Exim RCE via Heap Buffer Overflow in EHLO
Actively exploited in the wild. Patch or mitigate immediately — don't wait for a maintenance window.
An unauthenticated attacker, remotely over the network, with no user interaction, can fully compromise the system — steal data, tamper with it and take it offline.
Exim 4.92 through 4.92.2 allows remote code execution, a different vulnerability than CVE-2019-15846. There is a heap-based buffer overflow in string_vformat in string.c involving a long EHLO command.
Inferred from the weakness type (CWE) and how the flaw is reached — the techniques an attacker would chain to exploit this. Real-world corroboration appears here as threat intel is folded in.
Corroborated threat activity referencing this CVE — with source and confidence, not inference.
Named threat actors and observed ATT&CK techniques appear here as intel feeds are folded in — the ATT&CK chain above stays inferred until then.
Generated by Wavense from the CVE record — an immediate mitigation and the permanent fix.
Implement network-level rate limiting on incoming SMTP connections to port 25 to slow down potential brute-force or exploitation attempts. Deploy an Intrusion Detection/Prevention System (IDPS) to monitor for unusually long EHLO commands or known exploit signatures targeting Exim. Consider restricting direct internet access to Exim from unknown sources, if feasible, by routing through a hardened mail gateway.
The permanent resolution is to immediately upgrade all affected Exim installations to version 4.92.3 or later. This patched version addresses the heap-based buffer overflow vulnerability, eliminating the remote code execution risk. Review and ensure your package manager or official Exim sources are configured to provide the latest secure release.
An unauthenticated remote attacker can exploit a heap-based buffer overflow by sending a crafted, excessively long EHLO command. This allows the attacker to achieve remote code execution, gaining full control over the compromised Exim mail server. This can lead to sensitive data exfiltration, disruption of mail services, and the use of the server as a platform for further attacks.
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