Attackers Exploit Critical Zimbra Vulnerability Using CC'd Email Addresses

An anonymous reader quotes a report from Ars Technica: Attackers are actively exploiting a critical vulnerability in mail servers sold by Zimbra in an attempt to remotely execute malicious commands that install a backdoor, researchers warn. The vulnerability, tracked as CVE-2024-45519, resides in the Zimbra email and collaboration server used by medium and large organizations. When an admin manually changes default settings to enable the postjournal service, attackers can execute commands by sending maliciously formed emails to an address hosted on the server. Zimbra recently patched the vulnerability. All Zimbra users should install it or, at a minimum, ensure that postjournal is disabled.

On Tuesday, Security researcher Ivan Kwiatkowski first reported the in-the-wild attacks, which he described as 'mass exploitation.' He said the malicious emails were sent by the IP address 79.124.49[.]86 and, when successful, attempted to run a file hosted there using the tool known as curl. Researchers from security firm Proofpoint took to social media later that day to confirm the report. On Wednesday, security researchers provided additional details that suggested the damage from ongoing exploitation was likely to be contained. As already noted, they said, a default setting must be changed, likely lowering the number of servers that are vulnerable.

Proofpoint has explained that some of the malicious emails used multiple email addresses that, when pasted into the CC field, attempted to install a webshell-based backdoor on vulnerable Zimbra servers. The full cc list was wrapped as a single string and encoded using the base64 algorithm. When combined and converted back into plaintext, they created a webshell at the path: /jetty/webapps/zimbraAdmin/public/jsp/zimbraConfig.jsp. Proofpoint went on to say: 'Once installed, the webshell listens for inbound connection with a pre-determined JSESSIONID Cookie field; if present, the webshell will then parse the JACTION cookie for base64 commands. The webshell has support for command execution via exec or download and execute a file over a socket connection.'

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