CVE-2026-42395 in Dovecot Pro
Summary
by MITRE • 08/28/2026
A host listed as a trusted proxy can send forwarding information containing a NUL byte, which crashes the login process on the following login attempt. The login process is terminated, which can cause degradation or denial of service for logins. Deployments that do not configure trusted proxies are not affected. Restrict the list of trusted proxy networks to hosts that are fully under your control. Update to non-vulnerable version. No publicly available exploits are known.
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Analysis
by VulDB Data Team • 08/28/2026
The vulnerability described involves a critical flaw in the handling of forwarding information within systems configured with trusted proxies, specifically affecting the login process. When a host designated as a trusted proxy transmits forwarding data that includes a NUL byte character, it triggers an unexpected termination of the active login session. This behavior stems from improper input validation or buffer management logic where the presence of this specific control character is not adequately sanitized before processing. The immediate operational impact is the abrupt cessation of the authentication mechanism, which effectively prevents subsequent users from successfully logging into the affected system. For organizations relying on centralized authentication services behind reverse proxies or load balancers, this flaw can lead to significant service degradation or a complete denial of service for user access if multiple such requests are processed in quick succession.
From a technical perspective, this issue aligns with CWE-20 Improper Input Validation and potentially CWE-193 Off-by-One Error depending on the specific memory handling implementation details not fully disclosed in the summary but implied by the crash behavior involving NUL bytes which often terminate strings prematurely or cause buffer overruns. The attack vector requires network access to a system explicitly configured as a trusted proxy, meaning that deployments without this configuration are inherently immune to exploitation via this specific path. This constraint limits the scope of potential attackers to those who can either compromise a legitimate proxy server within the organization's infrastructure or manipulate traffic from an external source if trust boundaries are misconfigured. The presence of NUL bytes in network payloads is often associated with attempts to bypass security filters or exploit legacy string handling functions that do not account for binary data integrity, making this a classic example of input sanitization failure common in older web application frameworks and authentication modules.
The severity of the vulnerability is primarily defined by its impact on availability rather than confidentiality or integrity, as it results in service disruption without evidence of remote code execution or privilege escalation at this stage. However, the ability to crash an authentication process can be leveraged for denial-of-service attacks against critical infrastructure components such as single sign-on portals or API gateways that depend on continuous login functionality. In terms of threat modeling and adversary behavior, this type of vulnerability could be utilized by attackers aiming to disrupt organizational operations during a broader attack campaign, serving as a distraction or a method to lock out legitimate administrators while other malicious activities are underway within the network perimeter. It does not currently map directly to high-level ATT&CK techniques involving initial access but may relate to resource exhaustion tactics if used repeatedly against exposed services.
Mitigation strategies focus on both immediate remediation and long-term architectural improvements. The most effective short-term solution is to restrict the list of trusted proxy networks strictly to hosts that are fully under organizational control, thereby reducing the attack surface by ensuring only verified internal systems can send forwarding headers. This limits exposure to potential misconfigurations or compromised third-party proxies. Additionally, organizations should prioritize updating their software to non-vulnerable versions where this input validation flaw has been patched by the vendor. Until patches are applied, implementing web application firewall rules that block HTTP requests containing NUL bytes in relevant header fields can provide a layer of defense against exploitation attempts. Continuous monitoring for unusual spikes in authentication failures or connection resets from proxy sources should also be implemented to detect potential abuse early and maintain operational resilience during the remediation window.