A bug in an obscure but widely used email program may be putting as many as 400,000 servers around the world at risk of serious attack until they install an update.

The flaw—which is in all releases of the Exim message transfer agent except for version 4.90.1—opens servers to attacks that can execute malicious code, researchers who discovered the vulnerability warned in an advisory published Tuesday . The buffer overflow vulnerability, which is indexed as CVE-2018-6789, resides in base64 decode function. By sending specially manipulated input to a server running Exim, attackers may be able to remotely execute code.

A single byte of data resulting from an exploit "overwrites some critical data when the string fits some specific length," the researchers, from Devcore Security Consulting, wrote. "In addition, this byte is controllable, which makes exploitation more feasible. Base64 decoding is such a fundamental function, and therefore this bug can be triggered easily, causing remote code execution."

For their part, developers of Exim wrote in an advisory: "Currently, we're unsure about the severity" of the vulnerability. "We *believe* an exploit is difficult. A mitigation isn't known."

The Devcore researchers said as many as 400,000 servers are at risk. Queries on the Shodan computer search engine identified a large number of servers that reported running vulnerable versions. The developers published a fix with version 4.90.1, which was released on February 10. Any organization that uses Exim should ensure it's using the latest version.