Troubleshooting & Root Cause Remediation13 Verified Diagnostic Guides • Instant Code Fixes

Problem → Solution Guides

Engineering-grade troubleshooting for web protocols, browser security boundaries, authentication traps, and performance lockups. Every guide provides diagnostic reasoning (GEO) and an immediate copy-paste fix (AEO) up front.

Showing 13 of 13 solutions

CORS & CSP Security6 min

CORS Error: No Access-Control-Allow-Origin Header

Access to fetch at "https://api.example.com/data" from origin "https://app.example.com" has been blocked by CORS policy: No "Access-Control-Allow-Origin" header is present on the requested resource.

The browser’s Same-Origin Policy blocked client JavaScript from reading the API response because the target server failed to return the Access-Control-Allow-Origin HTTP response header matching the requesting origin.

HTTP/1.1HTTP/2Fetch API

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CORS & CSP Security7 min

How to Fix CORS Preflight Errors

Response to preflight request doesn't pass access control check: It does not have HTTP ok status. / Method Not Allowed (405).

A CORS preflight error occurs when your backend fails to return a successful HTTP status (200 OK or 204 No Content) on the automatic preflight HTTP OPTIONS request, or fails to include Access-Control-Allow-Methods and Access-Control-Allow-Headers.

HTTP OPTIONSFetch APIAxios

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CORS & CSP Security7 min

Why CSP Blocks JavaScript

Refused to execute inline script because it violates the following Content Security Policy directive: "script-src 'self'". Either the 'unsafe-inline' keyword, a hash ('sha256-...'), or a nonce ('nonce-...') is required to enable inline execution.

The browser blocked your JavaScript because your server’s Content-Security-Policy (CSP) header does not permit inline scripts or the external domain hosting the script in its script-src directive.

Content-Security-PolicyDOM Script ExecutionWeb Workers

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Authentication & JWT6 min

Why JWT Works in Postman but Not in Browser

Access to fetch at "https://api.example.com/me" from origin "https://app.example.com" has been blocked by CORS policy. OR: 401 Unauthorized (Missing token).

The request succeeds in Postman because Postman does not enforce the browser Same-Origin Policy (SOP). In a web browser, adding an Authorization: Bearer header triggers an automatic CORS preflight (HTTP OPTIONS) probe. If the backend fails the preflight or omits authorization from Access-Control-Allow-Headers, the browser blocks the token before the server ever reads it.

Fetch APIAxiosCORS

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Authentication & JWT7 min

Why JWT Signature Validation Fails

JsonWebTokenError: invalid signature / io.jsonwebtoken.security.SignatureException: JWT signature does not match locally computed signature.

A JWT signature verification failure means the cryptographic hash of the Header + Payload calculated by the receiver does not match the Signature segment attached to the token, almost always caused by a secret key mismatch, algorithm mismatch (HS256 vs RS256), or key formatting errors.

RFC 7519 (JWT)RFC 7515 (JWS)RS256 / HS256

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Networking, DNS & HTTPS7 min

Why DNS Works on One Device but Not Another

DNS_PROBE_FINISHED_NXDOMAIN or ERR_NAME_NOT_RESOLVED (Device A: Resolves to 198.51.100.1; Device B: Resolves to 203.0.113.5).

Devices resolve domains differently because they query different recursive resolvers (e.g., local ISP router DNS vs. public 1.1.1.1 or browser-native DoH), adhere to cached records governed by the previous TTL, or have local /etc/hosts overrides.

DNS (UDP 53)DoH (DNS-over-HTTPS)mDNS

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Networking, DNS & HTTPS6 min

Why DNS Returns NXDOMAIN

DNS_PROBE_FINISHED_NXDOMAIN (RCODE 3: Non-Existent Domain).

NXDOMAIN (Non-Existent Domain) means the authoritative nameserver confirmed that no record exists for that domain or subdomain. Common causes include missing A/CNAME records, domain expiration at the registrar, broken NS delegation, or DNSSEC validation failures where the parent DS record points to outdated keys.

DNS (RFC 1035)DNSSEC (RFC 4034)UDP/TCP 53

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Networking, DNS & HTTPS7 min

Why HTTPS Shows Mixed Content

Mixed Content: The page at "https://example.com" was loaded over HTTPS, but requested an insecure resource "http://cdn.example.com/lib.js". This request has been blocked.

Mixed content happens when an initial HTML document is loaded securely over HTTPS, but subresources (scripts, images, stylesheets, iframes, fonts, or AJAX endpoints) are requested using unencrypted HTTP:// URLs.

HTTPS (TLS 1.2 / 1.3)HTTP/1.1Content Security Policy (CSP)

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Networking, DNS & HTTPS8 min

Why HTTPS Certificate Errors Occur

NET::ERR_CERT_COMMON_NAME_INVALID or NET::ERR_CERT_AUTHORITY_INVALID or SEC_ERROR_UNKNOWN_ISSUER.

HTTPS certificate errors occur when the browser cannot establish a cryptographically valid trust path from the server’s certificate to a pre-installed root Certificate Authority (CA), when the certificate has expired, or when the domain name does not match the Subject Alternative Names (SAN) on the certificate.

TLS 1.3TLS 1.2X.509 Certificates (RFC 5280)

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HTTP Status Codes7 min

Why a Website Returns 403

403 Forbidden (Nginx: "403 Forbidden", Apache: "You don’t have permission to access this resource", Cloudflare: "Error 1020: Access Denied").

An HTTP 403 Forbidden response indicates the server understood the request but intentionally refused to fulfill it. Common root causes include inadequate RBAC user permissions, Linux file system permission issues (e.g. Nginx worker cannot read the file), Web Application Firewall (WAF) rule triggers, disabled directory indexing without an index file, or missing CSRF tokens on state-changing requests.

HTTP/1.1HTTP/2REST APIs

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HTTP Status Codes7 min

Why a Website Returns 401

401 Unauthorized (Response includes: WWW-Authenticate: Bearer error="invalid_token", error_description="The access token expired").

An HTTP 401 Unauthorized response means the target resource requires user authentication and the request either omitted credentials or provided credentials that are expired, revoked, or malformed. By RFC specification, a 401 response must include a WWW-Authenticate header explaining how the client can authenticate.

HTTP/1.1HTTP/2OAuth 2.0

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HTTP Status Codes6 min

Why a Website Returns 404

404 Not Found (Nginx: "404 Not Found", Apache: "The requested URL was not found on this server", Cloudflare: "Error 404").

An HTTP 404 Not Found status means the server found no file or route matching the requested URI path. In Single Page Applications (SPAs), 404s happen on page refresh because the web server looks for a physical file (e.g., /dashboard/index.html) that does not exist. On Linux web hosts, 404s also frequently stem from case-sensitive filenames (e.g. Logo.PNG vs logo.png).

HTTP/1.1HTTP/2HTML5 History API

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Regex & Performance8 min

Why Regex Causes Catastrophic Backtracking

RangeError: Maximum call stack size exceeded OR Thread Hang / Unresponsive Event Loop (Execution time exceeds 30,000ms).

Catastrophic backtracking occurs when a traditional NFA (Nondeterministic Finite Automaton) regex engine encounters an "evil regex" pattern—typically containing nested quantifiers like `(a+)+` or overlapping alternations like `(a|a)+`—evaluating an input string that fails near the end. The engine exhaustively tries every exponential combinatorial permutation (O(2^n) steps) before finally declaring that the string does not match.

JavaScript V8 RegExpPython reJava java.util.regex

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