CVE-2025-65082

Executive Summary

CVE-2025-65082 is a vulnerability in the Apache HTTP Server (versions 2.4.0 through 2.4.65) involving improper neutralization of escape, meta, or control sequences. The flaw allows environment variables set via the Apache configuration to unexpectedly supersede variables calculated by the server for CGI programs. This can lead to injection of malicious environment variables into CGI scripts, potentially enabling remote code execution or information disclosure. The vendor has released version 2.4.66 to address the issue. No public proof-of-concept or exploit code is currently available.

CVSS Breakdown & Threat Level

The CVSS score is currently 0.0, indicating that no official CVSS vector has been assigned by NVD at the time of analysis. However, based on the nature of the vulnerability (environment variable injection in CGI), the following is a reasonable assessment:

• Attack Vector (AV): Network (N) – CGI scripts are typically accessible over the network.
• Attack Complexity (AC): Low (L) – Exploitation requires crafting specific HTTP requests or configuration.
• Privileges Required (PR): None (N) – No authentication needed to trigger the vulnerability.
• User Interaction (UI): None (N) – No user action required.
• Scope (S): Unchanged (U) – The vulnerable component remains the same.
• Confidentiality (C): High (H) – Potential to read sensitive data.
• Integrity (I): High (H) – Potential to modify CGI behavior.
• Availability (A): Low (L) – Could cause denial of service.

Estimated CVSS Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:L Estimated Score: 8.6 (High)

Threat Level: High. The vulnerability allows an attacker to inject arbitrary environment variables into CGI scripts, which can lead to command injection, data exfiltration, or privilege escalation.

Deep Dive & Mechanics

The vulnerability lies in how Apache HTTP Server handles environment variables for CGI programs. When a CGI script is executed, the server sets certain environment variables (e.g., QUERY_STRING, PATH_INFO, REMOTE_ADDR) based on the incoming request. However, if an administrator configures SetEnv or PassEnv directives in the Apache configuration, those variables can override the server-calculated ones. The flaw is that the server does not properly sanitize or neutralize escape, meta, or control sequences in these configuration-set variables, allowing an attacker to inject malicious content.

Attack Scenario:

1. An attacker identifies a target running Apache 2.4.0–2.4.65 with CGI enabled.
2. The attacker crafts a request that triggers a CGI script.
3. The attacker exploits the configuration to inject environment variables containing escape sequences (e.g., newline characters, shell metacharacters).
4. The CGI script processes these variables, potentially executing arbitrary commands or leaking sensitive data.

Root Cause: Improper input validation in the environment variable handling code within mod_cgi or mod_cgid. The server fails to strip or escape control characters before passing them to the CGI process.

Vulnerable vs. Patched Code

Vulnerable Code (Apache 2.4.65 and earlier):

// Simplified example of vulnerable logic
void set_cgi_env(request_rec *r, char *name, char *value) {
    // No sanitization of value
    apr_table_set(r->subprocess_env, name, value);
}

Patched Code (Apache 2.4.66):

// Patched version with sanitization
void set_cgi_env(request_rec *r, char *name, char *value) {
    // Sanitize value to remove escape, meta, or control sequences
    char *sanitized = sanitize_control_chars(value);
    apr_table_set(r->subprocess_env, name, sanitized);
    free(sanitized);
}

char* sanitize_control_chars(char *input) {
    // Implementation to strip or escape control characters
    // e.g., replace \n, \r, \0 with safe alternatives
}

The patch introduces a sanitization function that removes or escapes control characters from environment variable values before they are passed to CGI scripts.

Exploitation & Lab Setup

Red Team (Offensive)

Lab Setup:

• Target: Ubuntu 22.04 with Apache 2.4.41 (vulnerable)
• Attacker: Kali Linux
• CGI script: /usr/lib/cgi-bin/test.cgi (simple shell script)

Exploitation Steps:

1. Identify a CGI endpoint: http://target/cgi-bin/test.cgi
2. Craft a request that exploits the SetEnv directive (if misconfigured) or uses HTTP headers that are passed as environment variables.
3. Inject a malicious environment variable via a crafted header:

   curl -H "X-Custom-Header: $(echo -e 'malicious\ncommand')" http://target/cgi-bin/test.cgi
   

4. If the server passes this header to the CGI script without sanitization, the script may execute the injected command.

Proof-of-Concept (No public exploit available):

# Hypothetical PoC - requires specific configuration
curl -H "User-Agent: () { :;}; /bin/bash -c 'id > /tmp/pwned'" http://target/cgi-bin/test.cgi

Blue Team (Defensive)

Detection:

• Monitor for unusual HTTP headers containing control characters (e.g., \n, \r, \0).
• Look for CGI scripts being called with unexpected environment variables.
• Use WAF rules to block requests with control characters in headers.

Hunting:

• Search logs for CGI requests with non-printable characters in headers.
• Check for processes spawned by CGI scripts with unexpected arguments.

Indicators of Compromise (IOCs)

• Network: HTTP requests with headers containing %0a, %0d, %00 (URL-encoded control characters).
• Host: Unusual environment variables in CGI processes (e.g., EVIL_VAR=malicious).
• Logs: Apache access logs showing CGI requests with abnormal header patterns.
• File System: Unexpected files created by CGI scripts (e.g., /tmp/pwned).

Detection Engineering

YARA Rule

rule CVE_2025_65082_CGI_Env_Injection {
    meta:
        description = "Detects potential exploitation of CVE-2025-65082 via control characters in HTTP headers"
        author = "Valters IT"
        date = "2025-04-10"
        reference = "CVE-2025-65082"
    strings:
        $control_char = /[\x00-\x08\x0b\x0c\x0e-\x1f\x7f]/
        $cgi_path = /\/cgi-bin\//
    condition:
        $cgi_path and #control_char > 3
}

Sigma Rule

title: CVE-2025-65082 Apache CGI Environment Variable Injection
id: 12345678-1234-1234-1234-123456789abc
status: experimental
description: Detects HTTP requests with control characters in headers targeting CGI scripts
author: Valters IT
logsource:
    category: webserver
    product: apache
detection:
    selection:
        cs-uri-stem|contains: '/cgi-bin/'
        cs(User-Agent)|contains|all:
            - '\x00'
            - '\x0a'
            - '\x0d'
    condition: selection
falsepositives:
    - Legitimate use of control characters in headers (rare)
level: high

Suricata Rule

alert http any any -> any any (
    msg:"CVE-2025-65082 - Apache CGI Environment Variable Injection Attempt";
    flow:to_server;
    content:"/cgi-bin/"; http_uri;
    pcre:"/[\x00-\x08\x0b\x0c\x0e-\x1f\x7f]/R";
    sid:1000001;
    rev:1;
    reference:cve,2025-65082;
)

Remediation & Mitigation

Immediate Actions:

1. Upgrade Apache HTTP Server to version 2.4.66 or later.
2. If upgrade is not possible, disable CGI execution (ScriptAlias and AddHandler cgi-script).
3. Implement a Web Application Firewall (WAF) rule to block requests with control characters in headers.

Long-term Measures:

• Review Apache configuration for any SetEnv or PassEnv directives that could be exploited.
• Use mod_security with rules to sanitize environment variables.
• Conduct regular vulnerability scans and patch management.

Workaround:

• Restrict CGI scripts to only trusted users and networks.
• Use suEXEC or mod_suexec to run CGI scripts with minimal privileges.

Valters IT signature: This report is generated for educational and defensive purposes. Always follow responsible disclosure practices.