CVE-2024-47252: Apache HTTP Server Log Injection Flaw

CVE-2024-47252 Overview

CVE-2024-47252 is a log injection vulnerability in the mod_ssl module of Apache HTTP Server versions 2.4.63 and earlier. The vulnerability stems from insufficient escaping of user-supplied data, allowing an untrusted SSL/TLS client to insert escape characters into log files when specific logging configurations are in use.

In logging configurations where CustomLog is used with %{varname}x or %{varname}c directives to log variables provided by mod_ssl (such as SSL_TLS_SNI), neither mod_log_config nor mod_ssl performs proper escaping. This allows unsanitized data provided by malicious clients to appear directly in server log files.

Critical Impact

Attackers can inject malicious escape sequences into Apache log files, potentially enabling log tampering, log analysis tool exploitation, or terminal escape sequence attacks against administrators reviewing logs.

Affected Products

• Apache HTTP Server versions 2.4.63 and earlier
• Systems using mod_ssl with CustomLog configurations logging SSL variables
• Environments with %{SSL_TLS_SNI}x or similar SSL variable logging enabled

Discovery Timeline

• 2025-07-10 - CVE-2024-47252 published to NVD
• 2025-11-04 - Last updated in NVD database

Technical Details for CVE-2024-47252

Vulnerability Analysis

This vulnerability is classified under CWE-150 (Improper Neutralization of Escape, Meta, or Control Sequences). The issue arises when Apache administrators configure logging to capture SSL/TLS-related variables using the %{varname}x or %{varname}c format specifiers in CustomLog directives.

When a client establishes an SSL/TLS connection and provides a Server Name Indication (SNI) value or other SSL-related data, this information can be logged. However, the mod_ssl module fails to sanitize or escape special characters within these client-supplied values before they are written to log files. This creates an opportunity for attackers to inject escape sequences, control characters, or other potentially malicious data.

Root Cause

The root cause lies in the absence of output encoding between the mod_ssl module and the mod_log_config module. When SSL variables are passed from mod_ssl to the logging system, neither module assumes responsibility for sanitizing the data. The mod_ssl module does not escape the data before providing it, and mod_log_config does not perform additional sanitization when writing SSL-derived variables to log files.

This gap in input validation means that whatever data a client sends in the SNI field or other SSL handshake parameters is written verbatim to the log file, including any embedded escape sequences or control characters.

Attack Vector

The attack is network-accessible and requires no authentication or user interaction. An attacker can exploit this vulnerability by crafting malicious SSL/TLS handshake requests containing specially crafted SNI values or other SSL parameters. When the server logs these values, the injected escape sequences or control characters become part of the log entry.

Potential attack scenarios include:

• Terminal Escape Sequence Injection: Injecting ANSI escape codes that could affect terminal output when administrators use tools like cat, tail, or less to view logs, potentially hiding malicious entries or manipulating displayed output
• Log Analysis Tool Exploitation: Crafting payloads that could trigger vulnerabilities in log parsing tools, SIEM systems, or log management platforms
• Log Tampering: Inserting control characters that could alter how log entries are interpreted or displayed, potentially masking other malicious activity

Detection Methods for CVE-2024-47252

Indicators of Compromise

• Unusual or non-printable characters appearing in Apache access or SSL logs
• Log entries containing ANSI escape sequences (e.g., \\x1b[ patterns)
• Anomalous SNI values in SSL handshake logs that contain control characters
• Log parsing errors or corruption in log analysis tools

Detection Strategies

• Implement regular expressions to detect escape sequences in log files, particularly in SSL-related fields
• Monitor for unusual byte patterns in SNI values during SSL/TLS handshake processing
• Deploy log integrity monitoring to detect unexpected characters or formatting in log entries
• Use hexdump or similar tools to inspect raw log content for hidden control characters

Monitoring Recommendations

• Configure SIEM rules to alert on non-printable ASCII characters in web server logs
• Implement automated log sanitization checks as part of log ingestion pipelines
• Review Apache CustomLog configurations to identify instances using %{varname}x or %{varname}c with SSL variables
• Enable network-level monitoring for abnormal SNI values in SSL/TLS connections

How to Mitigate CVE-2024-47252

Immediate Actions Required

• Review Apache configurations for CustomLog directives using %{SSL_TLS_SNI}x, %{SSL_CLIENT_S_DN}x, or similar SSL variable logging
• Consider temporarily removing or modifying logging configurations that log unsanitized SSL variables
• Upgrade to the latest Apache HTTP Server version that addresses this vulnerability
• Implement input validation on SNI values at the network perimeter if possible

Patch Information

Apache has addressed this vulnerability in versions after 2.4.63. Organizations should update to the latest stable release of Apache HTTP Server. Detailed security information is available in the Apache HTTPD Vulnerabilities documentation.

Additional security advisories have been published by Openwall OSS-Security and Debian LTS for distributions tracking this CVE.

Workarounds

• Modify CustomLog configurations to avoid logging SSL variables using %{varname}x or %{varname}c format specifiers until patching is complete
• Implement a log post-processing pipeline that sanitizes escape sequences before logs are stored or analyzed
• Use secure log viewing tools that do not interpret escape sequences (e.g., cat -v or dedicated log viewers)
• Consider restricting access to raw log files to prevent terminal escape sequence attacks against administrators

# Example: Safer log viewing to prevent terminal escape injection
# Use cat -v to display control characters visibly
cat -v /var/log/apache2/ssl_access.log

# Or use hexdump for raw inspection
hexdump -C /var/log/apache2/ssl_access.log | less

# Review CustomLog configurations for SSL variable logging
grep -r "CustomLog" /etc/apache2/ | grep -E "%\{.*\}[xc]"