PHP Timing Attacks: Real Risks & Fixes

TL;DR

Yes, timing attacks are used in practice against PHP applications, though they’re often a secondary attack vector. They exploit the fact that different amounts of time are taken to process data depending on its content. This can leak information about passwords, API keys, or other sensitive data. The main defence is using functions designed to avoid timing differences (like password_verify) and being careful with custom comparison code.

What are Timing Attacks?

Imagine you’re guessing a password. If the server takes longer to respond when you guess correctly than when you guess wrong, an attacker can measure these tiny differences in response time. Over many guesses, they can statistically determine the correct password character by character.

Are They Common?

Direct timing attacks against well-written PHP code are less common than other vulnerabilities like SQL injection. However, they’re often used as part of a broader attack strategy – for example, to bypass rate limiting or after an initial compromise to extract data.

How Do They Work in PHP?

The core issue is predictable execution time based on input. Here are some examples:

• String Comparison: Comparing strings character-by-character can take longer if the strings differ early on.
• Hashing: If you’re comparing hashes directly (bad practice!), timing differences in hash calculations can reveal information.

Step-by-Step Fixes

1. Use password_verify for Password Checking: This is the single most important step. It’s specifically designed to resist timing attacks.

2. Avoid Direct String Comparison for Sensitive Data: Don’t compare passwords or API keys directly using == or similar operators.

3. Constant-Time Comparison Functions (If Necessary): If you absolutely *must* implement custom comparison logic, use functions designed for constant time operation. These are rare in standard PHP but can be found in security libraries.

   Example (using a hypothetical function – check library documentation!):

4. Be Careful with Loops: Avoid loops where the number of iterations depends on the input data. This can create timing variations.

5. Regular Security Audits: Include timing attack testing in your regular security audits. Tools can help automate this process.
   • Consider using tools like timeattack (though setup and interpretation require expertise).

Further Considerations

• Caching: Caching can mask timing differences, but it doesn’t eliminate the underlying vulnerability.
• Hardware: The CPU and server environment also play a role in timing variations.