Enterprise security company Barracuda has warned its customers against using email security gateway (ESG) appliances impacted by a recently disclosed zero-day exploit and to replace them immediately.A patch for the vulnerability,\u00a0which has been exploited since October 2022, had been issued by Barracuda last month to stop the exploit from allowing ESG backdooring.\u201cThe vulnerability existed in a module which initially screens the attachments of incoming emails,\u201d the company had said previously. \u201cNo other Barracuda products, including our SaaS email security services, were subject to the vulnerability identified.\u201dUsers whose appliances Barracuda believed were impacted are being notified via the ESG user interface of actions to take. Barracuda has also reached out to these specific customers.Replacement advised despite patchesThe vulnerability, dubbed CVE-2023-2868, was identified on May 19, 2023, and reportedly affected versions 5.1.3.001 through 9.2.0.006, allowing a remote attacker to achieve code execution on susceptible installations.Consequently, Barracuda released patches on May 20 and May 21 for all ESG appliances worldwide. In the latest update on the incident, however, the company has advised to replace the appliance irrespective of their patch status.\u201cImpacted ESG appliances must be immediately replaced regardless of patch version level,\u201d the company said in an update, adding that its \u201cremediation recommendation at this time is a full replacement of the impacted ESG.\u201dMultistrained malware usedThree different malware strains have been discovered to date on a subset of appliances allowing for persistent backdoor access, according to the company. Evidence of data exfiltration was identified on a subset of impacted appliances, the company said in a previous update.The different strains used \u2014 Saltwater, Seaspy, and Seaside \u2014 were all backdoor modules affecting data exfiltration. While both Saltwater and Seaside help establish a hack for the Barracuda SMTP daemon (bsmtpd) equipped to upload and download arbitrary files, execute commands, and tunnel malicious traffic, Seasspy is an x64 executable and linkable format (ELF) backdoor offering persistence capabilities, activated through a magic (remote, wake-on-LAN) packet.Mandiant, the Google-owned cybersecurity intelligence firm investigating the incident, has revealed source code overlaps between SEASPY and an open source backdoor called cd00r. Attacks have not been attributed to any known threat actor or group.