CVE-2026-6637: 
PostgreSQL 脆弱性の分析と軽減

CVE-2026-6637 is a stack-based buffer overflow vulnerability in the PostgreSQL refint contrib module that allows an unprivileged database user to execute arbitrary code as the operating system user running the database. A secondary attack vector involves SQL injection when an application declares a user-controlled column as a refint cascade primary key and permits user-controlled updates, enabling arbitrary SQL execution as the database user. Affected versions span PostgreSQL 14 through 18, specifically before 14.23, 15.18, 16.14, 17.10, and 18.4. It was published on May 14, 2026, with a CVSS v3.1 base score of 8.8 (High) (GitHub Advisory, PostgreSQL Security).

The vulnerability has two distinct root causes: a stack-based buffer overflow (CWE-121) in the refint contrib module, and an SQL injection flaw (CWE-89) in the same module's handling of cascade primary key updates. The buffer overflow can be triggered by any unprivileged database user who can invoke the refint trigger functions, requiring no special schema or table ownership. The SQL injection attack path requires a more specific precondition: the application must declare a user-controlled column as a refint cascade primary key and allow user-controlled updates to that column, at which point an attacker can inject arbitrary SQL executed under the database user's privileges. Both attack vectors are network-accessible with low attack complexity (GitHub Advisory, PostgreSQL Security).

Successful exploitation of the stack buffer overflow path allows an unprivileged database user to execute arbitrary code as the OS-level user running the PostgreSQL process (typically postgres), resulting in full confidentiality, integrity, and availability compromise of the database host. The SQL injection path enables arbitrary SQL execution as the database user performing the primary key update, potentially exposing all data accessible to that user and enabling further privilege escalation within the database. Both attack paths carry high impact across all three CIA triad dimensions, and OS-level code execution could facilitate lateral movement to other systems on the network (GitHub Advisory, PostgreSQL Release).

1. Reconnaissance: Identify PostgreSQL instances running versions before 14.23, 15.18, 16.14, 17.10, or 18.4 using network scanning tools (e.g., Nmap, Shodan) or by querying SELECT version(); after obtaining low-privilege database access.
2. Obtain low-privilege database access: Acquire any valid database user credentials — the vulnerability requires only PR:L (low privileges), meaning any authenticated database user can attempt exploitation.
3. Identify refint trigger usage: Query the database catalog to find tables using refint trigger functions: SELECT tgname, tgrelid::regclass FROM pg_trigger WHERE tgfoid IN (SELECT oid FROM pg_proc WHERE proname IN ('check_primary_key', 'check_foreign_key'));
4. Buffer overflow path: Craft a malicious input that triggers the stack buffer overflow in the refint module by supplying an oversized or specially crafted value to a column monitored by a refint trigger, causing arbitrary code execution as the OS postgres user.
5. SQL injection path (if applicable): If the application exposes a user-controlled column defined as a refint cascade primary key with update capability, supply a crafted primary key update value containing SQL metacharacters to inject and execute arbitrary SQL as the database user performing the update.
6. Post-exploitation: With OS-level code execution, establish persistence (e.g., cron job, SSH key injection), exfiltrate data, or pivot to other network hosts accessible from the database server (GitHub Advisory, PostgreSQL Security).

• Logs: PostgreSQL server logs (postgresql.log) showing unexpected errors or crashes in refint trigger functions; unusual ERROR or FATAL entries related to check_primary_key or check_foreign_key functions; SQL statements containing unusual metacharacters or concatenated SQL in primary key update operations.
• Process: Unexpected child processes spawned by the postgres OS user (e.g., /bin/bash, curl, wget, python, nc) that are not part of normal PostgreSQL operation; new cron jobs or scheduled tasks created under the postgres user account.
• File System: New or modified files in the PostgreSQL data directory or home directory of the postgres OS user; unexpected scripts or binaries written to world-writable directories; new SSH authorized keys added to the postgres user's ~/.ssh/authorized_keys.
• Network: Outbound connections from the PostgreSQL server process to unexpected external IP addresses or unusual ports; reverse shell connections originating from the database host.

PostgreSQL has released patched versions addressing this vulnerability: 18.4, 17.10, 16.14, 15.18, and 14.23. All users running affected versions should upgrade immediately (PostgreSQL Release, PostgreSQL Security). As interim mitigations, database administrators should review and restrict which users have privileges to invoke refint trigger functions, and audit application schemas to identify any user-controlled columns declared as refint cascade primary keys. Applications that facilitate user-controlled updates to such columns should implement strict input validation and sanitization until the patch is applied. Note that PostgreSQL 14 is approaching end-of-life; users on that branch should plan migration to a supported major version.

The PostgreSQL project released a coordinated security update on May 14, 2026, addressing 11 CVEs simultaneously, with CVE-2026-6637 among the higher-severity issues (PostgreSQL Release). Security media including The Hacker Wire, GBHackers, CyberSecurityNews, and SecurityOnline.info covered the release, highlighting the RCE potential of the refint buffer overflow (The Hacker Wire, SecurityOnline). The Belgian Centre for Cybersecurity (CCB) issued an advisory warning about the vulnerabilities and the announced EOL date for PostgreSQL 14 (CCB Advisory). Linux distributions including Debian, SUSE, openSUSE, Ubuntu, and Amazon Linux 2 rapidly issued updated packages, reflecting broad ecosystem response to the disclosure.