“Updating Failed. The response is not a valid JSON response” in WordPress: Cause and Solution (Dangerous Keyword in Title Case)

2025年12月4日

Summary: Sudden “The response is not a valid JSON response” Error

When trying to save a post in the WordPress block editor, the following error suddenly appeared:

Updating failed. The response is not a valid JSON response.

I hadn’t changed any settings and was just writing blog posts as usual. This article summarizes:

The cause in my environment
Steps taken to isolate the problem
Final solution
Why certain keywords can be security issues

Conclusion: Specific Keyword (eval) in Title and URL Caused It

In short, the cause was including the string “eval” in the article title and URL slug.

Article Title: “Python’s eval() Function: How to Execute Strings as Code and Security Risks”
Permalink (Slug): /python-eval-function-dynamic-execution-security

Because the title and URL contained “eval,” the server-side security feature (WAF) flagged it as a “suspicious request” and blocked it. Consequently, WordPress displayed the error: “Updating failed. The response is not a valid JSON response.“

In fact:

Changing the title to “aaaa”
Changing the URL slug to “aaaaa”

…allowed the post (with the same body content) to be updated without issues.

Further testing revealed:

Removing “eval” from the title and slug resolved the error.
Writing “eval” in the post body caused no issues.

This confirmed that the “keywords in the title and URL” were the cause.

Isolation Steps Taken

Before identifying the cause, I tried standard troubleshooting steps:

Wait and retry: Tried updating again after a day.
Re-login: Logged out and logged back in.
PC Restart: Restarted the PC to rule out client-side issues.
REST API Check: Accessed https://mydomain/wp-json/ directly in the browser to ensure JSON was returned.
Change Browser: Tried updating using a different browser.
Clear Cache/Cookies: Deleted browser data to rule out cache issues.
Disable Security Plugins: Temporarily disabled plugins like XO Security, BBQ Firewall, SAKURA RS WP SSL.
Check Site Health: Confirmed “Unexpected result in REST API” and “403 Forbidden” for endpoint wp-json/wp/v2/types/post?context=edit.
Simple String Test: Created a test post with title “aaaa,” body “aaaaa,” slug “aaaaa,” which saved successfully.
Incremental Keyword Test: Added words from the original title back one by one. The error recurred only when “eval” was included.

This process of elimination led to the conclusion that the string itself in the title/slug was suspicious.

Why Keywords in Titles/URLs Get Blocked

Many hosting environments have WAF (Web Application Firewall) enabled. WAF inspects request contents and automatically blocks patterns associated with malicious attacks, such as:

SQL Injection
Command Injection
Cross-Site Scripting (XSS)
Code Injection leading to Remote Code Execution (RCE)

WAF checks not only the URL path but also Query Strings, POST Data, JSON Body, and HTTP Headers.

When saving a post in the block editor, WordPress uses the REST API, sending the title and slug strings within a JSON body. In this case:

Title and slug contained “eval.”
WAF inspected the JSON and flagged the keyword associated with dangerous code execution.
WordPress received a blocked response instead of valid JSON from the REST API, resulting in the error.

What is the `eval` Function? (Python Example)

A brief explanation of eval, the keyword in question: Python’s eval() function evaluates a string expression as Python code and returns the result.

Simple Usage:

expression = "2 + 3 * 4"
result = eval(expression)
print(result)  # Outputs 14

While flexible, it poses major security risks.

Typical Example of `eval` Abuse

Attackers can use eval() as an entry point for Arbitrary Code Execution.

1. Code Injection If a web app blindly passes user input to eval():

from flask import request
user_input = request.args.get("value")
result = eval(user_input)

An attacker could send: __import__("os").system("rm -rf /"). eval() executes this, potentially deleting server files.

2. Remote Code Execution (RCE) This vulnerability allows attackers to execute arbitrary commands on the server remotely, often flagged as a critical issue. WAFs are vigilant against “eval” to prevent such RCE attacks.

3. JavaScript eval and XSS JavaScript also has eval(), often linked to XSS (Cross-Site Scripting) or used for obfuscation by malicious scripts. Hence, security products are sensitive to the string “eval.”

Checklist for Similar Errors

If you encounter this error, try checking in this order:

Browser Check: Clear cache, try another browser/device.
WordPress Session: Log out and log back in.
REST API Check: Access https://domain/wp-json/ to check for errors.
Site Health: Check for REST API errors.
Plugins/WAF: Temporarily disable security plugins; check WAF logs in your hosting panel.
Simple String Test: Change title/slug to “test” to see if it saves.
Dangerous Keywords: Check for strings like eval, union select, drop table.

Final Solution & Policy

My operational policy to address this:

Body: Using “eval” for technical explanation is allowed.
Title/Permalink: Avoid using “eval”.
- Slug: Replace with a synonym (e.g., /python-dynamic-execution-security/).

While tweaking WAF rules is an option, simply avoiding dangerous keywords in titles and URLs is a simpler and effective solution for most hosting environments.