CVE-2026-94054 in Exim
Summary
by MITRE • 09/20/2026
Exim before 4.100.1, when Proxy-Protocol is used with an attacker-controlled proxy, has an out-of-bounds write.
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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 memory corruption flaw specifically triggered by the interaction between the mail transfer agent and the PROXY protocol when operating under untrusted network conditions. This issue arises from an out-of-bounds write, which occurs during the parsing or handling of proxy protocol headers that are supplied by a compromised or malicious intermediary server. In environments where Exim is configured to accept connections via proxies such as HAProxy or AWS Network Load Balancers using version 1 or 2 of the PROXY protocol, the application relies on the integrity of data transmitted from these intermediaries. When an attacker controls this proxy layer, they can craft malformed or excessively long header fields that exceed the allocated buffer boundaries within Exim's memory space.
From a technical perspective, the root cause lies in insufficient boundary checking during the processing of incoming connection metadata. The PROXY protocol is designed to preserve client identity information across load balancers and proxies by prepending specific headers to TCP connections. However, if the implementation fails to validate the length or structure of these headers against predefined limits before writing them into internal data structures, it results in a buffer overflow condition. This out-of-bounds write allows an attacker to overwrite adjacent memory locations, potentially corrupting critical program state variables, control flow pointers, or other sensitive data stored on the heap or stack. Such corruption can lead to arbitrary code execution if the overwritten memory contains function return addresses or object vtables, effectively granting the attacker full control over the Exim process running with the privileges of the mail server user account.
The operational impact of this vulnerability is severe, particularly for organizations that rely heavily on reverse proxies or load balancers in front of their email infrastructure. An successful exploitation could allow a remote attacker to execute arbitrary code on the affected system without requiring authentication. This compromises the confidentiality, integrity, and availability of the mail server. Beyond direct compromise, such memory corruption can destabilize the service, leading to denial-of-service conditions through crashes or unpredictable behavior. Furthermore, if Exim is running with elevated privileges or has access to sensitive internal networks due to its role as a mail gateway, an attacker could pivot from this initial foothold to attack other systems within the organization's infrastructure.
This vulnerability aligns with CWE-787: Out-of-bounds Write, which describes writing data outside of the intended buffer boundaries, and can be mapped to MITRE ATT&CK techniques related to memory corruption exploitation such as T1203 Exploitation for Client Execution or potentially T1496 Remote Service Vulnerability depending on the specific vector used. It highlights the risk associated with trusting unverified input from network intermediaries, a common architectural pattern in modern cloud-native and distributed systems.
To mitigate this vulnerability, organizations must immediately upgrade Exim to version 4.100.1 or later, where the issue has been resolved through improved validation of proxy protocol headers. Until an upgrade is feasible, administrators should consider restricting access to the PROXY protocol functionality by ensuring that only trusted and verified proxies are allowed to connect using this feature. Implementing strict network segmentation rules can also help limit exposure by preventing untrusted hosts from establishing connections that utilize the PROXY header mechanism. Additionally, deploying intrusion detection systems or web application firewalls capable of detecting anomalous buffer sizes in TCP headers may provide a layer of defense against exploitation attempts while patch management is being executed. Regular security audits and penetration testing focusing on input validation for network protocol implementations are recommended to identify similar weaknesses across other services within the infrastructure.