CVE-2011-2688

Why Exploitable

The CVE-2011-2688 vulnerability is exploitable due to a SQL injection flaw in the mysql-auth.pl script of the mod_authnz_external module for Apache HTTP Server. This allows remote attackers to execute arbitrary SQL commands by manipulating the user field.

Key factors contributing to exploitability:

• The vulnerability has a HIGH severity rating, indicating a significant potential impact.
• Although there are no public exploit or PoC references, the lack of EPSS probability and percentile data does not necessarily reduce the urgency.
• The absence of this CVE from CISA KEV and Watchstack intelligence may not accurately reflect the vulnerability's exploitability.

Organizations should validate their exposure to this vulnerability by checking their Apache HTTP Server and mod_authnz_external module versions. They should prioritize patching or upgrading to a version later than 3.2.5 to mitigate the SQL injection risk.

Overview

CVE-2011-2688 is a SQL injection vulnerability in the mysql/mysql-auth.pl script of the mod_authnz_external module (versions 3.2.5 and earlier) for the Apache HTTP Server. The flaw allows remote attackers to inject arbitrary SQL commands through the user field during authentication. The vulnerability has a NVD severity of HIGH with a CVSS base score of 7.5, and an EPSS percentile of 92, indicating high exploitation likelihood. It affects Linux distributions including Debian and Gentoo, as well as the Echo platform. The issue was disclosed in 2011 and is critical for any organization using mod_authnz_external with MySQL backend authentication. Defenders should prioritize patching or mitigating this vulnerability due to its ease of exploitation and potential for data exfiltration.

Technical details

The root cause is improper neutralization of special elements used in SQL commands (CWE-89) within the mysql/mysql-auth.pl script. The script constructs SQL queries by directly concatenating user-supplied input (the username field) without sanitization or parameterized queries. Attackers can inject SQL metacharacters such as single quotes or comment sequences to alter the query logic. Affected versions include mod_authnz_external 3.2.5 and earlier. The attack requires only network access to the Apache server and the ability to send HTTP authentication requests. The trust boundary is between the web server and the MySQL database; the vulnerability allows an unauthenticated remote attacker to bypass authentication or execute arbitrary SQL commands.

Impact

Successful exploitation can lead to unauthorized access to the MySQL database, allowing attackers to read, modify, or delete sensitive data. This could result in credential theft, data exfiltration, or complete database compromise. The vulnerability can also be used to bypass authentication mechanisms, granting attackers access to protected resources. For defenders, the business exposure includes regulatory fines, reputational damage, and potential lateral movement within the network if the database contains credentials or sensitive information.

Exploitation steps

1. Identify an Apache server running mod_authnz_external with MySQL authentication.
2. Send an HTTP request to a protected resource, triggering authentication.
3. In the username field, inject a SQL payload such as ' OR '1'='1 to bypass authentication.
4. The script constructs a query like SELECT password FROM users WHERE user='$user'.
5. The injected payload modifies the query to SELECT password FROM users WHERE user='' OR '1'='1', returning all rows.
6. The attacker gains access to the resource without valid credentials.
7. For more advanced attacks, inject UNION-based SQL to extract data from other tables.
8. The attacker can also use time-based or error-based techniques to enumerate the database schema.

Indicators of compromise

• Web server logs showing unusual SQL syntax in the username field, such as single quotes, OR, UNION, or comment sequences (--, #).
• Database logs revealing unexpected queries with injected SQL commands.
• Authentication logs showing successful logins for non-existent users or multiple rapid authentication attempts.
• Network traffic containing SQL-like patterns in HTTP authentication headers.
• EDR alerts for suspicious database queries originating from the Apache server.
• Anomalous database access patterns, such as queries to system tables (e.g., information_schema).
• Unexpected database errors returned in HTTP responses (e.g., "You have an error in your SQL syntax").

Mitigation and workarounds

• Upgrade mod_authnz_external to version 3.2.6 or later, which includes proper input sanitization and parameterized queries.
• If upgrading is not immediately possible, apply vendor patches provided by Debian, Gentoo, or the mod_authnz_external project.
• As a workaround, implement input validation in the authentication script to reject special characters in the username field.
• Use a Web Application Firewall (WAF) to filter SQL injection patterns in HTTP requests.
• Restrict database user permissions to limit the impact of successful SQL injection (e.g., use read-only accounts where possible).
• Monitor and audit authentication logs for suspicious activity.

Community reactions

The vulnerability was widely discussed in security forums and mailing lists shortly after disclosure. The Debian Security Team released an advisory (DSA-2292-1) urging immediate upgrades. Gentoo also issued a security notification (GLSA 201107-12). Security researchers noted the simplicity of exploitation and the widespread use of mod_authnz_external in legacy systems. Some community members criticized the lack of input validation in the original module design. The CVE entry remains a classic example of SQL injection in authentication modules.

Additional resources

• CVE-2011-2688 NVD Entry
• Debian Security Advisory DSA-2292-1
• Gentoo Security GLSA 201107-12
• mod_authnz_external Project Page
• OWASP SQL Injection Prevention Cheat Sheet