CVE-2025-65403 Overview
CVE-2025-65403 is a buffer overflow vulnerability (CWE-120) discovered in the g_cfg.MaxUsers component of LightFTP version 2.0. This vulnerability allows remote attackers to cause a Denial of Service (DoS) condition by sending crafted input to the affected FTP server. With a CVSS v3.1 score of 6.5 (Medium severity), this vulnerability poses a significant risk to organizations running unpatched versions of LightFTP.
The vulnerability can be exploited over the network without requiring authentication, though user interaction is required for successful exploitation. While the confidentiality and integrity of the system remain unaffected, the availability impact is rated as High, meaning successful exploitation can render the FTP service completely unavailable.
Critical Impact
Remote attackers can crash LightFTP servers through crafted input targeting the g_cfg.MaxUsers configuration parameter, causing complete service disruption.
Affected Products
- LightFTP v2.0 (hfiref0x)
Discovery Timeline
- 2025-12-01 - CVE-2025-65403 published to NVD
- 2025-12-05 - Last updated in NVD database
Technical Details for CVE-2025-65403
Vulnerability Analysis
This buffer overflow vulnerability exists in the g_cfg.MaxUsers component of LightFTP v2.0. Buffer overflows occur when a program writes data beyond the boundaries of allocated memory, potentially corrupting adjacent memory regions. In this case, the vulnerability is classified under CWE-120 (Buffer Copy without Checking Size of Input), indicating that the application fails to properly validate the size of input data before copying it to a fixed-size buffer.
The CVSS vector CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:N/I:N/A:H reveals the following characteristics:
- Attack Vector (AV:N): Network-accessible, meaning the vulnerability can be exploited remotely
- Attack Complexity (AC:L): Low complexity required for exploitation
- Privileges Required (PR:N): No authentication needed
- User Interaction (UI:R): Some user interaction is required
- Impact: High availability impact with no confidentiality or integrity impact
The EPSS (Exploit Prediction Scoring System) probability is 0.046% with a percentile ranking of 14.39%, indicating a relatively low likelihood of exploitation in the wild at this time.
Root Cause
The root cause of this vulnerability lies in improper bounds checking within the g_cfg.MaxUsers configuration handling code. When processing user-supplied input related to the maximum users configuration parameter, LightFTP v2.0 fails to validate the length of the input before copying it into a fixed-size buffer. This allows an attacker to provide input that exceeds the buffer's capacity, resulting in a buffer overflow condition.
The vulnerability specifically affects the configuration parsing mechanism where the MaxUsers parameter is processed. Without proper input validation, oversized input data can overflow the allocated buffer space, corrupting memory and causing the application to crash.
Attack Vector
The attack can be executed remotely over the network by sending specially crafted input to the LightFTP server. The attacker targets the g_cfg.MaxUsers component with malformed or oversized data designed to trigger the buffer overflow condition.
The attack flow involves:
- Establishing a connection to the vulnerable LightFTP server
- Sending crafted input that targets the g_cfg.MaxUsers configuration parameter
- The oversized input overflows the buffer boundary
- Memory corruption occurs, leading to service crash
The vulnerability mechanism involves malicious input being processed by the g_cfg.MaxUsers component without proper size validation. When the input exceeds the expected buffer size, adjacent memory is overwritten, causing the server process to terminate abnormally. For detailed technical information, refer to the security advisory at the external references.
Detection Methods for CVE-2025-65403
Indicators of Compromise
- Unexpected crashes or service restarts of the LightFTP process
- Abnormally large or malformed FTP requests in server logs
- Memory access violation errors in system logs associated with LightFTP
- Repeated connection attempts followed by immediate service failures
Detection Strategies
Organizations can implement the following detection strategies to identify exploitation attempts:
Network Traffic Analysis: Monitor FTP traffic for unusually large configuration-related requests or malformed packets targeting the LightFTP service on ports 20/21 or custom configured ports.
Log Analysis: Review LightFTP server logs and system logs for crash events, segmentation faults, or memory access violations that coincide with incoming network connections.
Process Monitoring: Implement process monitoring to detect unexpected termination of the LightFTP service process, which may indicate successful exploitation.
Endpoint Detection: Deploy endpoint detection and response (EDR) solutions capable of identifying buffer overflow exploitation patterns and memory corruption attempts.
Monitoring Recommendations
- Configure alerts for LightFTP service crashes or unexpected restarts
- Implement network intrusion detection rules for anomalous FTP traffic patterns
- Enable verbose logging on LightFTP servers to capture detailed request information
- Monitor system resource utilization for unusual memory consumption patterns
- Deploy SentinelOne Singularity platform for real-time behavioral analysis and automated threat detection of exploitation attempts targeting buffer overflow vulnerabilities
How to Mitigate CVE-2025-65403
Immediate Actions Required
- Identify all instances of LightFTP v2.0 in your environment
- Implement network segmentation to limit exposure of vulnerable FTP servers
- Consider temporarily disabling LightFTP services if not business-critical
- Apply input validation at the network perimeter using a Web Application Firewall (WAF) or FTP proxy
- Monitor for updates from the hfiref0x project on GitHub
Patch Information
As of the last modification date (2025-12-05), users should check the official LightFTP GitHub repository at https://github.com/hfiref0x/LightFTP for the latest security updates and patched versions. Organizations are strongly advised to upgrade to a patched version as soon as one becomes available.
Review the technical advisory referenced at https://shimo.im/docs/9030JMJpv4IM4Nkw for additional details regarding the vulnerability and potential fixes.
Workarounds
If patching is not immediately possible, organizations can implement the following workarounds to reduce risk:
Network Access Controls: Restrict network access to the LightFTP server to trusted IP addresses only using firewall rules or network ACLs.
Input Filtering: Deploy network-based security controls that can filter or block malformed FTP requests before they reach the server.
Service Isolation: Run LightFTP in a containerized or sandboxed environment to limit the impact of potential crashes.
Alternative FTP Server: Consider migrating to an alternative FTP server solution until a patch is available.
Example firewall configuration to restrict FTP access:
# Restrict FTP access to trusted networks only
# Example using iptables
# Allow FTP from trusted network
iptables -A INPUT -p tcp --dport 21 -s 192.168.1.0/24 -j ACCEPT
# Drop all other FTP connections
iptables -A INPUT -p tcp --dport 21 -j DROP
# Allow established connections for data transfer
iptables -A INPUT -p tcp --dport 20 -m state --state ESTABLISHED,RELATED -j ACCEPT
Disclaimer: This content was generated using AI. While we strive for accuracy, please verify critical information with official sources.
