Security readout for executives and security teams
Plain-English summary
CVE-2021-35223 affects SolarWinds Serv-U File Server. A feature that runs commands for audit events, such as failed logins, can accept user-controlled string variables as parameters. Under the right conditions, this can let an authenticated remote attacker run commands on the server.
Executive priority
Prioritize remediation for internet-facing or business-critical Serv-U servers. The vulnerability can lead to full server compromise, but available evidence requires low privileges and high attack complexity, so focus first on exposed older deployments.
Technical view
The issue is improper input handling in Serv-U’s event execute-command function. The CVSS vector requires network access and low privileges, with high attack complexity and no user interaction. Successful exploitation can cross trust boundaries and compromise confidentiality, integrity, and availability.
Likely exposure
Organizations running SolarWinds Serv-U 15.2.3 or earlier are potentially exposed, especially where the service is reachable over a network and event actions execute commands using user string variables.
Exploitation context
The source bundle does not show CISA KEV listing or cited evidence of active exploitation. Treat this as a serious authenticated RCE risk, but do not assume confirmed in-the-wild exploitation from the provided evidence.
Researcher notes
Key constraints are affected version scope, event execute-command configuration, and user-controlled variables in command parameters. Public evidence provided here is insufficient to confirm exploit activity, exploit reliability, or complete workaround guidance.
Mitigation direction
- Inventory all SolarWinds Serv-U deployments and versions.
- Upgrade affected systems according to SolarWinds advisory and 15.2.4 release notes.
- Review event actions that execute commands with user string variables.
- Restrict network access to Serv-U management and file-transfer services where feasible.
- Monitor vendor guidance for supported fixed versions and additional mitigations.
Validation and detection
- Confirm whether any Serv-U instance is version 15.2.3 or earlier.
- Review Serv-U event audit settings for execute-command actions.
- Check whether command parameters include user-controlled string variables.
- Review logs for suspicious failed-login events or unexpected command execution.
- Verify upgraded systems report a vendor-supported fixed version.
Public sources used
Generated from the cited source records. This long-tail analysis has not been individually reviewed by a named human.
Conservative CVE-to-ATT&CK context
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ATT&CK lookup starting points
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CWE-20: Exact CWE lookup
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Open ATT&CK lookupExecution behavior lookup
The CVE wording references code or command execution, so execution technique review may help defensive triage. This is a Glexia inferred lookup path, not an official MITRE, ATT&CK, or CVE Program mapping.
Open ATT&CK lookupCVE-2021-35223 mapping review
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Open ATT&CK lookup- Severity
- High
- CVSS
- 8.5 (3.1)
- Known Exploited
- No
- Published
Vector: CVSS:3.1/AV:N/AC:H/PR:L/UI:N/S:C/C:H/I:H/A:H
CNA and ADP enrichment extracted from CVE v5
These fields come from the CVE record and ADP containers, not from Glexia's Take. They preserve time-varying source decisions such as CISA SSVC, KEV status, CVSS metrics, and provider references.
CVSS vector scores
1 official scoreWe collect every scored CVSS vector available in the official CNA and ADP containers. When more than one version is present, the table keeps the source vectors side by side instead of collapsing them into the highest score.
CVSS:3.1/AV:N/AC:H/PR:L/UI:N/S:C/C:H/I:H/A:H1.86Primary CVE scoreVulnerability scoring details
Base CVSS 3.1 score
8.5HighVector: CVSS:3.1/AV:N/AC:H/PR:L/UI:N/S:C/C:H/I:H/A:H
Source materials
- CVE List V5 sourceCVE List V5
- https://support.solarwinds.com/SuccessCenter/s/article/Execute-Command-Function-Allows-Remote-Code-Execution-RCE-Vulnerability-CVE-2021-35223?language=en_USCVE reference · x_refsource_MISC
- https://documentation.solarwinds.com/en/success_center/servu/content/release_notes/servu_15-2-4_release_notes.htmCVE reference · x_refsource_MISC
- https://www.solarwinds.com/trust-center/security-advisories/cve-2021-35223CVE reference · x_refsource_MISC
Products and packages named in the record
CWE details
CWE links open Glexia weakness intelligence pages with official CWE context, developer remediation guidance, and related CVE mappings.