A dangerous security flaw has been discovered in Exim, a widely used email server software. The vulnerability, officially tracked as CVE-2025-26794, allows hackers to inject harmful commands into the system, potentially leading to data theft or even complete control over the email server. This issue affects Exim version 4.98 when used with a specific database system called SQLite. Experts warn that this is one of the biggest email security threats of 2025.
How This Vulnerability Works
The problem occurs because of the way Exim handles database queries under certain settings. It mainly affects systems that:
1. Use SQLite for storing email-related data – This happens when Exim is set up with a special feature called `USE_SQLITE`.
2. Enable the ETRN command – This is a function that allows users to request email deliveries, but it can be misused if not properly restricted.
3. Have weak protections against command execution – Some default settings make it easier for attackers to sneak in harmful database commands.
If all these conditions are met, a hacker can send specially designed emails to the server, tricking it into running unauthorized commands. This could allow them to access sensitive information, modify system settings, or even take control of the entire email system.
How Attackers Can Use This Flaw
For this security risk to be exploited, three things need to be true:
1. The system must be running Exim 4.98 with SQLite enabled.
2. The ETRN command must be set to "accept" instead of the safer "deny" mode.
3. A specific security setting, smtp_etrn_serialize, must be left at its default value, which can create a loophole for hackers.
Even though Exim’s default settings provide some level of security, many organizations adjust them to work with older systems, unknowingly making their servers more vulnerable.
Steps to Stay Safe
To protect email systems from this issue, cybersecurity experts recommend taking the following steps immediately:
1. Check which version of Exim is installed using the command `exim -bV`.
2. Disable SQLite integration if it’s not necessary.
3. Modify ETRN settings to prevent unauthorized use.
4. Update to the latest Exim version (4.98.1), which includes a fix for this problem.
For organizations that must continue using SQLite, additional security measures should be in place, such as filtering out risky commands and monitoring unusual activity in the database.
How Exim Developers Responded
The Exim development team acted quickly by releasing a patched version within 72 hours after confirming the issue. The flaw was originally reported by cybersecurity researcher Oscar Bataille, who followed responsible disclosure guidelines. This allowed Exim’s developers to fix the problem before it became public, reducing the chances of widespread attacks.
Why This Matters
Exim is used by over 60% of email servers on the internet, meaning this flaw could have affected millions of systems worldwide. This incident is a reminder that even well-established software can have hidden weaknesses, especially when newer features interact with older components.
To stay safe, organizations must regularly update their email software, follow security best practices, and stay informed about new threats. The faster vulnerabilities like this are addressed, the lower the risk of cyberattacks.
Companies are particularly susceptible to cyberattacks during public holidays and weekends due to reduced security manpower. A recent report on ransomware assaults, published by Semperis, a provider of identity-based cyber resilience, confirms this vulnerability.
The study revealed that an average of 86% of organizations assessed across the United States, United Kingdom, France, and Germany were targeted during public holidays or weekends. The findings also indicate that 75% of businesses reduced their security workforce by up to 50% during these periods, leaving critical systems exposed.
Half of the respondents who experienced cyberattacks reported being targeted during major business events such as mergers or acquisitions. For instance, after UnitedHealth acquired Change Healthcare, cybercriminals exploited a security flaw in remote access systems to breach the company’s infrastructure.
The report highlighted that 90% of ransomware attacks compromised a firm’s identity service, such as Microsoft Active Directory (AD) or Entra ID, as these are widely used and vulnerable. Additionally:
While 81% of respondents stated they possess the knowledge to defend against identity-related threats, 83% admitted to experiencing a successful ransomware assault within the past year. This disconnect underscores the need for better implementation of security measures.
The US Cybersecurity and Infrastructure Security Agency (CISA) has repeatedly emphasized the need for vigilance during weekends and public holidays. Notably, the ransomware group Clop exploited a long weekend to take advantage of a vulnerability in the MOVEit data exchange software. This attack affected over 130 companies in Germany, leading to significant data breaches and blackmail attempts.
To address these vulnerabilities, enterprises must take the following measures:
Prioritizing security as an essential component of business resilience can make the difference between surviving and thriving in the face of catastrophic cyber incidents.