CVE-2019-25683: FileZilla Client DoS Vulnerability

CVE-2019-25683 Overview

CVE-2019-25683 is a denial of service vulnerability affecting FileZilla Client version 3.40.0. The vulnerability exists in the local search functionality, where a malformed path string can crash the application. Local attackers can exploit this flaw by entering a specially crafted path containing 384 'A' characters followed by 'BBBB' and 'CCCC' sequences in the search directory field and initiating a local search operation.

Critical Impact

Local attackers can cause a complete denial of service by crashing the FileZilla application, disrupting file transfer operations and potentially causing data loss for in-progress transfers.

Affected Products

• FileZilla Client 3.40.0
• filezilla-project filezilla_client

Discovery Timeline

• 2026-04-05 - CVE CVE-2019-25683 published to NVD
• 2026-04-09 - Last updated in NVD database

Technical Details for CVE-2019-25683

Vulnerability Analysis

This vulnerability is classified under CWE-532 and represents a denial of service condition in the FileZilla Client's local search functionality. The application fails to properly validate and handle path input strings before processing them in the search operation. When an attacker provides an overly long or malformed path string with specific character patterns, the application crashes due to improper input handling.

The vulnerability requires local access to the system where FileZilla is installed. An attacker must be able to interact with the FileZilla application interface to input the malicious path string into the search directory field. While this limits the attack surface to local scenarios, it could be leveraged in multi-user environments or as part of a broader attack chain.

Root Cause

The root cause of this vulnerability is improper input validation in the local search functionality. The application does not adequately sanitize or validate the length and content of path strings entered into the search directory field. When processing a specially crafted string containing 384 'A' characters followed by specific byte sequences ('BBBB' and 'CCCC'), the application encounters an unhandled condition that leads to a crash.

Attack Vector

The attack vector is local, requiring the attacker to have access to the system where FileZilla Client is installed. The exploitation process involves:

1. Opening FileZilla Client version 3.40.0
2. Accessing the local search functionality
3. Entering a malformed path string containing 384 'A' characters followed by 'BBBB' and 'CCCC' sequences
4. Initiating the local search operation
5. The application crashes, causing denial of service

The attack does not require any special privileges and can be executed by any user with access to the FileZilla application. Technical details and a proof-of-concept are available in the Exploit-DB #46484 entry.

Detection Methods for CVE-2019-25683

Indicators of Compromise

• Unexpected FileZilla application crashes during search operations
• Application crash logs showing errors related to path processing or string handling
• Multiple rapid restarts of the FileZilla process
• User reports of FileZilla becoming unresponsive when using search functionality

Detection Strategies

• Monitor for FileZilla process crashes and terminations, particularly those associated with search operations
• Implement endpoint detection rules that flag unusual application behavior patterns in FileZilla Client
• Review Windows Event Logs or system logs for application crash events related to filezilla.exe
• Deploy SentinelOne agents to detect and alert on application crashes that may indicate exploitation attempts

Monitoring Recommendations

• Enable crash reporting and logging for FileZilla Client installations
• Configure endpoint protection solutions to monitor FileZilla process stability
• Establish baseline metrics for normal application behavior to identify anomalies
• Review the VulnCheck FileZilla DoS Advisory for additional detection guidance

How to Mitigate CVE-2019-25683

Immediate Actions Required

• Update FileZilla Client to a version newer than 3.40.0 from the FileZilla Project Homepage
• Restrict access to systems running vulnerable FileZilla versions to trusted users only
• Consider temporarily disabling or restricting access to the local search functionality if upgrade is not immediately possible
• Deploy endpoint protection solutions to monitor for potential exploitation attempts

Patch Information

The recommended mitigation is to upgrade FileZilla Client to the latest available version. The vulnerability affects version 3.40.0 specifically. Users should download the latest stable release from the official FileZilla Project Homepage to ensure they have all security patches applied.

Workarounds

• Avoid using the local search functionality in FileZilla version 3.40.0 until the application is upgraded
• Restrict physical and remote access to systems running the vulnerable version
• Implement application whitelisting policies that prevent unauthorized users from interacting with FileZilla
• Consider using alternative file transfer clients temporarily while planning the upgrade