CVE-2021-27890 in MyBB
Summary
by MITRE • 03/16/2021
SQL Injection vulnerablity in MyBB before 1.8.26 via theme properties included in theme XML files.
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Analysis
by VulDB Data Team • 12/21/2024
The vulnerability CVE-2021-27890 represents a critical SQL injection flaw discovered in MyBB versions prior to 1.8.26, specifically affecting the handling of theme properties within theme XML files. This vulnerability resides in the theme management system of the popular bulletin board software, where user-supplied data from XML configuration files is inadequately sanitized before being processed in database queries. The flaw allows remote attackers to execute arbitrary SQL commands through maliciously crafted theme properties that are parsed and stored in the database without proper input validation or escaping mechanisms.
The technical implementation of this vulnerability stems from improper handling of user-controllable data within the theme import functionality. When administrators or users upload theme XML files containing maliciously crafted properties, the system fails to properly escape or validate these inputs before incorporating them into SQL queries. This creates an exploitable condition where attacker-controlled data can directly influence the structure of database queries, potentially allowing full database access, data exfiltration, or even system compromise. The vulnerability specifically affects the theme properties processing logic that parses XML elements and translates them into database operations, making it particularly dangerous in environments where administrators might import third-party themes.
The operational impact of CVE-2021-27890 extends beyond simple data theft, as it provides attackers with the capability to manipulate the entire MyBB installation. Successful exploitation could enable attackers to escalate privileges, modify user accounts, access confidential forum data, or even gain shell access to the underlying server. The vulnerability is particularly concerning because it can be exploited through theme uploads, which are common administrative tasks that may be performed by users with limited privileges, potentially allowing privilege escalation attacks. Additionally, the vulnerability affects both local and remote exploitation scenarios, making it applicable across various deployment models including shared hosting environments and dedicated servers.
Security mitigations for CVE-2021-27890 primarily focus on immediate patching of MyBB installations to version 1.8.26 or later, which includes proper input sanitization and escaping mechanisms for theme properties. Organizations should implement strict theme upload policies that require verification of theme authenticity and source before deployment. Network-level protections including web application firewalls and database query monitoring can help detect and prevent exploitation attempts. The vulnerability aligns with CWE-89 which categorizes SQL injection flaws, and maps to ATT&CK techniques such as T1190 for exploitation of vulnerabilities and T1078 for valid accounts to maintain access. Regular security audits of uploaded content and database query logging should be implemented to detect anomalous behavior indicative of exploitation attempts. System administrators should also consider implementing principle of least privilege for theme upload capabilities and establish automated scanning of XML files for potentially malicious content before import operations.