CVE-2025-27111: Rack Log Injection Vulnerability

CVE-2025-27111 Overview

CVE-2025-27111 is a log injection vulnerability affecting Rack, the modular Ruby web server interface. The Rack::Sendfile middleware logs unsanitized header values from the X-Sendfile-Type header, allowing attackers to inject escape sequences (such as newline characters) into the header. This results in log injection, which can be exploited to manipulate log files, forge log entries, or potentially bypass security monitoring systems.

Critical Impact
Attackers can inject malicious escape sequences via the X-Sendfile-Type header, enabling log manipulation, log forging, and potential evasion of security monitoring in Ruby web applications using Rack middleware.

Affected Products

Rack versions prior to 2.2.12
Rack versions 3.0.x prior to 3.0.13
Rack versions 3.1.x prior to 3.1.11

Discovery Timeline

2025-03-04 - CVE-2025-27111 published to NVD
2025-11-03 - Last updated in NVD database

Technical Details for CVE-2025-27111

Vulnerability Analysis

This vulnerability exists in the Rack::Sendfile middleware component, which handles file serving optimization in Ruby web applications. When the middleware encounters an unknown X-Sendfile-Type header value, it logs an error message containing the unsanitized header value directly. The lack of proper input sanitization allows attackers to inject special characters, including newline sequences (\n, \r\n), into log files.

Log injection vulnerabilities (CWE-93) can have cascading security implications. Attackers may forge legitimate-looking log entries to cover their tracks, inject malicious data that could be interpreted by log analysis tools, or manipulate security information and event management (SIEM) systems that consume these logs.

Root Cause

The root cause is improper neutralization of CRLF sequences in HTTP headers (CWE-93). The vulnerable code directly interpolates the X-Sendfile-Type header value into a log message using string interpolation without sanitization. The original code used simple string interpolation ('#{type}') which preserved any control characters in the input, allowing them to be written directly to log output.

Attack Vector

The attack is network-based and requires no authentication or user interaction. An attacker can craft HTTP requests with malicious X-Sendfile-Type header values containing escape sequences. When processed by the vulnerable Rack middleware, these sequences are written to application logs, enabling:

Log forging to insert fake entries
Log poisoning to inject malicious content
Evasion of log-based security monitoring
Potential exploitation of log processing pipelines

The following patch demonstrates how the fix sanitizes the header value using Ruby's #inspect method:

ruby

           end
         when '', nil
         else
-          env[RACK_ERRORS].puts "Unknown x-sendfile variation: '#{type}'.\n"
+          env[RACK_ERRORS].puts "Unknown x-sendfile variation: #{type.inspect}"
         end
       end
       response

Source: GitHub Commit

The fix replaces direct string interpolation with #inspect, which escapes special characters and prevents control sequences from being interpreted literally in log output.

Detection Methods for CVE-2025-27111

Indicators of Compromise

Unusual multi-line entries in application logs originating from Rack middleware
Log entries containing unexpected escape sequences or control characters
Anomalous X-Sendfile-Type header values in web server access logs
Evidence of log file tampering or unexpected log entry formatting

Detection Strategies

Monitor web application logs for irregular formatting or injected content in Rack-related entries
Implement header validation rules to detect non-standard X-Sendfile-Type values
Deploy web application firewall (WAF) rules to inspect and block requests with control characters in headers
Review SIEM alerts for log injection patterns or anomalous log entry structures

Monitoring Recommendations

Enable verbose logging for Rack middleware to capture detailed request information
Configure log integrity monitoring to detect unauthorized modifications
Implement centralized log aggregation with anomaly detection capabilities
Set up alerts for requests containing escape sequences in HTTP headers

How to Mitigate CVE-2025-27111

Immediate Actions Required

Upgrade Rack to version 2.2.12, 3.0.13, or 3.1.11 or later immediately
Audit application logs for signs of exploitation or log injection attempts
Implement input validation on the X-Sendfile-Type header at the web server or WAF level
Review and harden log processing pipelines to handle potentially malicious input

Patch Information

Rack maintainers have released security patches across all affected version branches. The fix applies proper sanitization using Ruby's #inspect method to escape special characters before logging. Updates are available through standard Ruby gem distribution channels.

Official patch commits:

For additional details, refer to the GitHub Security Advisory GHSA-8cgq-6mh2-7j6v.

Workarounds

Deploy WAF rules to strip or reject requests with control characters in the X-Sendfile-Type header
Configure reverse proxy or load balancer to sanitize or block malicious header values
Disable the Rack::Sendfile middleware if not required for your application
Implement custom middleware to validate and sanitize headers before they reach Rack::Sendfile

bash

# Update Rack gem to patched version
gem update rack

# Or specify minimum version in Gemfile
# gem 'rack', '>= 2.2.12'  # for 2.x
# gem 'rack', '>= 3.0.13'  # for 3.0.x
# gem 'rack', '>= 3.1.11'  # for 3.1.x

bundle update rack