CVE-2026-1467: libsoup CRLF Injection Vulnerability

CVE-2026-1467 Overview

A CRLF (Carriage Return Line Feed) Injection vulnerability has been identified in libsoup, a widely-used HTTP client library. The flaw exists in how the library handles URL-decoded input when constructing the Host header while an HTTP proxy is configured. This improper input handling allows remote attackers to inject malicious CRLF sequences into specially crafted URLs, enabling them to inject additional HTTP headers or complete HTTP request bodies that can be forwarded by the proxy to downstream services.

Critical Impact

Remote attackers can exploit this vulnerability to inject unauthorized HTTP headers or request bodies through the HTTP proxy, potentially enabling request smuggling, cache poisoning, or unauthorized actions on downstream services.

Affected Products

• libsoup HTTP client library (versions affected - refer to vendor advisory)
• Applications and services utilizing libsoup for HTTP proxy communications
• Linux distributions shipping vulnerable libsoup versions

Discovery Timeline

• 2026-01-27 - CVE-2026-1467 published to NVD
• 2026-01-28 - Last updated in NVD database

Technical Details for CVE-2026-1467

Vulnerability Analysis

This vulnerability is classified as CWE-93 (Improper Neutralization of CRLF Sequences). The core issue lies in how libsoup processes user-controlled input when generating the Host header for HTTP requests made through a proxy server. When a URL containing encoded CRLF characters (%0d%0a) is provided, the library decodes these sequences and includes them in the Host header without proper sanitization.

The attack is possible when an HTTP proxy is configured, as the vulnerable code path specifically handles proxy-related header construction. An attacker who can control URLs processed by applications using libsoup can inject arbitrary HTTP headers or even craft entirely new HTTP request bodies that the proxy will forward to target servers.

Root Cause

The root cause is insufficient input validation and sanitization of URL-decoded data before incorporating it into HTTP headers. The libsoup library fails to strip or escape CRLF sequences (\r\n) from user-supplied URLs when constructing the Host header for proxied requests. This allows the attacker-controlled data to break out of the intended header context and inject additional header fields or request bodies.

Attack Vector

The attack requires network access and targets applications using libsoup with HTTP proxy configurations. An attacker crafts a URL containing encoded CRLF sequences (e.g., %0d%0a) followed by additional HTTP headers or request body content. When the victim application processes this URL through libsoup's proxy handling code, the decoded CRLF characters terminate the Host header prematurely, and the injected content becomes part of the HTTP request forwarded by the proxy.

A typical attack URL might contain sequences like example.com%0d%0aX-Injected-Header: malicious-value%0d%0a which, after URL decoding, would result in additional headers being injected into the HTTP request sent to the proxy server.

Detection Methods for CVE-2026-1467

Indicators of Compromise

• Unusual HTTP headers appearing in proxy logs that were not expected from the client application
• Unexpected HTTP requests to downstream services originating from the proxy server
• Evidence of cache poisoning or response manipulation on proxied services
• Anomalous URL patterns in application logs containing encoded CRLF sequences (%0d%0a)

Detection Strategies

• Monitor HTTP proxy logs for requests containing suspicious header patterns or unexpected header counts
• Implement URL pattern analysis to detect encoded CRLF sequences in incoming requests
• Deploy web application firewalls (WAF) with CRLF injection detection rules
• Use intrusion detection systems (IDS) to identify HTTP header injection attempts

Monitoring Recommendations

• Enable verbose logging on HTTP proxy servers to capture full request headers
• Implement alerting for requests containing encoded newline characters in URLs
• Monitor downstream services for unexpected or malformed requests originating from proxy infrastructure
• Review application logs for errors related to malformed HTTP requests or header parsing failures

How to Mitigate CVE-2026-1467

Immediate Actions Required

• Review applications in your environment that use libsoup for HTTP communications
• Identify deployments where HTTP proxy configurations are in use with libsoup
• Apply vendor patches as they become available from your Linux distribution or libsoup maintainers
• Consider implementing input validation at the application layer to sanitize URLs before passing them to libsoup

Patch Information

Consult the Red Hat CVE-2026-1467 Advisory for official patch information and affected package versions. Additional technical details and bug tracking information can be found in Red Hat Bug Report #2433174. Organizations should monitor their respective Linux distribution security channels for updated libsoup packages.

Workarounds

• Implement application-level input validation to strip or reject URLs containing CRLF sequences before processing
• Deploy a reverse proxy or WAF in front of vulnerable applications to filter malicious URL patterns
• Temporarily disable HTTP proxy functionality if not required for business operations
• Use network segmentation to limit the impact of potential request smuggling attacks on downstream services

Application-level URL validation should sanitize input by removing or encoding CRLF characters (\r, \n, %0d, %0a) before passing URLs to the libsoup library for processing.