CVE-2019-25646: Tabslab Mailcarrier Buffer Overflow Flaw

CVE-2019-25646 Overview

CVE-2019-25646 is a critical buffer overflow vulnerability affecting Tabs Mail Carrier 2.5.1. The vulnerability exists in the MAIL FROM SMTP command handler, allowing remote attackers to execute arbitrary code by sending a crafted MAIL FROM parameter. By connecting to the SMTP service on port 25 and sending a malicious MAIL FROM command with an oversized buffer, attackers can overwrite the EIP register and execute a bind shell payload, gaining complete control over the affected system.

Critical Impact

Remote attackers can achieve arbitrary code execution on vulnerable Tabs Mail Carrier servers without authentication, potentially leading to complete system compromise through a bind shell payload.

Affected Products

• Tabslab Mailcarrier 2.5.1
• cpe:2.3:a:tabslab:mailcarrier:2.5.1:*:*:*:*:*:*:*

Discovery Timeline

• 2026-03-24 - CVE-2019-25646 published to NVD
• 2026-03-25 - Last updated in NVD database

Technical Details for CVE-2019-25646

Vulnerability Analysis

This buffer overflow vulnerability (CWE-787: Out-of-bounds Write) occurs in the SMTP service component of Tabs Mail Carrier when processing the MAIL FROM command. The application fails to properly validate the length of user-supplied input before copying it into a fixed-size buffer on the stack. When an attacker sends an excessively long string as the MAIL FROM parameter, the buffer overflows, corrupting adjacent memory including the saved return address (EIP register). This classic stack-based buffer overflow allows attackers to redirect program execution to arbitrary code, such as a bind shell payload that provides remote command execution capabilities.

Root Cause

The root cause of this vulnerability is improper input validation in the MAIL FROM command handler. The application does not perform adequate boundary checking on the length of the email address parameter, allowing attackers to supply data that exceeds the allocated buffer size. This results in a stack-based buffer overflow that overwrites critical control structures, enabling arbitrary code execution.

Attack Vector

The attack is network-based and requires no authentication or user interaction. An attacker can exploit this vulnerability by:

1. Establishing a TCP connection to the target SMTP server on port 25
2. Sending a MAIL FROM command with a specially crafted oversized parameter
3. The malicious payload overwrites the EIP register with a controlled address
4. Execution is redirected to shellcode that establishes a bind shell

The vulnerability allows for unauthenticated remote code execution, making it highly dangerous for any exposed Tabs Mail Carrier instances. Technical details and exploitation methods are documented in the Exploit-DB #46547 entry and the VulnCheck Advisory.

Detection Methods for CVE-2019-25646

Indicators of Compromise

• Abnormally long MAIL FROM commands in SMTP server logs exceeding typical email address lengths
• Unexpected bind shell connections or new listening ports on affected servers
• SMTP service crashes or unexpected restarts indicating exploitation attempts
• Network traffic containing large payloads directed at port 25

Detection Strategies

• Deploy network intrusion detection rules to identify oversized MAIL FROM commands in SMTP traffic
• Monitor for unusual process spawning from the Mail Carrier service process
• Implement application-level logging to capture SMTP command parameters and lengths
• Use SentinelOne Singularity to detect and block memory corruption exploitation attempts

Monitoring Recommendations

• Enable detailed SMTP transaction logging on all Mail Carrier instances
• Monitor network connections from mail servers for unexpected outbound connections
• Set up alerts for SMTP service crashes or restarts that may indicate exploitation attempts
• Review system logs for evidence of bind shell activity or unauthorized command execution

How to Mitigate CVE-2019-25646

Immediate Actions Required

• Disable or restrict network access to the SMTP service on port 25 until patching is possible
• Implement network segmentation to limit exposure of vulnerable Mail Carrier servers
• Deploy network-based intrusion prevention systems to block exploitation attempts
• Consider migrating to a modern, actively maintained mail transfer agent

Patch Information

No vendor patch information is currently available in the CVE data. Tabs Mail Carrier 2.5.1 is a legacy application, and users should verify with the vendor whether security updates are available. Given the age of this software, organizations are strongly advised to migrate to actively supported mail server solutions.

For additional technical details, refer to the VulnCheck Advisory and Exploit-DB #46547.

Workarounds

• Restrict access to SMTP port 25 to trusted IP addresses only using firewall rules
• Deploy a web application firewall or network IDS/IPS with rules to detect oversized MAIL FROM parameters
• Run the Mail Carrier service in a sandboxed or isolated environment to limit impact
• Consider replacing Tabs Mail Carrier with a modern, actively maintained SMTP server solution

# Example firewall rule to restrict SMTP access to trusted networks only
# Linux iptables example
iptables -A INPUT -p tcp --dport 25 -s 192.168.1.0/24 -j ACCEPT
iptables -A INPUT -p tcp --dport 25 -j DROP

# Windows Firewall example (PowerShell)
New-NetFirewallRule -DisplayName "Block SMTP Except Trusted" -Direction Inbound -Protocol TCP -LocalPort 25 -RemoteAddress 192.168.1.0/24 -Action Allow
New-NetFirewallRule -DisplayName "Block SMTP Default" -Direction Inbound -Protocol TCP -LocalPort 25 -Action Block