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Reject JSON Patch move when from is a proper prefix of path

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RFC 6902 (section 4.4) forbids "from" from being a proper prefix of
"path" for a "move" operation: "a location cannot be moved into one
of its children." "move" is implemented as remove-then-add with no
check for this. For object targets, the subsequent "add" happened to
throw as a side effect of resolving through the now-removed parent,
but for array targets, removing the "from" element shifts subsequent
indices, so "path" silently re-resolves to a different element and
the operation "succeeds" with a silently corrupted document.

Add a check, before performing the remove/add, for whether "from" is
a proper prefix of "path" at the reference-token level. This compares
json_pointer's already-unescaped reference_tokens vectors (basic_json
is a friend of json_pointer) rather than the raw pointer strings, so
that tokens containing escaped '/' or '~' characters are compared
correctly, and a token that merely looks like a string prefix (e.g.
"/ab" vs "/abc/x") is not mistaken for a pointer-token prefix. When
"from" is a proper prefix of "path", throw out_of_range.414.

Fixes #5397.

Stacked on top of the fix for #5396 (branch
issue-5396-patch-remove-primitive-parent), since both touch the same
patch_inplace move/remove handling in include/nlohmann/json.hpp.

Signed-off-by: Niels Lohmann <mail@nlohmann.me>
This commit is contained in:
Niels Lohmann
2026-09-05 22:14:56 +02:00
parent eca15ca0e5
commit f826b1e9f9
6 changed files with 129 additions and 0 deletions
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single_include/nlohmann/json.hpp
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@@ -26450,6 +26450,19 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
const auto from_path = get_value("move", "from", true).template get<string_t>();
json_pointer from_ptr(from_path);
// RFC 6902 (section 4.4) forbids "from" from being a
// proper prefix of "path": a location cannot be moved
// into one of its own children. Compare the pointers'
// reference tokens (already unescaped by json_pointer's
// parser) rather than the raw pointer strings, since a
// token may itself contain an escaped '/' or '~' that
// would defeat a naive string-prefix comparison.
if (JSON_HEDLEY_UNLIKELY(from_ptr.reference_tokens.size() < ptr.reference_tokens.size()
&& std::equal(from_ptr.reference_tokens.begin(), from_ptr.reference_tokens.end(), ptr.reference_tokens.begin())))
{
JSON_THROW(out_of_range::create(414, detail::concat("cannot move value: 'from' path '", from_path, "' is a proper prefix of 'path' '", path, "'"), &result));
}
// the "from" location must exist - use at()
basic_json const v = result.at(from_ptr);