SSIs (Server-Side Includes) are directives present on web applications. These directories are used to feed an HTML page with dynamic page content.
SSIs are used to execute some actions before a page is loaded and while the page is being visualized.
For performing this action, the web server analyses the SSI before showing the page to the user.
What is SSI injection?
SSI injection allows an attacker to access and manipulate the file system of web servers that permit SSI execution without any proper validation.
The attacker can then process with the permission of the web server’s administrator to completely exploit the system.
Example of SSI injection
An attacker might inject:
<!--#exec cmd="ls"-->
into an input field if the server processes it, revealing directory contents.
What are the impacts of Server-Side Includes (SSI) injection attacks?
The impacts of a Server-Side Includes (SSI) injection attack can be severe, depending on the level of access the attacker gains and the context in which the vulnerability exists. Here are the key impacts:
1. Server compromise
Attackers can execute malicious commands on the server, potentially leading to full server control.
If the server is poorly configured, attackers may gain higher privileges, allowing further exploitation.
2. Data breach
Attackers can access environment variables, configuration files, or other sensitive server data.
If database credentials are stored in accessible files, they can be exfiltrated and used for further attacks.
3. Website defacement
Attackers can modify the content of web pages dynamically, damaging the reputation of the website or spreading misinformation.
4. Denial of Service (DoS)
Execution of resource-intensive commands can overload the server, causing it to crash or become unresponsive.
5. Spread of malware
Malicious scripts can be injected into the website, which may redirect users to phishing pages or infect their devices with malware.
6. Reputational damage and financial loss
Data leaks, website defacement, or server downtime can erode user trust and result in financial repercussions, such as loss of customers or regulatory penalties.
7. Pivoting to internal networks
If attackers gain a foothold on the server, they can use it to launch lateral attacks on other internal systems or networks.
How can you prevent Server-Side Includes (SSI) injection attacks?
Preventing Server-Side Includes (SSI) Injection requires a combination of secure coding practices, proper server configurations, and robust input handling. Here are the key prevention techniques:
1. Disable SSI If not needed
If your application does not require SSI, disable it entirely in the server configuration to eliminate the attack vector.
2. Sanitize and validate user input
Validate all user inputs against a whitelist of acceptable values.
Sanitize input to escape or remove characters that could be interpreted as part of an SSI directive, such as <!–#.
3. Restrict server permissions
Limit the permissions of the web server process to reduce the potential impact of an attack.
Ensure the web server runs with the least privilege required.
4. Use secure coding practices
Avoid including user input directly in SSI directives.
Use templating systems or frameworks that automatically escape potentially dangerous characters.
5. Disable command execution
Restrict or disable server-side commands and file inclusion functionality in SSI.
6. Apply Content Security Policies (CSPs)
Implement CSPs to limit the execution of malicious scripts injected into your pages.
7. Regular security testing
Conduct routine penetration testing and vulnerability scanning to identify SSI injection vulnerabilities.
Use automated tools to test for injection flaws during development.
8. Monitor and log activities
Log server activities and monitor for unusual patterns that could indicate an SSI injection attempt, such as unexpected directives in inputs.
9. Update and patch regularly
Keep your web server and related software up to date to protect against known vulnerabilities.
The best defense against SSI injection is to eliminate the feature if not necessary, sanitize all inputs, and harden server configurations. These proactive measures significantly reduce the risk of exploitation.
