Serv-U Remote Memory Escape Vulnerability (CVE-2021-35211)

What is affected, what to do

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.

The vulnerability is being actively exploited; treat it as a priority. CISA lists it as used in ransomware campaigns. Update serv-u to version 15.2.3. Immediate actions, mitigation and detection rules are in the runbook below.

Affected

• SolarWinds Serv-U Managed File Transfer Server and Serv-U Secured FTP gte SolarWinds Serv-U..*

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Metrics

Response and mitigation

1. 01

   Runbook step 1

   Immediate response (0-24 h)

   • Inventory every Serv-U instance (Managed File Transfer and Secure FTP for Windows) and check the version. Anything below 15.2.3 HF2 is vulnerable. Deploy vendor hotfix 15.2.3 HF2 or a later release immediately. Check the SolarWinds advisory for exact build details.
   • If you cannot patch right away, disable the SSH/SFTP service in Serv-U. The flaw sits in the SSH protocol handler, and vendor guidance states that instances without SSH enabled are not exploitable. Alternatively, block the SFTP listener (default TCP/22 or your configured port) at the perimeter firewall.
   • Run a compromise check both before and after patching: search DebugSocketLog.txt in the Serv-U install directory for EXCEPTION: C0000005; CSUSSHSocket::ProcessReceive();. A hit is a strong indicator of compromise. If you find one, isolate the host and preserve evidence (memory image, Serv-U directory, event logs).
   • Review Serv-U users and domain configuration for unknown or recently created accounts. Pay particular attention to global administrators and accounts with a home directory of C:\. Remove anything you cannot account for.
   • If compromise is suspected, rotate every credential stored on or used from the host: Serv-U admin, service account, local admins, and any partner or service credentials held in Serv-U. Treat the host as fully owned, since code execution runs with service-level privileges.
2. 02

   Runbook step 2

   Mitigation layers

   • Network segmentation: Place Serv-U in a dedicated DMZ with default-deny egress (no direct internet egress, no SMB/RDP/WinRM into the internal network). This cuts off tool staging and lateral movement.
   • Access restriction: Allowlist the SFTP port to known partner IP addresses only. Expose the Serv-U web/admin console exclusively to a management network.
   • IPS/network: Enable vendor IDS signatures for CVE-2021-35211 or for malformed SSH key-exchange traffic against the Serv-U port by updating your Suricata/Snort rulesets. Alert on repeated aborted SSH handshakes from a single source.
   • Endpoint hardening: Use an EDR custom block, ASR rule or AppLocker/WDAC to stop Serv-U.exe from spawning cmd.exe, powershell.exe, rundll32.exe, mshta.exe and similar binaries.
   • Least privilege: Run the Serv-U service under a dedicated low-privilege service account rather than LocalSystem. Restrict write access to the program and configuration directories to that account.
   • Configuration: Keep debug socket logging enabled, because it is your primary compromise-assessment source. Ship the logs to the SIEM.
3. 03

   Runbook step 3

   Detection rules

   • Serv-U application log: Run a recurring search of DebugSocketLog.txt for C0000005 together with CSUSSHSocket::ProcessReceive, and configure it as a high-severity SIEM alert.
   • Sysmon EID 1 / Windows 4688 (process ancestry): Alert when ParentImage ends with \Serv-U.exe and the child is a shell or LOLBin. KQL example: DeviceProcessEvents | where InitiatingProcessFileName =~ "Serv-U.exe" and FileName in~ ("cmd.exe","powershell.exe","rundll32.exe","whoami.exe","net.exe")
   • Sysmon EID 11 / file integrity: Flag new or modified files in the Serv-U directory outside maintenance windows. Focus on changed .Archive or user configuration files (a sign of injected admin accounts) and on dropped .txt or script files.
   • Windows Security 4720/4732: Alert on local account creation or Administrators group additions on the Serv-U host. SPL example: index=wineventlog host=<servu_host> EventCode IN (4720,4732)
   • Zeek/Suricata: Watch for unexpected outbound connections from the Serv-U host. Also watch for bursts of short, abnormally terminated SSH sessions on the SFTP port (ssh.log entries with no auth_success and an aborted key exchange).

Detection rules

No NEOSEC detection rule is available for this vulnerability yet.

Classification under NIS2 and CRA

Rule-based classification from exploitation status and severity. It does not replace a case-by-case assessment.

NIS2

NIS2 is EU Directive (EU) 2022/2555 to raise the level of cybersecurity across the Union. It substantially broadens obligations compared with the first NIS directive: more sectors, mandatory risk management, staged reporting duties and personal accountability of senior management.

Read in glossary

Actively exploited: treat the vulnerability as a priority in vulnerability handling under Art. 21(2)(e) NIS2. If it is exploited in your environment, check the reporting obligation under Art. 23 (early warning within 24 hours).

CRA

The CRA is EU Regulation (EU) 2024/2847 setting horizontal cybersecurity requirements for „products with digital elements“ — from industrial controllers and software to connected consumer devices. For the first time manufacturers must ensure security across the entire product lifecycle, handle vulnerabilities and provide security updates.

Read in glossary

Since 11.09.2026, manufacturers of affected products with digital elements must report actively exploited vulnerabilities under Art. 14 CRA, with an early warning within 24 hours via ENISA's single reporting platform.

ISO/IEC 27001

ISO/IEC 27001 is the internationally leading standard for an information security management system (ISMS). It describes a risk-based, process-oriented framework to systematically plan, implement, review and improve information security.

Read in glossary

Control A.8.8, management of technical vulnerabilities: assess relevance to your systems, plan remediation and keep evidence.

Description

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.

Source: CSA/SingCERT, NVD (NIST), cvelistv5, CISA KEV Catalog, CVE.org (MITRE)

Vendors and products

• SolarWinds SolarWinds Serv-U

Weakness classes (CWE)

• CWE-787Out-of-bounds Write

  Base

  The product writes data past the end, or before the beginning, of the intended buffer.

  Entry at MITRE CWE-787

References & sources

• solarwinds.comtrust-center/security-advisories/cve-2021-35211x_refsource_MISC, Patch, Vendor advisory
• microsoft.comsecurity/blog/2021/07/13/microsoft-discovers-threat-actor-targe…x_refsource_MISC, Patch, Vendor advisory
• cisa.govknown-exploited-vulnerabilities-catalog?field_cve=CVE-2021-35211government-resource, US government resource
• cert.cyber.gouv.frANSSI / CERT-FR: CERTFR-2021-ALE-015 – Multiples vulnérabilités dans SolarWinds Serv-U (clôturée 2021-10-19)cert-fr