Summary
Nginx UI improperly validates configuration lines by checking only the first directive on each line, even though Nginx accepts multiple semicolon-separated directives. An authenticated user may abuse this behavior or unrestricted includes to introduce restricted directives, potentially enabling code execution when a suitable executable module is available.
Nginx UI improperly validates configuration lines by checking only the first directive on each line, even though Nginx accepts multiple semicolon-separated directives. An authenticated user may abuse this behavior or unrestricted includes to introduce restricted directives, potentially enabling code execution when a suitable executable module is available.
Affected context
The issue can affect Nginx UI deployments where authenticated users are allowed to modify Nginx configurations through REST, site, stream, or related configuration-write paths. Risk is higher when the Nginx process has access to executable modules or when configuration files can include content that bypasses the application's directive checks.
The issue can affect Nginx UI deployments where authenticated users are allowed to modify Nginx configurations through REST, site, stream, or related configuration-write paths. Risk is higher when the Nginx process has access to executable modules or when configuration files can include content that bypasses the application's directive checks.
Exploit preconditions
- Attack surface: Authenticated Nginx UI configuration-management endpoints
- Required access: Permission to modify supported Nginx configuration content
- Trigger point: Configuration validation and subsequent loading by Nginx
- Why exploitation is possible: Validation examines only the first directive token on each newline-delimited line, while Nginx also parses later semicolon-separated directives and included content
- Environmental dependency: Code execution generally requires an available executable module and sufficient Nginx process privileges
- Attack surface: Authenticated Nginx UI configuration-management endpoints
- Required access: Permission to modify supported Nginx configuration content
- Trigger point: Configuration validation and subsequent loading by Nginx
- Why exploitation is possible: Validation examines only the first directive token on each newline-delimited line, while Nginx also parses later semicolon-separated directives and included content
- Environmental dependency: Code execution generally requires an available executable module and sufficient Nginx process privileges
Why this happens
The validation logic models an Nginx configuration as one directive per line and inspects only the first token. This does not match Nginx's actual grammar, which permits multiple directives on a single line and supports include-based composition. Consequently, restricted directives such as module-loading or scripting directives can evade checks when placed after an accepted directive or loaded indirectly through an include. This behavior maps primarily to CWE-20: Improper Input Validation and is also related to CWE-184: Incomplete List of Disallowed Inputs, because the validator does not comprehensively account for all syntactic paths to restricted configuration.
The validation logic models an Nginx configuration as one directive per line and inspects only the first token. This does not match Nginx's actual grammar, which permits multiple directives on a single line and supports include-based composition. Consequently, restricted directives such as module-loading or scripting directives can evade checks when placed after an accepted directive or loaded indirectly through an include. This behavior maps primarily to CWE-20: Improper Input Validation and is also related to CWE-184: Incomplete List of Disallowed Inputs, because the validator does not comprehensively account for all syntactic paths to restricted configuration.
Potential impact
- An attacker may inject restricted Nginx directives.
- The Nginx worker process could execute attacker-controlled code when a compatible executable module is available.
- The attacker could alter routing, proxying, authentication, or request-processing behavior.
- Sensitive data accessible to the Nginx worker may be exposed or modified.
- The host or connected services could be affected depending on process privileges and network access.
- An attacker may inject restricted Nginx directives.
- The Nginx worker process could execute attacker-controlled code when a compatible executable module is available.
- The attacker could alter routing, proxying, authentication, or request-processing behavior.
- Sensitive data accessible to the Nginx worker may be exposed or modified.
- The host or connected services could be affected depending on process privileges and network access.
General mitigation guidance
Mitigation generally focuses on parsing configuration according to the complete Nginx grammar rather than relying on line-oriented token checks.
Defensive patterns include:
- Tokenizing and validating every directive, including semicolon-separated directives on the same line.
- Applying allowlists for permitted directives and rejecting restricted directives regardless of position or nesting.
- Resolving and validating include targets under a controlled policy before configuration is accepted.
- Separating configuration editing from unrestricted filesystem access and preventing unauthorized modification of included files.
- Running Nginx with the minimum required privileges and limiting access to executable modules.
- Validating the resulting effective configuration before deployment and using secure-by-default configuration templates.
Mitigation generally focuses on parsing configuration according to the complete Nginx grammar rather than relying on line-oriented token checks.
Defensive patterns include:
- Tokenizing and validating every directive, including semicolon-separated directives on the same line.
- Applying allowlists for permitted directives and rejecting restricted directives regardless of position or nesting.
- Resolving and validating include targets under a controlled policy before configuration is accepted.
- Separating configuration editing from unrestricted filesystem access and preventing unauthorized modification of included files.
- Running Nginx with the minimum required privileges and limiting access to executable modules.
- Validating the resulting effective configuration before deployment and using secure-by-default configuration templates.
How teams detect issues like this
Detection can combine static analysis, code review, runtime monitoring, and policy enforcement.
Useful signals include:
- Searches for validators that split configuration only by newline or inspect only the first token.
- Code review checks for incomplete denylist-based directive validation.
- Tests containing multiple directives on one line, nested contexts, and include chains.
- Policy checks that restrict module-loading, scripting, privilege-changing, and unrestricted include directives.
- Monitoring for unexpected configuration changes, newly referenced modules, and anomalous Nginx worker behavior.
- Automated security review tools that identify parser and validation mismatches.
Detection can combine static analysis, code review, runtime monitoring, and policy enforcement.
Useful signals include:
- Searches for validators that split configuration only by newline or inspect only the first token.
- Code review checks for incomplete denylist-based directive validation.
- Tests containing multiple directives on one line, nested contexts, and include chains.
- Policy checks that restrict module-loading, scripting, privilege-changing, and unrestricted include directives.
- Monitoring for unexpected configuration changes, newly referenced modules, and anomalous Nginx worker behavior.
- Automated security review tools that identify parser and validation mismatches.
