CVE-2026-3634 in libsoup
Summary
by MITRE • 03/17/2026
A flaw was found in libsoup. An attacker controlling the value used to set the Content-Type header can inject a Carriage Return Line Feed (CRLF) sequence due to improper input sanitization in the `soup_message_headers_set_content_type()` function. This vulnerability allows for the injection of arbitrary header-value pairs, potentially leading to HTTP header injection and response splitting attacks.
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Analysis
by VulDB Data Team • 04/08/2026
The vulnerability identified as CVE-2026-3634 resides within the libsoup library, a widely used HTTP client and server library for applications built on the glib framework. This flaw represents a critical security weakness that stems from inadequate input validation mechanisms within the soup_message_headers_set_content_type() function. The vulnerability manifests when an attacker can manipulate the Content-Type header value, which then gets processed without proper sanitization, allowing malicious actors to inject Carriage Return Line Feed (CRLF) sequences. This specific weakness aligns with CWE-113, which categorizes improper neutralization of CRLF characters in HTTP headers as a significant security concern. The improper handling of input data creates a pathway for attackers to manipulate HTTP responses through header injection techniques that can bypass security controls and potentially lead to more severe consequences.
The technical exploitation of this vulnerability enables attackers to perform HTTP header injection attacks by inserting CRLF sequences into the Content-Type header value. When the affected function processes this malicious input, it fails to properly sanitize the data before setting the header, allowing the injected sequences to be interpreted as actual header terminators. This creates opportunities for response splitting attacks where an attacker can inject additional HTTP headers or manipulate the response structure. The vulnerability's impact extends beyond simple header manipulation as it can enable more sophisticated attacks such as cross-site scripting through header-based XSS vectors, cache poisoning attacks, or session manipulation techniques that rely on controlling HTTP response headers. The attack vector is particularly concerning because it can be exploited through user-controllable input fields that set content type headers, making it accessible to attackers who can influence application behavior.
The operational impact of CVE-2026-3634 is substantial across systems that rely on libsoup for HTTP communication, particularly web applications, API gateways, and proxy servers that use this library for handling HTTP requests and responses. Applications using the vulnerable function are at risk of having their HTTP responses manipulated, potentially leading to unauthorized access, data leakage, or service disruption. The vulnerability can be exploited in various contexts including web applications that accept user input for content type specification, REST APIs that process content type headers, or any system that uses libsoup for HTTP communication. Attackers can leverage this weakness to perform man-in-the-middle attacks, manipulate cached responses, or bypass authentication mechanisms that rely on proper HTTP header handling. The vulnerability also poses risks to web application firewalls and security proxies that depend on predictable HTTP response structures, as the injected headers can cause these security controls to malfunction or be bypassed entirely.
Mitigation strategies for CVE-2026-3634 should prioritize immediate patching of affected libsoup versions, as this represents the most effective defense against exploitation. Organizations should implement input validation controls that sanitize all user-supplied content type values before they are processed by the soup_message_headers_set_content_type() function. The implementation of proper header validation mechanisms that reject or escape CRLF sequences in header values provides an additional layer of protection. Security teams should also consider implementing monitoring and alerting for unusual header patterns that might indicate exploitation attempts, particularly in web application firewalls and intrusion detection systems. The vulnerability's classification under ATT&CK technique T1566.001 for credential access through header injection highlights the need for comprehensive security monitoring. Organizations should also review their application code to identify any other functions that might be vulnerable to similar header injection attacks and implement consistent sanitization practices across all HTTP header handling components. Network segmentation and access controls should be reviewed to limit the potential impact of successful exploitation, while regular security assessments should be conducted to ensure that the patching process has been completed successfully across all affected systems.