CVE-2026-101201 in Qt
Summary
by MITRE • 10/20/2026
A malformed header in the binary file fed to QBinaryJson::fromBinaryData() can cause undefined behaviour because of out-of-bounds read.
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Analysis
by VulDB Data Team • 10/20/2026
The vulnerability identified within the Qt Binary JSON library, specifically affecting the QBinaryJson::fromBinaryData function, represents a critical memory safety issue rooted in improper input validation and bounds checking mechanisms. This flaw allows an attacker to craft a maliciously formatted binary file containing malformed headers that trigger undefined behavior during parsing operations. The core technical deficiency lies in the failure of the parser to adequately verify the integrity and boundaries of the data structures it processes before attempting to access memory locations based on header values. When the input contains corrupted or intentionally crafted header fields, such as incorrect length indicators or invalid type codes, the internal logic may calculate offsets that exceed the allocated buffer size, leading directly to an out-of-bounds read condition.
From a technical perspective, this vulnerability is classified under CWE-125, which denotes Out-of-Bounds Read. This category of weakness occurs when software reads data from memory beyond the intended boundary of a buffer or array. In the context of QBinaryJson::fromBinaryData(), the parser likely relies on values embedded within the binary header to determine how much data to read and where in the stream subsequent elements reside. If these values are manipulated by an attacker, they can direct the application to fetch bytes from arbitrary memory addresses outside the bounds of the input buffer. This behavior is particularly dangerous because it does not necessarily cause an immediate crash but instead results in undefined behavior, which can manifest as data leakage, logic errors, or serve as a precursor to more severe exploitation vectors such as information disclosure or remote code execution if combined with other vulnerabilities like use-after-free conditions during subsequent processing steps.
The operational impact of this vulnerability is significant for any application that utilizes the Qt framework and processes untrusted binary JSON data streams. An attacker who can supply a crafted input file could potentially read sensitive memory contents, including stack variables, heap metadata, or even parts of other applications if shared memory regions are involved. This capability facilitates information disclosure attacks where confidential data such as encryption keys, session tokens, or user credentials stored in adjacent memory locations might be exfiltrated. Furthermore, the presence of undefined behavior complicates debugging and security auditing efforts, as the resulting state of the application becomes unpredictable and difficult to reproduce consistently without specialized fuzzing techniques.
In alignment with industry standards for threat modeling, this vulnerability maps to the ATT&CK technique T1083, which covers File and Directory Discovery, although in a broader sense it relates to data exfiltration via memory scraping rather than direct file system access. It also aligns with CWE-20 regarding Improper Input Validation, as the root cause is the failure to sanitize or validate incoming binary structures against expected constraints before processing them. The lack of rigorous boundary checks on header-derived indices allows external actors to influence internal control flow in unintended ways, violating the principle of least privilege and safe memory access patterns required for robust software engineering.
To mitigate this risk, developers must implement strict validation routines within the QBinaryJson::fromBinaryData function or its underlying parsing logic. This includes verifying that all header fields contain values consistent with valid binary JSON specifications before proceeding to parse payload data. Specifically, length indicators and offset calculations should be checked against the total size of the input buffer to ensure no read operation exceeds available memory boundaries. Additionally, employing static analysis tools configured to detect out-of-bounds access patterns can help identify similar weaknesses in related parsing functions. For users unable to immediately patch the underlying library, implementing a pre-processing layer that validates binary inputs using known-good parsers or schema validation rules can provide an effective defense-in-depth strategy against malformed header attacks.