Last updated at Thu, 10 Aug 2023 21:41:04 GMT

Rapid7 incident response teams are investigating exploitation of physical Barracuda Networks Email Security Gateway (ESG) appliances dating back to at least November 2022. As of June 6, 2023, as part of an ongoing product incident response, Barracuda is urging ESG customers to immediately decommission and replace ALL impacted ESG physical appliances irrespective of patch level. Barracuda has indicated that impacted ESG customers will see a notification in their user interface (UI). Customers who have not replaced their appliances after receiving this UI notice should contact Barracuda support:


On May 18 and 19, 2023, Barracuda discovered anomalous traffic originating from their Email Security Gateway (ESG) appliances. Barracuda ESG is a solution for filtering inbound and outbound email and protecting customer data. ESG can be deployed as a physical or virtual appliance, or in a public cloud environment on AWS or Microsoft Azure.

On May 30, Barracuda disclosed CVE-2023-2868, a remote command injection vulnerability that the firm said had been exploited in the wild by threat actors since at least October 2022 across a subset of devices running versions According to the security bulletin, the vulnerability exists in a module that performs initial screens on attachments of incoming emails. Barracuda has indicated that, as of June 6, no other products, including SaaS email security services, are known to be affected.

The company indicated they had pushed patches to their global ESG customer base on May 20, 2023. On May 21, Barracuda deployed an additional script to “contain the incident and counter unauthorized access methods.” However, on June 6, the company updated their advisory to warn customers that impacted devices should be completely replaced, irrespective of firmware version or patch level.

The pivot from patch to total replacement of affected devices is fairly stunning and implies the malware the threat actors deployed somehow achieves persistence at a low enough level that even wiping the device wouldn’t eradicate attacker access. Barracuda has a full description of the incident so far in their advisory, including extensive indicators of compromise, additional vulnerability details, and information on the backdoored module for Barracuda’s SMTP daemon.

On June 15, 2023, Mandiant published an in-depth analysis of the incident, which they are attributing to an "aggressive and skilled actor" with suspected links to China, tracked as UNC4841. According to the analysis, Barracuda ESG devices were exploited "as a vector for espionage" in an extensive threat campaign dating back to at least October 10, 2022. Following initial compromise, "Mandiant and Barracuda observed UNC4841 aggressively target specific data of interest for exfiltration, and in some cases, leverage access to an ESG appliance to conduct lateral movement into the victim network, or to send mail to other victim appliances. Mandiant has also observed UNC4841 deploy additional tooling to maintain presence on ESG appliances." We encourage security teams to read the full analysis.

Baselining on a known ESG appliance, which runs the "Barracuda Networks Spam Firewall" SMTP daemon, there appeared to be roughly 11,000 appliances on the internet (Barracuda Networks Spam Firewall smtpd) as of June 8. Notably, if other Barracuda appliances also run this service, that number may be inflated.

Observed attacker behavior

Rapid7 services teams have so far identified malicious activity that took place as far back as November 2022, with the most recent communication with threat actor infrastructure observed in May 2023. In at least one case, outbound network traffic indicated potential data exfiltration. We have not yet observed any lateral movement from a compromised appliance.

Note: Although sharing malware indicators like hashes and YARA hunting rules can be very useful, in this case they may not be as relevant unless teams have direct access to the operating system of the appliance or VMDK image. Network indicators like the IP addresses shared by Barracuda and also observed by Rapid services teams are a good start for reviewing network logs (e.g., firewall or IPS logs).

Mitigation guidance

Customers who use the impacted Barracuda ESG appliance should take the device offline immediately and replace it. For physical device users, this means completely replacing hardware. Barracuda’s advisory has instructions for contacting support ( Users are also being advised to rotate any credentials connected to the ESG appliance, including:

  • Any connected LDAP/AD
  • Barracuda Cloud Control
  • FTP Server
  • SMB
  • Any private TLS certificates

ESG appliance users should check for signs of compromise dating back to at least October 2022 using the network and endpoint indicators Barracuda has released publicly (where possible):

If you have questions about next steps or impact to your organization, please contact Barracuda support.


June 15, 2023: Updated with Mandiant's in-depth analysis on CVE-2023-2868 and UNC4841.