External risk intelligence

SolarWinds Serv-U Remote Code Execution Vulnerability.

CVE advisoryKnown Exploit

CVE-2021-35211

SolarWinds Serv-U is a Managed File Transfer and Secure FTP solution. Such products are explicitly designed to be internet-facing to facilitate the external exchange of files and data, making public network exposure a standard and expected deployment pattern for this software.

Out-of-bounds Write

Solarwinds Serv U

before 15.2.315.2.3

Halo Surface Signal: 5 out of 5 — more likely to be public-facing.

External exposure likelihood

Horizon Alert

Summary of the vulnerability and why it matters

A remote memory escape vulnerability exists in the SolarWinds Serv-U product. This flaw could allow an unauthorized party to gain elevated access to the system hosting the Serv-U software. This could affect organizations using Serv-U for file transfer operations.

SolarWinds Serv-U software
Remote memory escape vulnerability
Unauthorized privileged access

Attack Path

How an attacker could exploit the issue

A remote code execution vulnerability exists in the SolarWinds Serv-U product. An attacker can exploit this to gain privileged access to the affected server. This allows for potential unauthorized control over the system and its data.

Exposure condition: Public network exposure.
Attacker starting point: Network.
Trigger and result: Remote code execution, privileged access.

Live Threat

Current exploitation, exposure, and threat context

This vulnerability presents a significant risk to organizations using the affected SolarWinds Serv-U software. Exploitation could allow attackers to gain privileged access to the server, potentially leading to widespread system compromise. The critical nature of this vulnerability and its known exploitation underscore the need for prompt attention.

Attackers require no special skill.
No access or conditions are required.
Business risk is critical and urgent.

Operational Fix

Recommended remediation, mitigation, and detection steps

This vulnerability in SolarWinds Serv-U allows an attacker to execute code remotely with privileged access on the affected system. Organizations should prioritize understanding their exposure to this threat. This requires identifying all instances of the vulnerable Serv-U software across the environment. After identification, immediate steps should be taken to limit or remove external access to these systems until a permanent fix can be applied. The final critical step involves applying the vendor-supplied patch, verifying its successful implementation, and establishing ongoing monitoring for any signs of compromise or related malicious activity.

Find affected SolarWinds Serv-U assets.
Reduce exposure or isolate risk.
Apply vendor fix, verify, and monitor.

Supplementary metadata

CVSS vector

Validate whether this threat affects your internet-facing exposure.

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Frequently asked questions

What is SolarWinds Serv-U and what is it used for?

SolarWinds Serv-U is a software product used for managed file transfer and secure file transfer (FTP) operations. It allows organizations to exchange files and data, often over the internet.

What kind of vulnerability does CVE-2021-35211 represent?

CVE-2021-35211 is a critical remote code execution vulnerability. It's classified as a buffer overflow, specifically a remote memory escape, allowing an attacker to potentially take control of the server running the software.

What preconditions are needed to trigger this Serv-U vulnerability?

An attacker does not need any special skills or prior access to exploit this vulnerability. The vulnerability can be triggered remotely over the network without any specific user interaction.

Who should be concerned about CVE-2021-35211, considering its exposure?

Organizations using SolarWinds Serv-U should be concerned. This vulnerability is classified as external, meaning it can be exploited over the internet. Anyone running Serv-U that is accessible from the public network is at risk.

What are the first steps for responding to this threat?

First, identify all instances of the affected SolarWinds Serv-U software within your environment. Then, take immediate action to limit or remove external access to these systems until a permanent fix can be applied. Finally, apply the vendor-supplied patch and confirm its successful implementation.

References

Cyber Threat Intelligence (CTI)

Sources: threatActor

Written by Nicholas Merritt

Nicholas Merritt is VP of Security Solutions at Halo Security. With more than 25 years across application security, vulnerability prioritization, and offensive security, he helps teams translate threat data into practical exposure validation and remediation focus. He authors Halo Threat Intelligence CVE advisories using Halo Surface Signal and H/A/L/O context to prioritize internet-facing risk.