CVE-2026-107121 in Keycloak Single Sign-On
Summary
by MITRE • 10/07/2026
A flaw was found in the SMTP email configuration handling of the keycloak-services component. When the STARTTLS option is enabled, Keycloak fails to strictly enforce an encrypted connection, allowing it to fall back to unencrypted communication if the encryption request is tampered with. An attacker who can intercept network traffic can exploit this to capture sensitive email credentials and message content in plain text.
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Analysis
by VulDB Data Team • 10/08/2026
The vulnerability identified within the keycloak-services component represents a critical failure in transport layer security enforcement, specifically concerning the Simple Mail Transfer Protocol (SMTP) configuration when STARTTLS is utilized. This flaw stems from an improper implementation of the TLS negotiation process, where the application fails to strictly mandate that the connection remains encrypted after the initial upgrade request. Instead of treating the encryption requirement as a hard constraint, the software allows for protocol fallback mechanisms that prioritize connectivity over security integrity. Consequently, if an attacker intercepts network traffic and manipulates or strips the STARTTLS command from the communication stream, Keycloak will accept this modified state and proceed to transmit data without establishing a secure channel. This behavior contradicts the fundamental principle of explicit trust in encrypted communications, where the presence of encryption parameters should necessitate their successful completion before any sensitive payload is transmitted.
From a technical perspective, this issue aligns with CWE-319, which classifies cleartext transmission of sensitive information over an unencrypted channel. The root cause lies in the application logic that handles SMTP server responses and connection states. Rather than terminating the session immediately upon failure to establish encryption or detection of tampering, the service defaults to a plaintext mode. This creates a significant window for man-in-the-middle attacks where adversaries can passively observe network traffic without needing to decrypt any cryptographic material because no such material is actually generated during the compromised exchange. The flaw effectively nullifies the security benefits provided by STARTTLS, rendering the configuration vulnerable despite being technically enabled in the settings.
The operational impact of this vulnerability is severe for organizations relying on Keycloak for identity and access management that utilizes email services for critical functions such as password resets, two-factor authentication code delivery, or administrative notifications. An attacker positioned within the network path can capture sensitive credentials including SMTP usernames and passwords, which may be reused across other systems due to credential stuffing risks. Furthermore, any message content transmitted during this period is exposed in plain text, potentially revealing personal identifiable information, internal communications, or security tokens. This exposure not only compromises confidentiality but also undermines the integrity of authentication workflows that depend on email-based verification steps.
This vulnerability maps directly to MITRE ATT&CK technique T1043, which describes Common Application Protocol Traffic as a method for data exfiltration and command-and-control communication. By exploiting this flaw, an adversary can leverage standard SMTP traffic to harvest credentials without triggering typical intrusion detection systems that might look for unusual ports or encrypted tunnel anomalies since the traffic appears legitimate but unencrypted. The ability to downgrade security protocols is also consistent with CWE-693, Protection Mechanism Failure, where a system fails to prevent unauthorized access due to incorrect implementation of protective controls.
To mitigate this risk, immediate action should be taken by administrators and developers responsible for Keycloak deployments. The primary remediation involves upgrading the keycloak-services component to a version that enforces strict TLS requirements, ensuring that connections are terminated if encryption cannot be successfully established. In environments where STARTTLS is mandatory, configuration settings should explicitly disable fallback options or enforce certificate validation rigorously. Additionally, organizations should implement network-level monitoring to detect anomalies in SMTP traffic patterns and consider deploying additional controls such as firewall rules that restrict direct internet access for mail servers, forcing all communication through secure proxies. Regular security audits of email configurations are essential to ensure compliance with industry standards like NIST SP 800-52 regarding TLS implementation guidelines.