CVE-2026-3633 in libsoup
Summary
by MITRE • 03/17/2026
A flaw was found in libsoup. A remote attacker, by controlling the method parameter of the `soup_message_new()` function, could inject arbitrary headers and additional request data. This vulnerability, known as CRLF (Carriage Return Line Feed) injection, occurs because the method value is not properly escaped during request line construction, potentially leading to HTTP request injection.
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Analysis
by VulDB Data Team • 04/08/2026
The vulnerability identified as CVE-2026-3633 resides within the libsoup library, a widely-used HTTP client and server library for GNOME applications and other software systems. This flaw represents a critical security weakness that can be exploited by remote attackers to manipulate HTTP requests through improper handling of the method parameter in the soup_message_new() function. The vulnerability stems from inadequate input validation and sanitization mechanisms that fail to properly escape special characters in the HTTP method field, creating an avenue for malicious injection attacks.
The technical implementation of this vulnerability manifests as a CRLF (Carriage Return Line Feed) injection flaw that occurs during the construction of HTTP request lines. When a remote attacker provides a crafted method parameter to the soup_message_new() function, the library fails to properly sanitize this input before incorporating it into the HTTP request structure. This insufficient escaping allows attackers to inject additional headers and request data that can be interpreted by the target server as legitimate HTTP components, effectively bypassing normal request parsing and validation mechanisms. The vulnerability operates at the protocol level where the HTTP request line construction process does not adequately filter or encode special characters that could alter the request structure.
From an operational perspective, this vulnerability presents significant risks to systems utilizing libsoup for HTTP communications, particularly web applications, API clients, and network services that depend on proper HTTP request handling. Attackers can exploit this weakness to perform various malicious activities including cache poisoning, cross-site scripting attacks, or even gain unauthorized access to protected resources by manipulating the HTTP request flow. The impact extends beyond simple data injection as it can potentially enable more sophisticated attacks such as HTTP response splitting, where attackers can inject multiple HTTP responses into a single connection, leading to session hijacking or content manipulation. The vulnerability is particularly concerning because it can be exploited without requiring authentication or specific user interaction, making it a high-severity threat in environments where libsoup is integrated.
Security mitigations for CVE-2026-3633 should focus on implementing proper input validation and sanitization mechanisms within the libsoup library and any applications that utilize it. The most effective immediate solution involves updating to patched versions of libsoup where the method parameter is properly escaped and validated before inclusion in HTTP request construction. Organizations should also implement network-level protections such as web application firewalls that can detect and block suspicious HTTP request patterns containing CRLF sequences. Additionally, developers should adopt defensive programming practices including strict input validation, parameterized queries, and proper encoding of all user-supplied data before it is processed in HTTP request contexts. This vulnerability aligns with CWE-113, which addresses improper neutralization of CRLF sequences in HTTP headers, and can be mapped to ATT&CK technique T1071.004 for Application Layer Protocol: DNS, representing the exploitation of protocol-level weaknesses to manipulate network communications and establish unauthorized access paths.