CVE-2026-116733 in flatcc
Summary
by MITRE • 10/27/2026
flatcc through 0.6.4 contains an out-of-bounds read vulnerability in __flatcc_json_parser_number that allows attackers to read past the input buffer via numbers ending in a decimal point. Attackers can supply untrusted JSON ending immediately after a decimal point to generated parsers, crashing applications when the buffer ends at an unmapped page.
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Analysis
by VulDB Data Team • 10/27/2026
The flatcc library, specifically through version 0.6.4, contains a critical out-of-bounds read vulnerability within its JSON parsing logic. This flaw is located in the __flatcc_json_parser_number function, which is responsible for processing numeric values during the deserialization of JSON data into C structures. The root cause of this issue stems from an improper boundary check when handling floating-point numbers that end with a decimal point but lack subsequent digits. When the parser encounters such a number at the very end of the input buffer, it attempts to read beyond the allocated memory region in search for additional characters or validation markers, leading to an invalid memory access operation.
From a technical perspective, this vulnerability represents a classic case of insufficient bounds checking during stream processing. The parser does not adequately verify whether there are sufficient bytes remaining in the buffer before attempting to consume them as part of the numeric value parsing routine. This oversight allows an attacker who controls the input data to trigger memory reads outside the intended boundaries. In many execution environments, particularly those utilizing virtual memory management with guard pages at the end of allocated buffers, this out-of-bounds read can result in a segmentation fault or access violation. Consequently, applications relying on flatcc for JSON parsing may experience abrupt termination when processing maliciously crafted input that exploits this specific edge case involving trailing decimal points.
The operational impact of this vulnerability is primarily centered around availability and potential information disclosure. While the immediate effect observed is often an application crash due to accessing unmapped memory pages, out-of-bounds reads can also lead to the leakage of sensitive data residing in adjacent memory locations if the read operation does not immediately fault. This makes the vulnerability particularly dangerous in network-facing services or applications that process untrusted JSON payloads from external sources. An attacker could potentially use this flaw to cause a denial of service by repeatedly crashing the target application, disrupting business operations and causing downtime for users relying on the affected software.
This vulnerability aligns with CWE-125, which describes out-of-bounds read conditions where data is accessed beyond the end or beginning of a buffer without proper validation. It also relates to CWE-20, indicating improper input validation, as the parser fails to correctly validate the structure and length of numeric tokens before processing them. In terms of attack vectors, this falls under ATT&CK technique T1496, which covers Resource Hijacking via denial-of-service attacks, although the underlying mechanism is a memory safety violation rather than resource exhaustion in the traditional sense. The exploitation requires only the ability to supply untrusted JSON input, making it accessible to remote attackers with network access to the vulnerable service.
Mitigation strategies for this vulnerability involve upgrading flatcc to version 0.6.5 or later, where the issue has been addressed by implementing stricter boundary checks within the number parsing logic. Developers should ensure that their build pipelines enforce dependency updates and regularly scan for known vulnerabilities in third-party libraries. Additionally, employing input validation layers at the application perimeter can help filter out malformed JSON structures before they reach the parser. Using static analysis tools configured to detect buffer overflows and bounds checking errors during development phases can further reduce the risk of introducing similar flaws into software systems that depend on flatcc for data serialization tasks.