CVE-2026-94055 in Exim
Summary
by MITRE • 09/20/2026
Exim before 4.100.1, when certain non-default TLS settings are used with GnuTLS, has a use-after-free.
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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 management flaw within the mail transfer agent's handling of Transport Layer Security (TLS) connections when utilizing the GnuTLS library. This specific issue manifests as a use-after-free error, which occurs when the software attempts to access memory that has already been deallocated by the system. Such vulnerabilities are particularly dangerous because they can lead to arbitrary code execution, denial of service conditions, or information disclosure depending on how an attacker manipulates the freed memory region and the timing of subsequent operations. The condition is specifically triggered under configurations where non-default TLS settings are employed, indicating that standard default installations may be less susceptible but still potentially vulnerable if administrators have customized security parameters for compliance or performance reasons.
From a technical perspective, this flaw falls squarely under CWE-416, which classifies use-after-free vulnerabilities as errors resulting from the improper handling of memory deallocation and subsequent access. In the context of Exim, the vulnerability likely arises during the complex handshake process or session resumption mechanisms inherent to GnuTLS integration. When specific TLS parameters are altered from their defaults, the internal state management may fail to correctly track object lifecycles, leading to a scenario where pointers remain valid in application logic even after the underlying memory has been returned to the heap allocator. An attacker who can interact with the SMTP service and influence these TLS negotiation phases could potentially exploit this race condition or logical error to execute malicious code on the mail server host.
The operational impact of such a vulnerability is severe for organizations relying on Exim for email infrastructure. A successful exploitation could allow an unauthenticated remote attacker to gain control over the underlying operating system, effectively compromising the confidentiality, integrity, and availability of all data processed by the mail server. This includes sensitive customer communications, internal corporate correspondence, and potentially credentials or tokens transmitted via email. Furthermore, even if code execution is not achieved, the instability caused by memory corruption can lead to service crashes, resulting in a denial of service that disrupts business operations. Given Exim's widespread deployment across Linux-based systems, this vulnerability poses a significant risk to a large portion of the internet's mail infrastructure.
Mitigation strategies primarily involve upgrading to version 4.100.1 or later, where these memory management issues have been addressed by the developers through rigorous code review and patching. For environments where immediate upgrades are not feasible due to compatibility constraints, administrators should consider restricting TLS configurations to default settings if possible, thereby avoiding the specific trigger conditions associated with non-default parameters. Additionally, deploying network-level intrusion detection systems that monitor for anomalous SMTP traffic patterns related to TLS handshakes can provide a layer of defense in depth. It is also recommended to review ATT&CK technique T1059, Command and Scripting Interpreter, as this vulnerability could serve as an initial access vector leading to further lateral movement within the network if left unpatched. Regular security audits and adherence to CIS benchmarks for mail servers can help identify misconfigurations that might exacerbate such risks.