CVE-2026-94056 in Exim
Summary
by MITRE • 09/20/2026
Exim before 4.100.1, when Proxy-Protocol is used with an attacker-controlled proxy, allows attackers to read certain uninitialized data from stack memory.
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Analysis
by VulDB Data Team • 09/20/2026
The vulnerability in Exim versions prior to 4.100.1 represents a critical information disclosure flaw rooted in improper handling of uninitialized stack variables during the processing of Proxy Protocol headers. When Exim is configured to accept connections using the PROXY protocol, it relies on external proxies or load balancers to convey client connection metadata such as source and destination IP addresses and ports. This feature allows mail servers behind NAT devices or reverse proxies to correctly identify the originating client for logging and access control purposes. However, in versions before 4.100.1, the implementation fails to properly initialize certain stack-based data structures when parsing these headers under specific conditions involving an attacker-controlled proxy.
The technical core of this vulnerability lies in how Exim allocates and populates memory buffers associated with connection parameters. When a malicious or compromised intermediary sends a Proxy Protocol header that triggers specific code paths within the mail transfer agent, the software may read from stack locations that have not been explicitly zeroed out by previous operations. Because these uninitialized regions of memory can contain remnants of data from prior function calls, including sensitive information such as authentication tokens, private keys, or other user credentials processed in earlier requests on the same thread, an attacker can exploit this behavior to leak arbitrary bytes from the server's stack space. This constitutes a classic case of reading uninitialized memory, which is particularly dangerous because it bypasses standard bounds checks that might otherwise prevent access to protected data regions.
From an operational perspective, this vulnerability poses a severe risk to the confidentiality and integrity of email communications. An attacker who controls or can manipulate the proxy layer between themselves and the Exim server could craft specific Proxy Protocol headers designed to trigger the uninitialized read condition. By analyzing the response packets returned by the mail server, which may inadvertently include fragments of stack memory in error messages or protocol responses depending on how the data is subsequently used or logged, the attacker can reconstruct sensitive internal state information. This capability effectively transforms a configuration feature intended for network transparency into an attack vector that undermines the security boundary between the external network and the internal mail infrastructure.
This flaw aligns with CWE-908, which describes the use of uninitialized resource, specifically highlighting scenarios where software uses memory or data structures without first initializing them to known safe values. Furthermore, in the context of the MITRE ATT&CK framework, this vulnerability facilitates Information Discovery techniques, allowing adversaries to gather intelligence about the internal workings and security posture of the target system. The exploitation path typically involves Lateral Movement if the proxy is within a compromised network segment or Remote Code Execution prerequisites for more advanced attacks where leaked memory contents aid in bypassing stack-based protections like ASLR by revealing base addresses.
Mitigation strategies must prioritize immediate software updates to version 4.100.1 or later, where this initialization flaw has been addressed through rigorous code review and testing of the Proxy Protocol handling logic. Organizations should also enforce strict network segmentation policies to ensure that only trusted and authorized proxies are permitted to communicate with Exim servers using the PROXY protocol. Additionally, implementing input validation on proxy headers can help detect malformed or maliciously crafted requests before they reach vulnerable code paths. Security teams should monitor logs for unusual patterns in connection metadata and consider deploying intrusion detection systems capable of identifying anomalies consistent with stack-based information leakage attempts. Regular audits of mail server configurations against industry best practices will further reduce the attack surface associated with third-party proxy integrations.