CISA KEV Critical Memory corruption

CVE-2021-35211 — SolarWinds Serv-U Remote Code Execution Vulnerability

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.

Published Jul 14, 2021 · Updated Oct 13, 2026 · Sources: CISA Known Exploited Vulnerabilities, NIST National Vulnerability Database, CVE Program (cve.org) e CISA ADP, GitHub Security Advisories · CNA: SolarWinds

Urgent

Who exploits it and how urgent it is

A 0–100 score combining confirmed exploitation, ransomware and APT use, public exploits, EPSS and severity.

CISA KEV · Nov 3, 2021 R clop Detection templates (Nuclei) · 1 Proofs of concept on GitHub · 3

How the score was calculated

Confirmed exploitation (CISA KEV)+35
Used in ransomware campaigns (CISA)+10
Public exploit/PoC (nuclei, github)+10
EPSS 91.2% (30-day exploitation probability)+15
Technical severity (critical)+10

The score orders the patch queue; it does not replace your asset context (internet exposure, criticality, compensating controls).

Triage

Is it exploited, how likely is exploitation, what does it touch, and how severe do the sources call it.

Exploitation

Confirmed

CISA KEV, listed Nov 3, 2021

EPSS FIRST

91.16%

ahead of 100% of scored CVEs · Oct 14, 2026

Affects

Solarwinds

Serv-u and 1 more

CVSS base

10.0

Critical · 4 source(s)

CISA SSVC assessment CISA

Three decision points CISA publishes for the CVEs it assesses (SSVC 2.0.3). A stakeholder decision, not a severity score.

Exploitation

Active

none · proof-of-concept · active

Automatable

can an attacker script the kill-chain steps

Technical impact

Total

partial · total control of the vulnerable component

Remediation

Fixes and mitigations documented by the sources — linked to the original advisory.

Documented patch — www.microsoft.com (NIST National Vulnerability Database)
Documented patch — www.solarwinds.com (NIST National Vulnerability Database)

CISA required action CISA

Apply updates per vendor instructions.

US federal deadline: Nov 17, 2021 — passed 1793 day(s) ago

First 24 hours

Ordered from the record’s own fields — exposure first, because you cannot patch what you have not found. A rule-generated starting point.

Identify exposed assets running the affected vendor/product/version combinations (Solarwinds Serv-u, Solarwinds Serv-U Managed File Transfer Server and Serv-U Secured FTP).
Treat as emergency remediation: the CVE is in CISA KEV (confirmed exploitation).
The attack is network-based and needs no privileges: restrict exposure (ACLs, VPN, WAF) until patched.
Apply the fixed version or the patch documented under Remediation.
EPSS ahead of 100% of scored CVEs: prioritize it in the patch queue.
Search available logs for exploit probes matching the vulnerability class (memory corruption), errors and authentication anomalies.

Affected scope

Vendor, product and versions as the sources word them.

Vendor	Product	Versions	Status	Source
Solarwinds	Serv-u	< 15.2.3version 15.2.3version 15.2.3-hotfix1	Vulnerable	NIST National Vulnerability Database
Solarwinds	Serv-U Managed File Transfer Server and Serv-U Secured FTP	≥ SolarWinds Serv-U and < 15.2.3 HF1	Vulnerable	CVE Program (cve.org) e CISA ADP

Attack characteristics

The CVSS vector, decoded. It describes the attack, not your exposure to it.

Attack vector

Network

Attack complexity

Low

Privileges required

None

User interaction

None

Scope

Changed

Confidentiality

High

Integrity

High

Availability

High

Every base score collected

Sources score independently and may disagree; each row says who scored it and under which version.

Score	Version	Severity	Expl.	Impact	Scored by
10.0	CVSS 3.1	Critical	3.9	6.0	nvd@nist.gov via NIST National Vulnerability Database
10.0	CVSS 3.1	Critical	—	—	GitHub, Inc. via GitHub Security Advisories
9.0	CVSS 3.1	Critical	2.2	6.0	psirt@solarwinds.com via NIST National Vulnerability Database
9.0	CVSS 3.1	Critical	—	—	SolarWinds via CVE Program (cve.org) e CISA ADP
10.0	CVSS 2.0	High	10.0	10.0	nvd@nist.gov via NIST National Vulnerability Database

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:H/I:H/A:H (score shown at the top: nvd@nist.gov)

Scoring entities (NIST, CNA, ADP) score independently; each score is shown with its origin. For prioritization we use 10.0 (NVD (NIST)) as a conservative provisional reference — not a definitive official score.

Weakness MITRE

Weakness class according to MITRE’s CWE catalog.

CWE-787 · Out-of-bounds Write

Other CVEs with this weakness →

CISA KEV details CISA

CISA lists a CVE once it has evidence of exploitation against real targets.

Listed

Nov 3, 2021

Federal deadline

Nov 17, 2021

passed 1793 day(s) ago

Ransomware use

Known

CISA records ransomware use only for campaigns it has confirmed, so “Unknown” means unrecorded, not ruled out. The deadline binds US federal agencies; for everyone else it is a reference date.

Public exploits, PoCs and detection templates

Public code that demonstrates or tests the flaw raises the odds of mass exploitation — and Nuclei templates help find vulnerable assets in your environment.

Detection templates (Nuclei)

SolarWinds Serv-U FTP - Remote Code Execution

Proofs of concept on GitHub

BishopFox/CVE-2021-35211 ★ 39 Jan 14, 2022
Simple Serv-U CVE-2021-35211 PoC ★ 12 Sep 30, 2021
Serv-U-FTP CVE-2021-35211 exploit ★ 1 Oct 24, 2021

Warning: public PoCs are sometimes disguised malware. Use them only for authorized testing in an isolated environment. This portal does not host exploit code — it only points to the sources.

Associated ransomware groups ransomware.live

clop

EPSS over time FIRST

Oct 13, 2026 → Oct 14, 2026 · 2 reading(s) · max 91.16%

Detection

Read off the CVSS vector and the weakness class. Starting points, not tested rules.

Prioritize edge telemetry for the network-reachable component (proxy, WAF, IDS/IPS).
Correlate service crashes/restarts with inbound traffic; repeated crashes may indicate attempts.
Hunt retroactively in logs from before Nov 3, 2021 (KEV listing): exploitation may have started earlier.

web/proxy/WAF logsnetwork IDS/IPSendpoint telemetry (EDR)crash dumps

Timeline

What happened to this CVE, newest first.

2026
Affected products added · solarwinds/serv-u managed file transfer server and serv-u secured ftp
Oct 13, 2026 · CVE Program (cve.org) e CISA ADP
SSVC assessment updated
Oct 13, 2026 · CVE Program (cve.org) e CISA ADP
Record published
Oct 13, 2026
Fix documented
Oct 13, 2026 · NIST National Vulnerability Database
Affected products added · solarwinds/serv-u
Oct 13, 2026 · NIST National Vulnerability Database
CVSS changed · 10.0
Oct 13, 2026 · NIST National Vulnerability Database
Source status changed · → Analyzed
Oct 13, 2026 · NIST National Vulnerability Database
Added to CISA KEV (detected)
Oct 13, 2026 · CISA Known Exploited Vulnerabilities
Record created in this database
Oct 13, 2026 · CISA Known Exploited Vulnerabilities
2021
Added to CISA KEV catalog
Nov 2, 2021 · CISA
CVE published
Jul 14, 2021 · SolarWinds

Source activity

Repeated readings are counted rather than listed.

2× References added · Oct 13, 2026 – Oct 13, 2026
1× Official description updated · Oct 13, 2026 – Oct 13, 2026

References

5 on the record

www.microsoft.com/security/blog/2021/07/13/microsoft-discovers-threat-actor-targeting-s
Patch, Vendor Advisory, x_refsource_MISC, NIST National Vulnerability Database
www.solarwinds.com/trust-center/security-advisories/cve-2021-35211
Patch, Vendor Advisory, x_refsource_MISC, NIST National Vulnerability Database
www.cisa.gov/known-exploited-vulnerabilities-catalog
US Government Resource, NIST National Vulnerability Database
nvd.nist.gov/vuln/detail/CVE-2021-35211
GitHub Security Advisories
github.com/advisories/GHSA-cpmg-9xq8-3934
GitHub Security Advisories

Elsewhere on this site

Solarwinds
every CVE for this vendor

2021-11
what else CISA listed that month

Memory corruption
same class

CWE-787
other pages naming this weakness

Related CVEs (same products)

Critical CVE-2026-28321 SolarWinds Serv-U Broken Access Control Vulnerability Jul 21, 2026
Critical CVE-2026-28317 SolarWinds Serv-U Insecure Direct Object Reference (IDOR) Vulnerability Jul 21, 2026
Critical CVE-2026-28313 SolarWinds Serv-U Insecure Direct Object Reference (IDOR) Vulnerability Jul 21, 2026
Critical CVE-2026-28312 SolarWinds Serv-U Privilege Escalation Vulnerability Jul 21, 2026
Critical CVE-2026-28314 SolarWinds Serv-U Insecure Direct Object Reference (IDOR) Vulnerability Jul 21, 2026
Critical CVE-2026-28316 SolarWinds Serv-U Insecure Direct Object Reference (IDOR) Vulnerability Jul 21, 2026

Not in any source we poll

Listed rather than left blank: an empty field and an unmeasured one look identical on screen, and only one is a reason to look elsewhere.

No confirmed IOCs, IP addresses, domains, file hashes, or malware artifacts supplied.
No organization-specific asset inventory, compensating-control status, or patch deployment evidence supplied.
No exploit packet captures, log samples, or incident case IDs supplied.

Answered from this record

Text assembled only from the fields above — no additional claims.

What should defenders know first?

CVE-2021-35211 is "SolarWinds Serv-U Remote Code Execution Vulnerability", rated Critical. It affects Serv-u (Solarwinds); versions are in the scope table. It is in CISA KEV: there is evidence of real-world exploitation. CVSS 10.0 describes technical severity. EPSS 91.16% estimates the probability of exploitation in 30 days; neither replaces your asset context. First verify whether exposed or business-critical assets run the affected versions, apply vendor fixes or mitigations, and preserve logs for review.

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