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@@ -1,7 +1,7 @@
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# Parsing Untrusted Input
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This page is for applications that parse JSON -- or one of the supported [binary formats](../binary_formats/index.md)
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(BJData, BON8, BSON, CBOR, MessagePack, UBJSON) -- from a source they do not fully control, such as a network
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This page is for applications that parse JSON — or one of the supported [binary formats](../binary_formats/index.md)
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(BJData, BON8, BSON, CBOR, MessagePack, UBJSON) — from a source they do not fully control, such as a network
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connection, an uploaded file, or another process. It summarizes what the library already does for such input and what
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remains the caller's responsibility, linking to the pages that cover each aspect in detail rather than repeating them.
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@@ -50,12 +50,12 @@ stack, however deeply it is nested.
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[SAX handler](sax_interface.md) (for the JSON parser or a binary format alike) and throwing once your limit is
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exceeded.
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Once a value has been parsed, operations that walk it recursively -- serializing it with
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Once a value has been parsed, operations that walk it recursively — serializing it with
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[`dump`](../../api/basic_json/dump.md), hashing it, copying it, comparing two values with `#!cpp ==`, `#!cpp <`, or (in
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C++20) `#!cpp <=>`, merging with [`update`](../../api/basic_json/update.md), applying a
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[`merge_patch`](../../api/basic_json/merge_patch.md), creating a patch with [`diff`](../../api/basic_json/diff.md) or
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[`merge_diff`](../../api/basic_json/merge_diff.md), [`flatten`](../../api/basic_json/flatten.md), and the binary
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writers (`to_cbor`, `to_msgpack`, ...) -- descend at most 128 levels on the call stack and continue
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writers (`to_cbor`, `to_msgpack`, ...) — descend at most 128 levels on the call stack and continue
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below that with an explicit stack instead, so none of them can exhaust the stack either, however deeply the value is
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nested. Destroying a value (its destructor) never recurses at all, regardless of nesting depth, for the same reason.
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@@ -63,12 +63,12 @@ nested. Destroying a value (its destructor) never recurses at all, regardless of
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The library does not limit the overall size of a JSON text; a value nested or wide enough will use memory
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proportional to the input. If you parse untrusted input of unbounded size, check the size of the file or stream
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yourself before -- or while -- handing it to `parse()`.
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yourself before — or while — handing it to `parse()`.
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For the binary formats, an announced size is never trusted outright:
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- Reading a string or binary value copies the input in bounded 4096-byte chunks and grows the result as bytes are
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actually consumed, rather than allocating the announced length up front -- a truncated input runs out of bytes
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actually consumed, rather than allocating the announced length up front — a truncated input runs out of bytes
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(reported as a parse error) instead of triggering an oversized allocation.
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- When an array announces its number of elements and the array container supports `reserve()` (as `#!cpp std::vector`,
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the default, does), the library reserves storage for at most 16384 of them upfront, regardless of how large the
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@@ -91,7 +91,7 @@ Invalid UTF-8 is rejected while parsing, not just while serializing:
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A `#!cpp '\0'` (NUL) byte *inside* a quoted JSON string is always rejected (it must be escaped as `\u0000`). A NUL byte
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*outside* of a string is different: by default it is silently treated as the end of the input, so trailing bytes after
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it -- including further, otherwise well-formed JSON -- are silently ignored rather than rejected. Since untrusted input
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it — including further, otherwise well-formed JSON — are silently ignored rather than rejected. Since untrusted input
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that happens to embed a NUL is a way to make part of it disappear without a parse error, see the
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[FAQ entry](../../home/faq.md#nul-bytes-in-the-input) and consider defining
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[`JSON_STRICT_NUL_HANDLING`](../../api/macros/json_strict_nul_handling.md) to `1` to reject a NUL byte like any other
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@@ -116,7 +116,7 @@ silently resolving them one way or another, see the
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## Numbers
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A number whose value cannot be represented -- for instance `1E1000`, which overflows `double` -- is rejected while
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A number whose value cannot be represented — for instance `1E1000`, which overflows `double` — is rejected while
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parsing as [`out_of_range.406`](../../home/exceptions.md#jsonexceptionout_of_range406) rather than silently becoming
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infinity. An integer that is syntactically valid but does not fit the 64-bit integer types is not rejected; it is
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instead stored as a `double`, which may lose precision for very large values. See
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@@ -139,7 +139,7 @@ conforming JSON consumers is exactly what the default rejects.
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[SAX handler](sax_interface.md); the library bounds the call stack but not memory use.
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- Bound the size of the input itself before parsing, independent of the library's own bounded allocations for binary
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format lengths.
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- Decide up front how you want a string with invalid UTF-8 -- from any source, not only parsing -- to be serialized
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- Decide up front how you want a string with invalid UTF-8 — from any source, not only parsing — to be serialized
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(`strict`, `replace`, or `ignore`), rather than discovering it from an uncaught `type_error.316`.
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- If a stray NUL byte silently truncating trailing input is a problem for your input format, define
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`JSON_STRICT_NUL_HANDLING`.
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@@ -160,3 +160,9 @@ report a security issue in the library itself, follow the [security policy](../.
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- [Number handling](../types/number_handling.md) - number ranges and overflow behavior
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- [Assurance case](../../community/assurance_case.md) - the library's threat model and countermeasures
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- [Security policy](../../community/security_policy.md) - how to report a vulnerability
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- [`parse`](../../api/basic_json/parse.md) - deserialize from a compatible input
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- [`accept`](../../api/basic_json/accept.md) - check if the input is valid JSON
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- [`sax_parse`](../../api/basic_json/sax_parse.md) - generate SAX events
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- [`parser_callback_t`](../../api/basic_json/parser_callback_t.md) - callback function type for the parser
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- [`JSON_STRICT_NUL_HANDLING`](../../api/macros/json_strict_nul_handling.md) - opt in to rejecting a NUL byte in the input
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- [`JSON_DIAGNOSTIC_POSITIONS`](../../api/macros/json_diagnostic_positions.md) - record start and end positions of parsed values
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