CVE-2026-102555 in Red Hat
Summary
by MITRE • 09/29/2026
A flaw was found in libsoup. The soup_uri_decode_data_uri() function incorrectly treated base64 data-URI payloads as NUL-terminated strings when calling g_base64_decode_inplace(). If the percent-decoded payload contained embedded NUL bytes, the decoded length could remain uninitialized and be used as the size of the returned GBytes. This can lead to an out-of-bounds read or application crash when processing a crafted data URI.
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Analysis
by VulDB Data Team • 09/29/2026
The vulnerability identified in libsoup represents a critical memory safety flaw within the URI parsing subsystem, specifically affecting the soup_uri_decode_data_uri function. This component is responsible for handling data URIs, which are commonly used to embed small files or text directly into web content and network requests. The core technical deficiency lies in how the library processes base64-encoded payloads embedded within these URIs. When a data URI containing percent-decoded data with embedded NUL bytes is processed, the function incorrectly assumes that the resulting string is NUL-terminated. This assumption leads to an incorrect calculation of the decoded length because standard C-string functions stop reading at the first NUL byte rather than processing the entire base64 payload as intended by the RFC 2397 specification for data URIs.
The immediate consequence of this logical error is that the variable storing the size of the decoded buffer remains uninitialized or holds an incorrect value derived from a truncated string length. This uninitialized or erroneous length is subsequently passed to the GBytes structure, which defines the boundaries of the memory block returned by the function. Because the actual allocated memory may be larger than the incorrectly calculated length, or because the system attempts to access data beyond the valid NUL-terminated boundary during subsequent operations, an out-of-bounds read condition occurs. This type of error falls squarely under CWE-125, which describes Out-of-Bounds Read vulnerabilities where software reads data past the end or before the beginning of the intended buffer.
From a security impact perspective, this flaw can lead to severe operational disruptions and potential information disclosure. The most immediate effect is an application crash due to segmentation faults when the program attempts to access memory addresses that are not mapped or protected. However, in more complex scenarios involving heap-based allocations, out-of-bounds reads can allow attackers to leak sensitive data from adjacent memory regions. This could include internal state variables, cryptographic keys, or other user credentials stored nearby in memory. Such information leakage is particularly dangerous as it provides adversaries with valuable intelligence about the target environment without triggering immediate detection mechanisms associated with crashes.
The exploitation of this vulnerability aligns with techniques documented in the MITRE ATT&CK framework, specifically under T1083 File and Directory Discovery or more broadly within data exfiltration tactics if sensitive memory contents are successfully read. Attackers could craft malicious web pages or network requests containing specially constructed data URIs to trigger this flaw when processed by applications relying on libsoup for HTTP communication. This is particularly relevant in environments where user-controlled input can influence the construction of data URIs, such as through file uploads that generate embedded resources or via phishing attacks involving crafted links.
Mitigation strategies must address both immediate remediation and long-term defensive coding practices. The primary solution involves updating to a patched version of libsoup that corrects the length calculation logic in soup_uri_decode_data_uri. Developers should ensure that base64 decoding functions are called with explicit length parameters rather than relying on string termination markers for binary data. Furthermore, implementing strict input validation and sanitization for all incoming URIs can prevent malformed payloads from reaching vulnerable code paths. Security teams should also consider deploying runtime application self-protection tools or memory safety analyzers during testing phases to detect similar uninitialized variable usage patterns before deployment. Regular security audits focusing on C/C++ memory management practices are essential to identify and rectify such low-level errors that compromise the integrity of network-facing applications.