⚠️ Overview

SynFlooder is a denial-of-service (DoS) malware family primarily used to perform SYN flood attacks, a network-layer assault that exploits the TCP three-way handshake by sending a high volume of SYN packets without completing the handshake. First observed in the wild around 2020, it is distributed as a standalone executable on underground forums and is not associated with a single persistent threat group but rather used by various hacktivists and script kiddies. It belongs to the DoS/DDoS tool category, distinct from ransomware or RATs.

🔧 Technical Capabilities

SynFlooder generates spoofed source IP addresses to obscure the attacker’s origin, making mitigation difficult. It targets a specified destination IP and port (commonly port 80 for HTTP servers) by sending crafted SYN packets using raw sockets or the WinPcap library on Windows systems. The malware does not require a command-and-control (C2) server; instead, operators provide target parameters via command-line arguments or a simple configuration file. It employs no persistence mechanisms, as it is typically run as a single-use payload from a compromised host or used offensively by an attacker. Evasion techniques include randomizing source ports and using randomized packet sizes to bypass shallow rate-limiting rules. According to MITRE ATT&CK, this activity maps to T1498 (Network Denial of Service) and T1499 (Endpoint Denial of Service).

📜 History & Notable Incidents

SynFlooder first appeared on a hacking forum in mid-2020 as an open-source tool published by a user under the alias "darkcoder." It was widely used in low-sophistication attacks against educational institutions and small businesses, with a notable campaign in late 2021 targeting online gaming servers in Eastern Europe. No specific CVEs are associated with the malware itself, as it exploits a fundamental protocol design weakness rather than a software vulnerability. Law enforcement actions have not been publicly documented against its distributors due to the decentralized nature of its use.

🔍 Detection Indicators

Behavioral signatures include an anomalous spike in TCP SYN packets from a single host (often thousands per second) with no corresponding SYN-ACK or RST responses. Network indicators involve packets with spoofed IP addresses from an ephemeral source port range (1024–65535) and a fixed destination port. No unique file hashes are publicly maintained because variants are custom-compiled, but typical file sizes range from 50–150 KB. On hosts, presence of a process named “synflood.exe” or similar, and the use of raw sockets (checkable via netstat -ano showing no listening TCP state) are common indicators.

☠️ Risk & Impact

SynFlooder can saturate network bandwidth and exhaust server resources, causing service unavailability for web applications, DNS servers, or any TCP-based service. While no data exfiltration occurs, financial losses from downtime can be substantial, particularly for e-commerce sites and online service providers. Sectors most affected include hosting providers, educational institutions, and small-to-medium businesses that lack enterprise-grade DDoS mitigation infrastructure.

🛡️ Mitigation

Defenses include enabling SYN cookies on servers (e.g., sysctl -w net.ipv4.tcp_syncookies=1 on Linux), deploying rate-limiting at the network perimeter, and using DDoS protection services such as Cloudflare or AWS Shield. No antivirus signatures specifically target SynFlooder, but generic detection for flood tools can be implemented via Snort rules (e.g., alert tcp $EXTERNAL_NET any -> $HOME_NET 80 flags:S,12.0.0.0.0).

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