Warning: hackers exploit SolarWinds Serv-U flaw to cause server crashes
- Evelyn Carter
- Jun 8
- 3 min read
The U.S. Cybersecurity and Infrastructure Security Agency (CISA) has issued an urgent warning regarding a newly patched, high-severity vulnerability within SolarWinds Serv-U. Cybercriminals are actively exploiting this flaw in the wild, with the primary intent of crashing enterprise servers. To stay updated on international security trends and protect your infrastructure, explore the IPSIP Vietnam Blog.
What is SolarWinds Serv-U and why is this flaw significant?
SolarWinds Serv-U is a dedicated file transfer application designed for both Windows and Linux environments. It offers Managed File Transfer (MFT) and FTP server functionalities, allowing organizations to securely share files via standard protocols such as HTTP/HTTPS, FTP, FTPS, and SFTP.
The recently uncovered security flaw, tracked as CVE-2026-28318, is classified as a Denial-of-Service (DoS) vulnerability. This type of security issue causes uncontrolled resource consumption, effectively overwhelming the server and forcing it to shut down or become unresponsive.
How do cybercriminals execute this server-crashing attack?
According to SolarWinds, the issue stems from a weakness in how the system handles internal resource consumption. Remote attackers can compromise the system by sending specially crafted POST requests (data submission commands) that utilize the Content-Encoding: deflate header.
What makes this threat particularly dangerous is its low complexity. The attack requires no prior privileges or user accounts, meaning anyone on the internet can trigger it remotely. Furthermore, it demands zero user interaction, meaning the server will crash instantly upon receiving the malicious request without anyone needing to click a link or open a file.
What is the scale of impact and the response from security agencies?
Internet intelligence networks indicate that a significant number of systems are currently exposed. The Shodan platform identifies over 12,000 Serv-U servers accessible online, while the Shadowserver foundation monitors just over 3,100 exposed instances. It remains unclear how many of these systems have adopted the latest security fix.
Due to active exploitation, CISA placed CVE-2026-28318 into its Known Exploited Vulnerabilities Catalog. Under Binding Operational Directive (BOD) 22-01, all U.S. Federal Civilian Executive Branch agencies are mandated to secure their infrastructure against these attacks by June 19. CISA also strongly advises private-sector network defenders to patch immediately, noting that such flaws are frequent targets that pose immense risks to organizational networks.
Has SolarWinds Serv-U been targeted by malicious actors before?
Historically, SolarWinds products - especially Serv-U have frequently been targeted by both cybercrime syndicates and state-backed hacking groups seeking to breach corporate networks.
In 2021: the Clop ransomware group leveraged a remote code execution flaw (CVE-2021-35211) to break into corporate networks. The same vulnerability was used in zero-day attacks by a Chinese hacking collective known as DEV-0322 during July of that year.
In June 2024: security firms GreyNoise and Rapid7 reported active exploitation of a path-traversal vulnerability (CVE-2024-28995) affecting Serv-U.
Overall Context: Over the past few years, CISA has added a total of 11 different vulnerabilities across various SolarWinds software lines to its active exploit tracking list, including flaws leveraged by ransomware operators.
What mitigation steps should network administrators take immediately?
SolarWinds has addressed the issue by releasing Serv-U 15.5.4 Hotfix 1. Deploying this update is the most effective way to eliminate the risk.
For administrators who cannot apply the patch immediately, the vendor recommends implementing the following temporary workarounds:
Restrict server access exclusively to known, trusted IP addresses.
Configure network defenses to block any incoming POST requests containing the "content-encoding" attribute, as the Serv-U service does not require this functionality for its standard operations.
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