SolarWinds Serv-U Remote Memory Escape Remote Code Execution (RCE) Vulnerability (CVE-2021-35211) Technical Advisory
Executive & SecOps Summary
SECOPS ASSESSMENTSolarWinds Serv-U Managed File Transfer and Serv-U Secure FTP for Windows before version 15.2.3 HF2
Unauthenticated remote attackers can exploit this vulnerability to gain SYSTEM-level privileged access and execute arbitrary code on the host machine
Immediately upgrade to Serv-U version 15.2.3 HF2 or later, and restrict external access to the service ports
Affected Products & Version Scope
Remediation & Live Workaround Commands
PATCH & WORKAROUND1. Official Patch & Version Upgrade Commands
Apply the official security update by upgrading to Serv-U 15.2.3 HF2 or later. Use the following PowerShell commands to stop the Serv-U service safely before applying the patch.
# Query and stop the Serv-U service to prepare for the upgrade
$Service = Get-Service -Name 'Serv-U' -ErrorAction SilentlyContinue
if ($Service) {
Write-Output 'Stopping Serv-U service...'
Stop-Service -Name 'Serv-U' -Force
Set-Service -Name 'Serv-U' -StartupType Disabled
Write-Output 'Serv-U service stopped and disabled. Proceed with the installer.'
} else {
Write-Output 'Serv-U service not found on this system.'
}2. Temporary Workaround & Hardening Configuration
If patching cannot be performed immediately, restrict access to the Serv-U ports using Windows Advanced Firewall to limit exposure to trusted IP addresses only.
# Block all public access to Serv-U ports (FTP/SFTP/HTTPS)
New-NetFirewallRule -DisplayName 'Block-Public-Serv-U' `
-Direction Inbound `
-Action Block `
-Protocol TCP `
-LocalPort 21, 22, 443 `
-Enabled True
# Allow access only from trusted internal or partner IP ranges
New-NetFirewallRule -DisplayName 'Allow-Trusted-Serv-U-Only' `
-Direction Inbound `
-Action Allow `
-Protocol TCP `
-LocalPort 21, 22, 443 `
-RemoteAddress '10.0.0.0/8', '192.168.1.100' `
-Enabled True3. Log Audit & IoC Verification Commands
Inspect the Serv-U installation directory for unexpected crash dumps or error logs indicating memory access violations or exploitation attempts.
# Scan for crash dump files and anomalous error patterns in Serv-U logs
$InstallDir = 'C:\Program Files\SolarWinds\Serv-U'
if (Test-Path $InstallDir) {
Write-Host 'Checking for crash dumps (.dmp)...' -ForegroundColor Yellow
Get-ChildItem -Path $InstallDir -Filter '*.dmp' -Recurse | Select-Object Name, Length, LastWriteTime
Write-Host 'Searching logs for memory access violations...' -ForegroundColor Yellow
Select-String -Path "$InstallDir\*.txt" -Pattern 'EXCEPTION_ACCESS_VIOLATION', 'crash' -ErrorAction SilentlyContinue | Select-Object Line, Path -First 5
} else {
Write-Host 'Default Serv-U directory not found.' -ForegroundColor Red
}Block Exploits Instantly with Managed Virtual Patching Rules
Can't patch production immediately? Deploy managed WAF rules at the edge to intercept RCE, SQLi, and deserialization payloads before they hit your origin server.
Technical Root Cause & Exploitation Mechanics
1. Vulnerability Overview & Root Cause
This vulnerability (CVE-2021-35211) is a critical out-of-bounds write/memory corruption vulnerability (CWE-787) discovered by Microsoft in the SolarWinds Serv-U product line. The root cause lies in a 'Remote Memory Escape' flaw within the service's handling of incoming connection packets. Due to insufficient validation of input data, an attacker can corrupt the application's memory space, leading to arbitrary code execution with the privileges of the Serv-U process.
2. Exploitation Mechanics & Attack Scenario
An attacker initiates the exploit by sending specially crafted packets to the open port of the Serv-U service (typically SSH/SFTP or HTTP/S ports). When the Serv-U process parses these malicious payloads, it triggers a memory corruption event. This allows the attacker to hijack the instruction pointer and redirect execution flow to an injected shellcode or payload. Because the Serv-U service on Windows typically runs under high-privilege accounts (such as LocalSystem), the successfully executed code inherits these administrative privileges, granting the attacker full control over the operating system.
3. Infrastructure & Business Impact
With a CVSS score of 10.0 and active exploitation documented in the wild (CISA KEV catalog), this vulnerability poses an extreme risk to enterprise infrastructure. Successful exploitation allows threat actors to establish a persistent foothold, exfiltrate sensitive files transferred via FTP/MFT, and perform lateral movement across the internal network, potentially leading to full domain compromise or ransomware deployment.
Official Vendor Advisories & References
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.
Frequently Asked SecOps Questions (CVE-2021-35211)
FAQQ1. How critical is CVE-2021-35211 and is it actively exploited in the wild?
CVE-2021-35211 is rated CVSS 10.0 (CRITICAL) with a 30-day EPSS exploitation probability of 91.2%. It is listed in the CISA Known Exploited Vulnerabilities (KEV) catalog, requiring immediate emergency remediation.
Q2. Which vendors and products are affected by CVE-2021-35211?
Primary impacted vendors include solarwinds across serv-u, serv-u managed file transfer, serv-u secure ftp. Scope summary: SolarWinds Serv-U Managed File Transfer and Serv-U Secure FTP for Windows prior to version 15.2.3 HF2
Q3. What temporary mitigations or workarounds exist for CVE-2021-35211 before patching?
[Mitigation] Immediately upgrade to Serv-U version 15.2.3 HF2 or later, and restrict external access to the service ports Follow the CLI remediation commands and interactive SecOps checklist on this page to restrict exposure prior to your maintenance window.
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