CVE-2026-27613: TinyWeb Auth Bypass Vulnerability

CVE-2026-27613 Overview

CVE-2026-27613 is a critical command injection vulnerability affecting TinyWeb, a lightweight HTTP/HTTPS web server written in Delphi for Windows 32-bit systems. This vulnerability allows unauthenticated remote attackers to bypass the web server's CGI parameter security controls. Depending on the server configuration and the specific CGI executable in use, successful exploitation can result in either source code disclosure or remote code execution (RCE).

The vulnerability is classified as CWE-78 (Improper Neutralization of Special Elements used in an OS Command), indicating that user-supplied input is not properly sanitized before being passed to CGI executables. This is particularly dangerous when hosting interpreted languages like PHP using php-cgi.exe.

Critical Impact

Unauthenticated attackers can achieve remote code execution or source code disclosure on TinyWeb servers hosting CGI scripts, particularly PHP applications, by bypassing CGI parameter security controls.

Affected Products

• TinyWeb versions prior to 2.01
• TinyWeb installations hosting CGI scripts (especially PHP via php-cgi.exe)
• Windows 32-bit systems running vulnerable TinyWeb configurations

Discovery Timeline

• 2026-02-25 - CVE CVE-2026-27613 published to NVD
• 2026-02-25 - Last updated in NVD database

Technical Details for CVE-2026-27613

Vulnerability Analysis

This vulnerability enables remote attackers to inject malicious command-line parameters into CGI executable invocations. The core issue lies in insufficient validation of URL query string parameters before they are passed to CGI executables. When an attacker crafts requests containing parameters beginning with a hyphen (-) or encoded double quotes (%22), these can be interpreted as command-line flags by the CGI executable rather than as legitimate data parameters.

For PHP-CGI specifically, this attack vector is well-documented and allows attackers to leverage dangerous flags that can expose source code or enable arbitrary code execution. The network-accessible nature of web servers combined with the lack of authentication requirements makes this vulnerability trivially exploitable by any attacker who can reach the server.

Root Cause

The root cause is improper input validation in TinyWeb's CGI parameter handling mechanism. Prior to version 2.01, the server did not adequately sanitize or validate query string parameters before passing them to CGI executables. This allows specially crafted URL parameters to be interpreted as command-line arguments by the target CGI executable (such as php-cgi.exe), effectively bypassing intended security controls.

The vulnerability exists when the STRICT_CGI_PARAMS configuration is not enabled, allowing parameter injection attacks to succeed.

Attack Vector

The attack is conducted remotely over the network without requiring authentication. An attacker can craft malicious HTTP requests targeting CGI-enabled endpoints with query string parameters designed to be interpreted as command-line flags. For PHP-CGI, attackers can leverage flags that:

1. Enable source code disclosure by modifying PHP's behavior
2. Execute arbitrary PHP code through configuration manipulation
3. Bypass security restrictions implemented in the web application

The following patch shows the version update addressing this vulnerability:

  SocksCount: Integer;

const
-  CServerVersion = '2.0';
+  CServerVersion = '2.01';
  CServerProductName = 'TinyWeb';
  CServerName = CServerProductName + '/' + CServerVersion;
  CMB_FAILED = MB_APPLMODAL or MB_OK or MB_ICONSTOP;

Source: GitHub Commit d9dbda8

Detection Methods for CVE-2026-27613

Indicators of Compromise

• HTTP requests containing query string parameters starting with hyphen (-) characters
• URL-encoded double quotes (%22) in query string parameters targeting CGI endpoints
• Unusual PHP-CGI process execution with unexpected command-line arguments
• Source code or configuration files being returned in HTTP responses

Detection Strategies

• Implement web server log analysis to identify requests with suspicious query string patterns (parameters beginning with - or containing %22)
• Monitor for anomalous responses from CGI endpoints that may indicate source code disclosure (look for PHP tags <?php in response bodies)
• Deploy network intrusion detection rules to flag HTTP requests targeting .cgi or .php endpoints with malicious parameter patterns
• Review TinyWeb process spawning behavior for CGI executables receiving unexpected arguments

Monitoring Recommendations

• Enable detailed access logging on TinyWeb servers and centralize logs for analysis
• Configure Web Application Firewall (WAF) rules to alert on query parameters matching known CGI parameter injection patterns
• Implement file integrity monitoring on web application source files to detect unauthorized access or modification
• Monitor outbound network connections from the web server that may indicate post-exploitation activity

How to Mitigate CVE-2026-27613

Immediate Actions Required

• Upgrade TinyWeb to version 2.01 or later immediately to address this vulnerability
• Verify that STRICT_CGI_PARAMS is enabled in define.inc if upgrading is not immediately possible
• Audit current CGI configurations and remove unnecessary CGI executables, particularly php-cgi.exe if not required
• Deploy a Web Application Firewall (WAF) in front of TinyWeb servers as an additional defense layer

Patch Information

The vulnerability has been patched in TinyWeb version 2.01. The fix addresses the CGI parameter injection vulnerability by implementing proper validation of query string parameters before they are passed to CGI executables. The security patch is available through the GitHub Release v2.01 and details can be found in the GitHub Security Advisory GHSA-rfx5-fh9m-9jj9.

Additional technical details about the vulnerability are available at the Masiutin Blog.

Workarounds

• Enable STRICT_CGI_PARAMS in define.inc (this is defined by default in the configuration include file)
• Avoid using CGI executables that natively accept dangerous command-line flags (such as php-cgi.exe)
• Deploy a WAF that explicitly blocks URL query string parameters beginning with a hyphen (-) or containing encoded double quotes (%22)
• Consider migrating from CGI to alternative PHP hosting methods such as FastCGI or PHP-FPM behind a reverse proxy

# Example WAF rule (ModSecurity format) to block malicious CGI parameters
# Block query strings starting with hyphen
SecRule ARGS "@rx ^-" "id:100001,phase:1,deny,status:403,msg:'Blocked potential CGI parameter injection'"

# Block encoded double quotes in query strings
SecRule ARGS "@contains %22" "id:100002,phase:1,deny,status:403,msg:'Blocked encoded quotes in CGI parameter'"