CVE-2026-66338 in Red Hat
Summary
by MITRE • 07/25/2026
A flaw was found in libsoup. The chunked transfer encoding parser uses a permissive parsing function for chunk sizes that silently accepts inputs violating RFC 9112, including leading whitespace, plus sign prefixes, and trailing invalid characters. When libsoup operates behind a strict frontend proxy, this parsing differential can be exploited to smuggle HTTP requests.
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Analysis
by VulDB Data Team • 07/25/2026
This vulnerability resides within the libsoup library's implementation of chunked transfer encoding parsing, representing a critical security weakness that enables HTTP request smuggling attacks. The flaw stems from an overly permissive parsing approach that accepts non-compliant input formats as defined in RFC 9112, creating a dangerous discrepancy between how libsoup processes chunk sizes and how strict proxies expect them to be formatted. The vulnerability specifically allows for leading whitespace characters, plus sign prefixes, and trailing invalid characters in chunk size declarations, all of which should be rejected according to web standards but are silently accepted by libsoup's parser.
The technical implementation of this vulnerability operates through a parsing differential that occurs when libsoup processes HTTP requests behind strict frontend proxies. When a malicious actor crafts HTTP requests with non-compliant chunk size specifications, the permissive parser in libsoup accepts these inputs while a strict proxy would reject them, creating an inconsistency that can be exploited for request smuggling. This parsing differential enables attackers to inject additional HTTP requests into the processing pipeline by manipulating the chunked encoding format in ways that appear valid to libsoup but violate the expected RFC 9112 compliance standards.
The operational impact of this vulnerability extends beyond simple data corruption or service disruption, as it creates a potential pathway for attackers to bypass security controls and manipulate HTTP traffic flows. The ability to smuggle requests means that an attacker could potentially inject malicious payloads, redirect traffic, or perform unauthorized operations by exploiting the inconsistent parsing behavior between libsoup and compliant proxies. This vulnerability particularly affects applications using libsoup as their HTTP library implementation in environments where strict proxy enforcement is present, making it a significant concern for web applications and services that rely on proper HTTP protocol adherence.
Security mitigations for this vulnerability should focus on updating to patched versions of libsoup that implement strict RFC 9112 compliance for chunked transfer encoding parsing. Organizations should also consider implementing additional HTTP traffic monitoring and validation mechanisms to detect anomalous chunked encoding patterns that might indicate exploitation attempts. Network administrators should ensure that proxy configurations properly validate all incoming HTTP requests regardless of the underlying library implementation, while developers should review their code for potential reliance on permissive parsing behaviors that could be exploited through similar attack vectors.
This vulnerability aligns with CWE-1297, which addresses improper handling of input validation in network protocols, and represents a specific instance of how protocol parsing inconsistencies can create security weaknesses. The attack vector corresponds to techniques described in the ATT&CK framework under T1071.004 for application layer protocol manipulation, where attackers exploit protocol implementation differences to gain unauthorized access or manipulate system behavior. Organizations should also consider implementing network segmentation and traffic inspection tools that can identify and block malformed HTTP chunked encoding patterns that deviate from RFC 9112 specifications.