CVE-2021-35247
CVE-2021-35247 is a security flaw in SolarWinds Serv-U, SolarWinds’ IT management software. Attackers are using a flaw in 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 21 Jan 2022.[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 15.3 of Serv-U.
Description
CISA calls it an improper input validation in SolarWinds Serv-U. In its words: “SolarWinds Serv-U versions 15.2.5 and earlier contain an improper input validation vulnerability that allows attackers to build and send queries without sanitization.”[1]
The CVE record adds: “Serv-U web login screen to LDAP authentication was allowing characters that were not sufficiently sanitized. SolarWinds has updated the input mechanism to perform additional validation and sanitization. Please Note: No downstream affect has been detected as the LDAP servers ignored improper characters. To insure proper input validation is completed in all environments. SolarWinds recommends scheduling an update to the latest version of Serv-U.”[2]
The underlying weakness is improper input validation (CWE-20), in MITRE’s list of common software weaknesses.
It scores 5.3 out of 10 on the Common Vulnerability Scoring System, which rates it medium, 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 | 15.3 |
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 21 Jan 2022, 11 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 4 Feb 2022 to fix it.[1]
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 4%. That’s in the top 11% 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 15.3 of Serv-U), or not in use. CISA gave US government agencies until 4 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 5.3.[2] | |
| CISA adds it to its catalog of flaws known to be used in attacks.[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 4%.[4] |
See also
Other SolarWinds Serv-U flaws
- CVE-2026-28318: uncontrolled resource consumption ·
- CVE-2024-28995: path traversal ·
- CVE-2021-35211: remote code execution ·
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 improper input validation flaws
- CVE-2026-93952, Arista VeloCloud Orchestrator: improper input validation ·
- CVE-2019-1068, Microsoft SQL Server: remote code execution ·
- CVE-2026-12569, PTC Windchill and FlexPLM: improper input validation ·
- CVE-2026-34910, Ubiquiti UniFi OS: improper input validation ·
- CVE-2026-6973, Ivanti Endpoint Manager Mobile (EPMM): improper input validation ·
Added the same day
- CVE-2018-8453, Microsoft Win32k: privilege escalation ·
- CVE-2012-0391, Apache Struts 2: improper input validation ·
- CVE-2006-1547, Apache Struts 1: actionForm denial-of-service ·
Every SolarWinds flaw on CISA’s list · Everything added in January 2022
References
- ^ Cybersecurity and Infrastructure Security Agency. “SolarWinds Serv-U Improper Input Validation Vulnerability”. Known Exploited Vulnerabilities Catalog, added 21 Jan 2022.
- ^ National Institute of Standards and Technology. “CVE-2021-35247 Detail”. National Vulnerability Database, published 10 Jan 2022.
- ^ SolarWinds. Advisories and fixes: documentation.solarwinds.com.
- ^ FIRST. “Exploit Prediction Scoring System”. EPSS 0.037, percentile 89.7, 31 Oct 2026.
- CVE Program. “CVE-2021-35247”. 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.