[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 91%.[4] |
See also
Other SolarWinds Serv-U flaws
- CVE-2026-28318: uncontrolled resource consumption ·
- CVE-2024-28995: path traversal ·
- CVE-2021-35247: improper input validation ·
Other SolarWinds flaws
- CVE-2025-26399, SolarWinds Web Help Desk: deserialization of untrusted data ·
- CVE-2025-40536, SolarWinds Web Help Desk: security control bypass ·
- CVE-2025-40551, SolarWinds Web Help Desk: deserialization of untrusted data ·
- CVE-2024-28987, SolarWinds Web Help Desk: hardcoded credential ·
- CVE-2024-28986, SolarWinds Web Help Desk: deserialization of untrusted data ·
Other out-of-bounds write flaws
- CVE-2026-86950, Apple devices: out-of-bounds write ·
- CVE-2026-53266, Linux Kernel: out-of-bounds write ·
- CVE-2026-87491, Chrome and other Chromium browsers: out of bounds write ·
- CVE-2026-53362, Linux Kernel: unspecified ·
- CVE-2022-0995, Linux Kernel: out-of-bounds write ·
Added the same day
- CVE-2021-42258, BQE BillQuick Web Suite: SQL injection ·
- CVE-2021-42013, Apache HTTP Server: path traversal ·
- CVE-2021-41773, Apache HTTP Server: path traversal ·
- CVE-2021-40539, Zoho ManageEngine: aDSelfService plus authentication bypass ·
- CVE-2021-40444, Microsoft MSHTML: remote code execution ·
Every SolarWinds flaw on CISA’s list · Everything added in November 2021
References
- ^ Cybersecurity and Infrastructure Security Agency. “SolarWinds Serv-U Remote Code Execution Vulnerability”. Known Exploited Vulnerabilities Catalog, added 3 Nov 2021.
- ^ National Institute of Standards and Technology. “CVE-2021-35211 Detail”. National Vulnerability Database, published 14 Jul 2021.
- ^ SolarWinds. Advisories and fixes: microsoft.com, solarwinds.com.
- ^ FIRST. “Exploit Prediction Scoring System”. EPSS 0.912, percentile 99.8, 31 Oct 2026.
- CVE Program. “CVE-2021-35211”. 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.