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A critical security vulnerability, SVD-2026-0603 (CVE-2026-20253), has been identified in Splunk Enterprise versions 10.0.0 through 10.0.6 and 10.2.0 through 10.2.3. This flaw allows unauthenticated, remote attackers to create or truncate arbitrary files on the host system by exploiting the PostgreSQL Sidecar Service endpoints. The vulnerability is actively exploited, with public proof-of-concept code available, and has been added to the CISA Known Exploited Vulnerabilities (KEV) list. Successful exploitation can lead to full remote code execution (RCE) as the Splunk user. The vulnerability arises from inadequate authentication controls on the PostgreSQL Sidecar Service endpoints, specifically /v1/postgres/recovery/backup and /v1/postgres/recovery/restore, which are accessible without authentication. It is classified under CWE-306: Missing Authentication for Critical Function and has a CVSS v3.1 base score of 9.8 (Critical). Attackers can exploit the vulnerability by sending crafted HTTP POST requests to the exposed endpoints, allowing them to create or truncate files and potentially execute malicious scripts. Indicators of compromise include unexpected files in directories such as /tmp/ or /opt/splunk/var/run/supervisor/pkg-run/, modified Splunk Python scripts, and unusual outbound connections from Splunk to unknown PostgreSQL servers. The vulnerability aligns with several MITRE ATT&CK techniques, including T1190 (Exploit Public-Facing Application) and T1059 (Command and Scripting Interpreter). Active exploitation of CVE-2026-20253 has been confirmed, and it is likely that both opportunistic cybercriminals and sophisticated threat actors will use this exploit. The affected versions of Splunk Enterprise are 10.2.0 through 10.2.3 and 10.0.0 through 10.0.6, with the issue resolved in versions 10.2.4 and 10.0.7. Organizations are advised to upgrade to fixed versions or disable the PostgreSQL Sidecar Service as a mitigation strategy.