json/docs/mkdocs/docs/community/assurance_case.md

Assurance case

This page argues why the library meets its security requirements. It describes the threats the library faces, where the trust boundaries lie, and how the library's design and the quality assurance counter these threats. To report a vulnerability, see the security policy.

Threat model

The library parses, stores, and serializes JSON values in memory. It does not open network connections, does not open files (it only reads from streams or std::FILE* handles that the caller has already opened), does not read environment variables, and does not implement cryptography or handle credentials.

The primary threat is therefore untrusted input: JSON text or binary data (BJData, BON8, BSON, CBOR, MessagePack, UBJSON) that an attacker controls, passed to parse, accept, sax_parse, or one of the from_* functions such as from_cbor. Such input may try to

• make the library read or write out of bounds (malformed lengths, truncated input, invalid UTF-8),
• trigger undefined behavior (integer overflow in sizes or numbers, invalid casts),
• exhaust memory (huge announced sizes), or
• exhaust the call stack (deeply nested arrays and objects).

Trust boundaries

• Untrusted: all serialized input read by the parser, the SAX interface, and the binary readers. The library must handle every possible input by either producing a value or throwing a parse_error (or returning false when exceptions are disabled for the call).
• Trusted: the C++ code that calls the library. Calling a function with violated preconditions, for instance accessing an array with operator[] out of range, is a programming error and not a security boundary. Such preconditions are checked with runtime assertions in debug builds; functions such as at offer checked access with exceptions.

flowchart LR
    A[Untrusted input] --> B[Parser]
    A --> C[SAX interface]
    A --> D[Binary readers]
    B --> E["Value tree (basic_json)"]
    C --> E
    D --> E
    E --> F[Trusted caller]

The trust boundary: untrusted input enters on the left through the parser, the SAX interface, or the binary readers, and only a value tree reaches the trusted caller on the right.

Secure design

• Strict parsing. The parser accepts exactly the JSON grammar of RFC 8259. Extensions such as comments and trailing commas must be enabled explicitly. Invalid UTF-8 is rejected.
• Errors are reported, not ignored. Malformed input results in a parse_error with the byte position of the error. Binary readers do not trust announced sizes: strings and binary values grow only as bytes are actually read, arrays reserve at most a fixed number of elements up front, and sizes that no container can hold are rejected.
• Memory is owned by values. Each basic_json value owns its content, and there is no manual memory management in user code. The destructor does not recurse, so destroying a deeply nested value does not exhaust the stack.
• Bounded recursion. The JSON parser and the binary readers keep their state in explicit stacks instead of recursing per nesting level. Operations that walk a value, such as dump, copying, comparison, hashing, merge_patch, diff, flatten, and the binary writers, recurse only up to a fixed depth and continue with an explicit stack below it. Applications that process untrusted input can limit its nesting depth with a parser callback.
• Invariants are checked. The class invariant (for instance, that the pointer for the stored type is never null) is checked with runtime assertions throughout the test suite.

Common weaknesses

The following table maps the relevant classes of the Common Weakness Enumeration to the measures that counter them. The measures are described in detail in Quality assurance.

Weakness	Countermeasures
Out-of-bounds read/write (CWE-125, CWE-787)	bounds checks on all reads from the input; AddressSanitizer and Valgrind on the test suite; OSS-Fuzz
Integer overflow (CWE-190)	UndefinedBehaviorSanitizer with integer overflow detection; Clang-Tidy; Cppcheck
Use after free, double free (CWE-416, CWE-415)	ownership of all memory by values; AddressSanitizer and Valgrind; Clang Static Analyzer
Memory leaks (CWE-401)	Valgrind (Memcheck) on the test suite
Uncontrolled recursion (CWE-674)	iterative parser, binary readers, and destructor; bounded recursion in value operations; tests with deeply nested inputs
Uncontrolled resource consumption (CWE-400)	allocations based on announced sizes are capped; OSS-Fuzz with memory limits
Undefined behavior in general (CWE-758)	UndefinedBehaviorSanitizer; runtime assertions; Clang-Tidy, Cppcheck, Clang Static Analyzer, Infer

In addition, every line of the library is covered by the unit tests, and all parsers are fuzz-tested around the clock by OSS-Fuzz.