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Deep Security Analysis of PSR-11 (php-fig/container)

1. Objective, Scope, and Methodology

Objective:

The objective of this deep security analysis is to thoroughly examine the php-fig/container (PSR-11) specification and its implications for the security of applications that utilize it. This includes analyzing the interface itself, potential vulnerabilities in implementations, and the broader security context of dependency injection. We will focus on identifying potential attack vectors, weaknesses, and providing actionable mitigation strategies. The key components under scrutiny are:

  • Psr\Container\ContainerInterface: The core interface defining get() and has().
  • Psr\Container\NotFoundExceptionInterface: Exception thrown when an entry is not found.
  • Psr\Container\ContainerExceptionInterface: General exception for container errors.
  • Representative Implementations (though not exhaustively): We will infer potential security issues based on how implementations could be built, given the interface. We will not analyze specific implementations in this document, as that would be a separate, implementation-specific audit.

Scope:

  • This analysis focuses on the PSR-11 specification itself and the potential security risks arising from its use.
  • We will consider the security implications within the context of a typical PHP application deployment, specifically a containerized deployment using Docker, as outlined in the design review.
  • We will not cover general PHP security best practices (e.g., SQL injection prevention) unless they are directly relevant to the use of PSR-11.
  • We will not perform a code audit of any specific PSR-11 implementation. This analysis is focused on the design and potential implementation pitfalls.

Methodology:

  1. Interface Analysis: We will analyze the ContainerInterface and related exception interfaces to identify any inherent security weaknesses in the specification itself.
  2. Implementation Risk Assessment: We will infer potential security vulnerabilities that could arise in implementations of the interface, based on common coding errors and attack patterns.
  3. Data Flow Analysis: We will analyze how data (specifically, the dependency identifiers) flows through the container and identify potential injection points.
  4. Deployment Context Analysis: We will consider the security implications of using PSR-11 within the chosen containerized deployment model.
  5. Mitigation Strategy Recommendation: We will provide specific, actionable recommendations to mitigate the identified risks, focusing on both secure implementation practices and secure usage of PSR-11 containers.

2. Security Implications of Key Components

2.1 Psr\Container\ContainerInterface

  • get(string $id):

    • Security Implication: The $id parameter is the primary attack vector. A malicious user could potentially inject code or manipulate the container's behavior by controlling this value. The type of attack depends heavily on the implementation.
      • Code Injection: If the implementation uses $id directly in file paths, class names, or other contexts without proper sanitization, it could lead to arbitrary code execution. For example, if an implementation uses $id to directly instantiate a class (new $id()), an attacker could provide a malicious class name.
      • Denial of Service (DoS): An attacker could provide a very long or complex $id that causes excessive resource consumption (memory, CPU) within the container's lookup mechanism.
      • Information Disclosure: Careless error handling or exception messages in the implementation could reveal information about the container's internal structure or the existence of specific dependencies.
      • Logic Flaws: Depending on how the container maps identifiers to dependencies, an attacker might be able to request a dependency they shouldn't have access to, bypassing intended access controls.
    • Mitigation:
      • Strict Input Validation: Implementations must validate the $id string. This should, at a minimum, involve checking the data type (it must be a string, as per the interface). Further validation should be performed based on the expected format of identifiers within the specific implementation. For example, if identifiers are expected to be alphanumeric, enforce that. Regular expressions can be useful here, but must be carefully crafted to avoid ReDoS vulnerabilities.
      • Whitelist Approach: If possible, maintain a whitelist of allowed identifiers. This is the most secure approach, as it prevents any unexpected input.
      • Avoid Direct Use in Sensitive Contexts: Never use the raw $id value directly in file paths, class instantiations, or other security-sensitive operations. Always use a mapping or lookup mechanism that provides an additional layer of abstraction.
      • Secure Error Handling: Implementations must throw NotFoundExceptionInterface for missing identifiers and ContainerExceptionInterface for other errors. Exception messages must not reveal sensitive information. Log errors securely, avoiding exposure of internal details.
      • Resource Limits: Implement resource limits (memory, execution time) to mitigate DoS attacks.

  • has(string $id):

    • Security Implication: Similar to get(), the $id parameter is the main point of concern. While has() only returns a boolean, vulnerabilities in its implementation could still lead to information disclosure or DoS.
      • Timing Attacks: If the implementation handles existing and non-existing identifiers in significantly different ways (e.g., different code paths, different execution times), an attacker could potentially use timing analysis to determine whether a specific identifier exists, even if they can't retrieve the associated dependency.
      • DoS: Similar to get(), a malicious $id could cause excessive resource consumption.
    • Mitigation:
      • Input Validation: Apply the same strict input validation as for get().
      • Consistent Timing: Strive to make the execution time of has() as consistent as possible, regardless of whether the identifier exists. This may involve adding dummy operations to the faster code path to equalize execution time. This is a defense-in-depth measure, as timing attacks are often difficult to exploit in practice.
      • Resource Limits: Implement resource limits, as with get().

2.2 Psr\Container\NotFoundExceptionInterface

  • Security Implication: This exception is thrown when an identifier is not found. The primary security concern is information disclosure through exception messages.
    • Mitigation:
      • Generic Error Messages: Exception messages must not reveal any information about the requested identifier or the internal state of the container. A generic "Entry not found" message is sufficient.

2.3 Psr\Container\ContainerExceptionInterface

  • Security Implication: This is a general exception for container errors. The same concerns about information disclosure apply.
    • Mitigation:
      • Generic Error Messages: Exception messages must not reveal sensitive information. Log detailed error information separately, but do not expose it to the user.

3. Architecture, Components, and Data Flow (Inferred)

Based on the PSR-11 interface and common dependency injection patterns, we can infer a likely architecture:

  • Components:

    • User Application: The code that uses the PSR-11 container.
    • Container Implementation: A concrete class implementing ContainerInterface. This likely contains:
      • Identifier Mapping: A mechanism (e.g., an array, a database table, a configuration file) that maps identifiers to dependency definitions (e.g., class names, factory functions, instances).
      • Dependency Resolution Logic: Code that takes a dependency definition and resolves it (e.g., instantiates a class, calls a factory function).
      • Caching (Optional): Some implementations might cache resolved dependencies to improve performance.
    • Dependencies: The actual objects managed by the container.

  • Data Flow:

    1. The user application calls $container->get('some_identifier').
    2. The container implementation receives the 'some_identifier' string.
    3. The implementation validates the identifier.
    4. The implementation uses its identifier mapping to look up the dependency definition associated with 'some_identifier'.
    5. If the identifier is not found, a NotFoundExceptionInterface is thrown.
    6. If the identifier is found, the implementation uses its dependency resolution logic to create or retrieve the dependency instance.
    7. If an error occurs during dependency resolution, a ContainerExceptionInterface is thrown.
    8. The dependency instance is returned to the user application.
    9. If caching is enabled, the resolved dependency might be stored in the cache for future use.

  • Security-Relevant Aspects:

    • The identifier mapping is a critical security component. If an attacker can control or influence this mapping, they can potentially control which dependencies are returned.
    • The dependency resolution logic is another critical area. Vulnerabilities here could lead to code injection or other attacks.
    • The caching mechanism (if present) could be a target for cache poisoning attacks.

4. Tailored Security Considerations

Given that PSR-11 is a standard for dependency injection containers in PHP, the following security considerations are particularly relevant:

  • Identifier Injection: This is the most significant threat. Since the container's primary function is to retrieve objects based on identifiers, any vulnerability that allows an attacker to control the identifier can lead to serious consequences.
  • Configuration-Based Attacks: Many container implementations rely on configuration files (e.g., YAML, XML, PHP arrays) to define the mapping between identifiers and dependencies. If an attacker can modify these configuration files, they can control the container's behavior.
  • Supply Chain Attacks: The container itself, and the dependencies it manages, are part of the application's supply chain. Vulnerabilities in any of these components can compromise the entire application.
  • Denial of Service: While less critical than code injection, DoS attacks targeting the container could disrupt application functionality.
  • Information Leakage: Careless error handling or logging could reveal information about the application's dependencies or internal structure.

These considerations are not general PHP security recommendations; they are specifically tailored to the context of using a PSR-11 container.

5. Actionable Mitigation Strategies

The following mitigation strategies are tailored to the identified threats and are applicable to both implementers of PSR-11 containers and developers using them:

For Implementers (of PSR-11 Containers):

  1. Input Validation (MUST):
    • Type Check: Enforce that the identifier is a string.
    • Format Validation: Validate the identifier against an expected format (e.g., alphanumeric, specific pattern). Use regular expressions cautiously, avoiding ReDoS.
    • Whitelist (Strongly Recommended): If possible, maintain a whitelist of allowed identifiers.
  2. Secure Identifier Mapping (MUST):
    • Avoid Direct Use of Identifiers: Never use the raw identifier directly in file paths, class names, or other security-sensitive contexts. Use a mapping mechanism.
    • Secure Configuration Storage: If using configuration files, store them securely with appropriate permissions. Consider using environment variables for sensitive configuration values.
    • Tamper-Proofing: Implement measures to detect and prevent unauthorized modification of the identifier mapping (e.g., digital signatures, checksums).
  3. Secure Dependency Resolution (MUST):
    • Avoid eval() and Similar Constructs: Never use eval() or similar functions to instantiate classes or execute code based on the identifier.
    • Use Factory Functions: Prefer using factory functions or closures to create dependencies. This provides better control over the instantiation process.
    • Parameterize: If instantiating classes directly, use a parameterized approach (e.g., $className = $mapping[$id]; $instance = new $className(...$params);) rather than string concatenation.
  4. Secure Error Handling (MUST):
    • Generic Exception Messages: Do not reveal sensitive information in exception messages.
    • Secure Logging: Log detailed error information securely, but do not expose it to users.
  5. Resource Limits (SHOULD):
    • Memory Limits: Set memory limits for container operations to prevent DoS attacks.
    • Execution Time Limits: Set execution time limits.
  6. Timing Attack Mitigation (SHOULD):
    • Consistent has() Implementation: Strive for consistent execution time in has(), regardless of whether the identifier exists.
  7. Secure Caching (If Applicable):
    • Cache Key Validation: If caching resolved dependencies, validate the cache key to prevent cache poisoning attacks.
    • Secure Cache Storage: Store the cache securely, with appropriate permissions.
  8. Regular Security Audits (Strongly Recommended): Conduct regular security audits of the container implementation.
  9. Follow Secure Coding Guidelines (MUST): Develop and adhere to secure coding guidelines specifically for PSR-11 implementations.

For Users (of PSR-11 Containers):

  1. Choose a Secure Implementation (MUST): Carefully evaluate the security of the chosen PSR-11 container implementation. Look for implementations that have undergone security audits and have a good track record.
  2. Secure Configuration (MUST):
    • Protect Configuration Files: Store container configuration files securely, with appropriate permissions.
    • Use Environment Variables: Use environment variables for sensitive configuration values (e.g., database credentials, API keys).
    • Avoid Hardcoding Secrets: Never hardcode secrets directly in the container configuration.
  3. Dependency Management (MUST):
    • Use Composer with composer.lock: Ensure consistent dependency versions.
    • Regularly Update Dependencies: Keep dependencies up to date to patch security vulnerabilities.
    • Audit Dependencies: Regularly audit dependencies for known vulnerabilities.
  4. Input Validation (in Application Code) (MUST): Even with a secure container implementation, validate any user-provided data before passing it to the container as an identifier. This provides an additional layer of defense.
  5. Monitor and Log (SHOULD): Monitor container activity and log any errors or suspicious behavior.
  6. Least Privilege (MUST): Configure the container with the principle of least privilege. Only grant the container access to the resources it absolutely needs.

These mitigation strategies provide a comprehensive approach to securing applications that use PSR-11 containers, addressing both implementation-level vulnerabilities and usage-related risks. The "MUST", "SHOULD", and "Strongly Recommended" designations indicate the relative importance of each strategy.