CVE-2025-25184: Rack Log Injection Vulnerability

CVE-2025-25184 Overview

CVE-2025-25184 is a log injection vulnerability affecting Rack, a modular Ruby web server interface that powers many Ruby web applications and frameworks. The vulnerability exists in Rack::CommonLogger and can be exploited by attackers who craft usernames containing newline (CRLF) characters to manipulate log entries.

When authentication is performed via Rack::Auth::Basic, successful authentication stores the username in env['REMOTE_USER'], which is subsequently used by Rack::CommonLogger for logging purposes. If a server allows user creation with usernames containing CRLF and whitespace characters, or if the server logs all login attempts (including failures), an attacker can inject malicious content into log files by using a specially crafted username with CRLF characters.

Critical Impact

Attackers can break log formats, insert fraudulent log entries, potentially obscure real malicious activity, or inject malicious data into log files used for security monitoring and forensic analysis.

Affected Products

• Rack versions prior to 2.2.11
• Rack versions 3.0.x prior to 3.0.12
• Rack versions 3.1.x prior to 3.1.10

Discovery Timeline

• 2025-02-12 - CVE-2025-25184 published to NVD
• 2025-11-03 - Last updated in NVD database

Technical Details for CVE-2025-25184

Vulnerability Analysis

This vulnerability is classified under CWE-93 (Improper Neutralization of CRLF Sequences), commonly known as CRLF Injection or HTTP Response Splitting. The root issue stems from insufficient input validation in the logging mechanism of Rack::CommonLogger.

The attack is network-accessible and requires low privileges to execute—specifically, an attacker needs the ability to authenticate or attempt authentication against the target application. No user interaction is required, making this vulnerability exploitable in automated attack scenarios.

The integrity impact is significant as attackers can corrupt log data integrity, potentially hiding evidence of malicious activity or injecting false evidence. While confidentiality and availability are not directly impacted, the ability to manipulate logs undermines security monitoring, incident response, and forensic investigations.

Root Cause

The vulnerability occurs because Rack::CommonLogger does not sanitize or validate the username stored in env['REMOTE_USER'] before writing it to log files. When usernames containing CRLF characters (\r\n) are logged, these control characters are interpreted literally by the logging system, causing unintended line breaks in log output.

This allows attackers to:

• Terminate the current log line prematurely
• Insert entirely new, fabricated log entries
• Break log parsing tools and SIEM integrations
• Obfuscate malicious activity within log files

Attack Vector

The attack requires network access and the ability to provide authentication credentials to the target application. An attacker creates or uses a username containing CRLF sequences such as attacker\r\n127.0.0.1 - admin [timestamp] "GET /admin HTTP/1.1" 200 to inject fake log entries.

When this malicious username is processed by Rack::Auth::Basic and subsequently logged by Rack::CommonLogger, the CRLF characters cause a line break in the log file, and everything following becomes a new, attacker-controlled log entry. This technique can be used to create fake entries that appear to show legitimate administrative access, masking actual unauthorized activities.

Detection Methods for CVE-2025-25184

Indicators of Compromise

• Log entries containing unexpected line breaks or formatting anomalies
• Usernames in authentication logs containing control characters or unusual whitespace patterns
• Log file corruption or parsing errors in SIEM tools processing application logs
• Discrepancies between application authentication logs and access control logs

Detection Strategies

• Implement log integrity monitoring to detect unexpected formatting changes or injected entries
• Configure SIEM rules to alert on usernames containing CRLF sequences, null bytes, or other control characters
• Monitor for authentication attempts using usernames with unusual character patterns or excessive length
• Deploy file integrity monitoring on critical log files to detect unauthorized modifications

Monitoring Recommendations

• Enable verbose logging for authentication events and cross-reference with network access logs
• Implement real-time log analysis to detect formatting anomalies as they occur
• Establish baseline patterns for normal log entries and alert on deviations
• Review user account creation processes to identify accounts with suspicious username characters

How to Mitigate CVE-2025-25184

Immediate Actions Required

• Upgrade Rack to patched versions: 2.2.11, 3.0.12, or 3.1.10 immediately
• Audit existing user accounts for usernames containing CRLF or other control characters
• Implement input validation at user creation to reject usernames with control characters
• Review existing log files for signs of exploitation or log injection attacks

Patch Information

The Rack development team has released security patches in versions 2.2.11, 3.0.12, and 3.1.10. The fix involves proper sanitization of username data before it is written to log files. The patch commit is available at the GitHub Commit Update. Additional details are provided in the GitHub Security Advisory. Debian users should also refer to the Debian LTS Announcement for distribution-specific guidance.

Workarounds

• Implement input validation at the application level to reject or sanitize usernames containing CRLF, null bytes, and other control characters before authentication processing
• Configure web application firewalls to block authentication requests containing control characters in credential fields
• Deploy a custom middleware before Rack::CommonLogger to sanitize env['REMOTE_USER'] by stripping or encoding control characters
• Consider using structured logging formats (JSON) that properly escape special characters

# Gemfile configuration to update Rack
# Update your Gemfile to specify minimum patched version
gem 'rack', '>= 2.2.11' # For Rack 2.x users
# OR
gem 'rack', '>= 3.0.12' # For Rack 3.0.x users
# OR
gem 'rack', '>= 3.1.10' # For Rack 3.1.x users

# Then run bundle update
bundle update rack