CVE-2026-94057 in Exim
Summary
by MITRE • 09/20/2026
Exim before 4.100.1 allows SMTP smuggling in which the received message does not match any sent message, and instead depends on crafted data sent after a rejection during DATA processing.
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Analysis
by VulDB Data Team • 09/20/2026
The vulnerability identified in Exim versions prior to 4.100.1 represents a critical failure in input validation and state management within the Simple Mail Transfer Protocol implementation. This flaw specifically manifests as SMTP smuggling, a technique where an attacker crafts maliciously formatted email data that exploits discrepancies between how different mail transfer agents interpret protocol boundaries. In this scenario, the vulnerability arises during the DATA command processing phase of the SMTP transaction. When Exim receives a message payload, it is designed to parse and validate the content before accepting or rejecting it based on policy checks such as spam filtering, virus scanning, or access control lists. However, due to insufficient handling of specific character sequences and line endings within the data stream, an attacker can inject crafted data that arrives after a rejection signal has been initiated but not fully processed by the server's internal state machine.
The technical core of this flaw lies in the ambiguity surrounding message termination and boundary detection during error conditions. Standard SMTP protocols rely on clear delimiters to distinguish between command headers and body content, as well as to mark the end of an email transmission via a specific sequence like CRLF.CRLF. In vulnerable versions of Exim, if the server encounters an error condition that triggers a rejection response while still processing the DATA block, it may fail to correctly reset its parsing state or properly discard subsequent input bytes associated with that transaction. Consequently, data sent immediately after the rejection signal can be misinterpreted as part of a new message rather than being discarded as invalid trailing content from the previous failed attempt. This allows an attacker to smuggle arbitrary email messages into the system without triggering standard validation checks that would normally apply to explicitly submitted emails.
The operational impact of this vulnerability is severe, primarily because it bypasses security controls designed to inspect incoming mail traffic. Since the smuggled message does not match any sent message in a way that triggers normal logging or inspection routines associated with new SMTP sessions, it can evade spam filters, antivirus engines, and content policy checks. An attacker could leverage this mechanism to deliver phishing emails containing malicious links, distribute malware attachments, or exfiltrate sensitive data through email channels without detection by standard security infrastructure. This undermines the integrity of the mail server as a trusted gateway for organizational communication and poses significant risks related to information leakage, credential theft, and initial access establishment in broader attack chains.
From a classification perspective, this issue aligns with CWE-20 Improper Input Validation, specifically regarding the failure to correctly validate input during error handling states. It also relates to CWE-754: Improper Check for Unusual or Exceptional Conditions, as the server does not adequately handle unexpected sequences of data following an abnormal termination signal. In terms of offensive security frameworks such as MITRE ATT&CK, this technique falls under T1071 Application Layer Protocol: SMTP, which describes how adversaries use standard email protocols to bypass defenses and establish communication channels that are less likely to be monitored by network-based intrusion detection systems.
Mitigation for this vulnerability requires immediate upgrading of the Exim mail transfer agent to version 4.100.1 or later, where these parsing logic errors have been corrected. Administrators should ensure that their patching processes include verification of the installed binary version against known good baselines. Additionally, organizations employing multiple layers of email security should review their configurations for any reliance on Exim alone as a gatekeeper and consider implementing additional inspection points at network boundaries or using dedicated secure email gateways that perform deep packet inspection independent of the underlying MTA's internal state handling. Regular monitoring of mail logs for anomalies in message sizes, unexpected rejections followed by successful deliveries, or irregularities in SMTP session durations can help detect potential exploitation attempts while patching is being implemented across the infrastructure.