[oschwald]

MMDB_open() looked up each metadata key with a first-match scan, so a repeated key had only its first value read and validated. A reader that keeps the last value could see different metadata for the same file. maxmind/MaxMind-DB#314 adds a spec rule that metadata keys must be unique.

MMDB_open() now returns MMDB_INVALID_METADATA_ERROR when a key repeats in the top-level metadata map or in the description map:

• It checks binary_format_major_version first, so a newer format still gets MMDB_UNKNOWN_DATABASE_FORMAT_ERROR.
• It walks the top-level map once, and it checks the description keys from the decoded list.
• It compares keys up to the first NUL, because some callers cut keys there. For example, the PHP extension would otherwise keep the later of database_type and database_type<NUL>x.
• It sorts the keys. Each map can have at most 1,024 keys, and each key's size counts against the decoder payload budget, so the sort stays fast for crafted metadata with any qsort.

Other effects:

• Every top-level metadata entry is now decoded. A bad entry after the required keys, which was never read before, now makes MMDB_open() fail.
• A top-level metadata map that claims more entries than it has returns MMDB_INVALID_DATA_ERROR.
• A top-level value nested past the depth limit, keys whose total size exceeds the payload budget, or a map with more than 1,024 keys returns MMDB_INVALID_METADATA_ERROR. Values are skipped from depth 1, the same as in MMDB_get_metadata_as_entry_data_list().
• Metadata whose root is not a map returns MMDB_INVALID_METADATA_ERROR.
• Values are skipped without following pointers, so a bad pointer target in a value that MMDB_open() does not use is not detected.

skip_map_or_array skipped only the header of a map or array key, so its contents were read as the keys and values that follow. MMDB_get_value() could return the wrong value, and the metadata walk could miss a repeated key. It now returns MMDB_INVALID_DATA_ERROR for such a key. A lookup benchmark that skips nested maps on GeoIP2-City-Test.mmdb was 0.5% slower, with default and with fixed function alignment. #516 makes MMDB_get_entry_data_list() reject the same keys.

Tests:

• t/duplicate_metadata_key_t.c builds small databases in memory. It covers each error case above, a data-section lookup past an array key, and both sides of the depth and key-count limits. The depth and budget tests use the build's limits.
• t/repeated_metadata_key_t.c checks the four repeated-key databases from Add bad-data databases that repeat a metadata key MaxMind-DB#314. It needs no temporary file, so it also runs on Windows.
• t/maxmind-db points at the The 'addresses' pointer was utilized before it was verified against nullptr #314 branch for now. Before this PR merges, The 'addresses' pointer was utilized before it was verified against nullptr #314 must merge and the submodule must move to the merged commit.

Follow-up: STF-2363 reads the metadata in one walk instead of a uniqueness walk plus first-match lookups.

Related: maxmind/MaxMind-DB-Reader-python#468.

🤖 Generated with Claude Code

Summary by CodeRabbit

• Bug Fixes
  • MMDB_open() now rejects repeated keys in the top-level metadata map and its description map with MMDB_INVALID_METADATA_ERROR.
  • A metadata root that is not a map also returns MMDB_INVALID_METADATA_ERROR. Malformed top-level keys or values, and metadata maps with inconsistent entry counts, return MMDB_INVALID_DATA_ERROR.
  • Validation now checks metadata beyond required fields. Pointers in unused values are not followed, so invalid pointer targets there are not detected.
• Documentation
  • Clarified metadata validation and the errors returned for invalid metadata.

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Walkthrough

MMDB_open() now checks the top-level metadata map and the description map for repeated keys. It reports errors for malformed metadata keys or values and invalid map structures. Tests, documentation, and release notes cover these checks.

Changes: Metadata Key Validation

Layer / File(s)	Summary
Metadata map validation src/maxminddb.c	New helpers validate top-level metadata map structure, decode keys, skip values, and detect repeated keys. The code also checks description language names for duplicates.
Validation coverage and documentation t/duplicate_metadata_key_t.c, t/CMakeLists.txt, t/Makefile.am, doc/libmaxminddb.md, Changes.md	Fixture tests cover valid metadata, repeated keys, malformed entries, and invalid map structures. Build files register the test. Documentation and release notes describe validation errors.

Estimated code review effort: 3 (Moderate) | ~20 minutes

Change: Bug fix

Possibly related PRs

• maxmind/libmaxminddb#514: Changes data-section bounds and decoder checks used when metadata values are skipped.
• maxmind/libmaxminddb#515: Changes decode_one handling of extended type bytes, which can affect errors when metadata values are decoded.

Merge Risk: Minimal · previous review 7bd42

No actionable merge-blocking risk was established.

Pre-merge checks: 4 · 1

Failed checks: 1 warning

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Passed checks: 4 passed

Check name	Status	Explanation
Description Check		Check skipped - CodeRabbit’s high-level summary is enabled.
Title check		The title clearly and concisely describes the main change: rejecting repeated metadata keys in MMDB metadata.
Linked Issues check		Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check		Check skipped because no linked issues were found for this pull request.

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Explanation

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_{A rabbit checks each map with care,
And spots repeated keys hiding there.
It skips each value, then checks the rest,
Malformed entries fail the test.
The notes now tell what errors appear,
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Actionable comments posted: 2

Additional comments: 1

src/maxminddb.c-780-781: 1

780-781: Minor · Functional Correctness · Quick win

Unverified finding
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Validate pointer targets in metadata values.

If a distinct extra metadata key has an out-of-range pointer value, CHECKED_DECODE_ONE accepts the pointer encoding and skip_map_or_array does not follow it. MMDB_open() can therefore accept that invalid entry after the required keys. Validate the pointer target while preserving the encoded value’s offset for the next-key scan. (raw.githubusercontent.com)

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Review comments at @Changes.md:
- Around line 8-10: Narrow the MMDB_open() claim in the metadata documentation
to describe the entries it actually validates: next_map_key() does not follow
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Review comments at @t/duplicate_metadata_key_t.c:
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t/duplicate_metadata_key_t.c-56-56: 1

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Use the configured temporary directory.

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🟡 Changes recommended

Incorrect skipping of container-valued map keys can desynchronize metadata traversal, allowing a duplicate top-level database_type key to evade detection.

1 open issue

• Nested map keys can hide duplicate top-level database_type entries · New

_{Review effort: Balanced}

[oschwald]

From Claude (Claude Opus 5.5), on Greg's behalf: This replies to CodeRabbit's TMPDIR comment on t/duplicate_metadata_key_t.c in this review. The comment is valid. The test now creates its fixture in $TMPDIR, and uses /tmp if TMPDIR is unset or empty, in 7bd424a. With TMPDIR set to a missing directory, the test now stops with "could not create a temporary file in

". The full ctest suite passes with TMPDIR set and unset.

[Copilot]

🔵 Needs a closer look

The validation pass can accept a truncated nested map because skip_map_or_array() skips only a compound key’s header and mistakes its contents for the missing value.

1 open issue

• Nested map keys can hide duplicate top-level database_type entries

_{Review effort: Balanced}

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only still-valid issues, skip the rest with a brief reason, keep changes
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Failed pre-merge checks:
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🟡 Changes recommended

Nested-map traversal can desynchronize validation and allow duplicate top-level keys, while overlapping pointer keys can cause quadratic scanning and excessive CPU use in MMDB_open().

2 open issues

• Overlapping pointer keys can make metadata validation quadratic · New
• Nested map keys can hide duplicate top-level database_type entries

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[oschwald]

From Claude (Claude Opus 5.5), on Greg's behalf: I skipped CodeRabbit's failed "Docstring Coverage" pre-merge check. Most static helpers in src/maxminddb.c have no doc comments, and the new helpers follow that pattern. Each one is short and has a descriptive name.

[horgh]

LGTM, with Claude feedback

[horgh]

Out-of-bounds read when the metadata section is shorter than 3 bytes.

This bug was already in the code before this PR. But check_metadata_keys_unique is now the first code that decodes the metadata, so it is now the first code to hit the bug. MMDB_open checks for a data section shorter than 3 bytes, but it does not do the same check for the metadata section.

Example: a 4096-byte file that ends in the metadata marker plus the two bytes \x9f\x03 gives a 2-byte metadata section. In decode_one, case 31 calculates data_section_size - 3. That value wraps to UINT32_MAX, so the bounds check passes, and get_uint24 reads 2 bytes past the end of the mapping. I reproduced this with ASan through mmdblookup --file. Without ASan, a page-aligned file can cause SIGBUS or SIGSEGV.

Suggested fix: reject metadata_section_size < 3 in MMDB_open.

🤖 Comment by Claude Opus 5.5.

[oschwald]

From Claude (Claude Opus 5.5), on Greg's behalf: Valid, and the bug is already in main: before this PR, the first required-key lookup decoded the same bytes. #514 fixes it. Its first commit guards the size-code 30 and 31 subtractions in decode_one, and its tests cover a 2-byte metadata section, which now returns MMDB_INVALID_DATA_ERROR. So I did not change this PR.

[horgh]

This check also rejects keys that are an empty map (0xE0) or an empty array (0x00 0x04). When the code skips the header of an empty map or empty array, it skips the whole key, so these keys never caused the misaligned read. Only keys that have contents caused it.

Before this change, MMDB_get_value on a path past such a nested map worked. Now it returns MMDB_INVALID_DATA_ERROR, so lookups on some lenient third-party databases start to fail for no reason. Should the check apply only to a map or array key that has contents?

🤖 Comment by Claude Opus 5.5.

[horgh]

Weird feedback. Seems like the change is an improvement.

[oschwald]

From Claude (Claude Opus 5.5), on Greg's behalf: I agree with you: the check stays. The spec says map keys are always UTF-8 strings, so a map or array key is invalid even when it is empty, and rejecting it in one place is simpler than a special case for empty containers.

[horgh]

MMDB_get_value and MMDB_get_entry_data_list now give different results for the same data.

MMDB_get_value and MMDB_aget_value reject a map or array key through skip_map_or_array. But MMDB_get_entry_data_list still accepts it, because get_entry_data_list decodes the whole key (around line 2051).

Example: a record has a nested map with an array key. MMDB_get_entry_data_list(&entry) succeeds and returns the full structure. MMDB_get_value(&entry, ..., "sibling", NULL) on the same entry returns MMDB_INVALID_DATA_ERROR. Changes.md describes only the second behavior.

🤖 Comment by Claude Opus 5.5.

[oschwald]

From Claude (Claude Opus 5.5), on Greg's behalf: Valid. #516 makes get_entry_data_list reject a non-string map key with MMDB_INVALID_DATA_ERROR, so the two functions agree once both PRs merge. I did not duplicate that change here.

[horgh]

The strict walk of all top-level keys now runs before the binary_format_major_version check. Also, the code now decodes the values of unknown keys that come after the required keys. Before this change, the code did not read them.

Example: a future major-version-3 database adds new metadata keys that the v2 decoder cannot skip. MMDB_open now returns MMDB_INVALID_DATA_ERROR or MMDB_INVALID_METADATA_ERROR, not MMDB_UNKNOWN_DATABASE_FORMAT_ERROR. A caller that uses the format error to tell the user to upgrade libmaxminddb gets a corruption error instead. Should the version check run first?

🤖 Comment by Claude Opus 5.5.

[oschwald]

From Claude (Claude Opus 5.5), on Greg's behalf: Valid. read_metadata now looks up binary_format_major_version before the walk and returns MMDB_UNKNOWN_DATABASE_FORMAT_ERROR for any version other than 0 or 2. A major-version-3 test file with an undecodable extra value now gets the format error, as on main. Fixed in edcbcb3.

[horgh]

The depth count here is one level different from MMDB_get_metadata_as_entry_data_list.

next_map_key skips each top-level metadata value starting at depth 0. MMDB_get_metadata_as_entry_data_list decodes the same values starting at depth 1.

Example: the metadata has an extra key whose value is a chain of exactly 512 nested nodes. MMDB_open accepts it. Then MMDB_get_metadata_as_entry_data_list (used by mmdblookup --verbose and by dumps) fails with MMDB_DECODER_LIMIT_ERROR. The new docs say that MMDB_open rejects values that are nested past the depth limit, so the two must agree.

🤖 Comment by Claude Opus 5.5.

[oschwald]

From Claude (Claude Opus 5.5), on Greg's behalf: Valid. Before the fix, a value with 511 nested arrays opened, and then MMDB_get_metadata_as_entry_data_list failed with MMDB_DECODER_LIMIT_ERROR. The walk now skips top-level values from depth 1, and the test checks both sides of the boundary, including that MMDB_get_metadata_as_entry_data_list reads the accepted file. Fixed in ca1e355.

[horgh]

The manual does not say that MMDB_get_value and MMDB_aget_value can now return MMDB_INVALID_DATA_ERROR when a nested map that they skip has a map or array key. Only Changes.md says this.

The MMDB_get_value section and the error code list (around line 383, outside this diff) are the places to add it. Users who read those sections do not learn that lookups which returned data before (possibly the wrong value) now fail.

🤖 Comment by Claude Opus 5.5.

[oschwald]

From Claude (Claude Opus 5.5), on Greg's behalf: Valid. The Data Lookup Functions section and the MMDB_INVALID_DATA_ERROR entry in the status code list now describe the map or array key error. Fixed in 522dd44.

[horgh]

Design: two separate parsers read the same map.

This function checks for unique keys in a separate walk. The 9 required fields are still read by first-match scans (value_for_key_as_*). The two parsers must stay in step. Commit de11e2a had to fix this kind of problem: a nested array key made the checker read something different from what the lookups read.

A single walk that dispatches on the key name, fills the metadata fields, and fails on a second occurrence of a key would remove that class of problem. It would also replace about 10 scans of the metadata with one.

🤖 Comment by Claude Opus 5.5.

[oschwald]

From Claude (Claude Opus 5.5), on Greg's behalf: Agreed that one walk is the better design, but it rewrites read_metadata, so I filed it as a follow-up: https://linear.app/maxmind/issue/STF-2363/read-libmaxminddb-metadata-in-one-walk. The two parsers now use the same skip_map_or_array, so the de11e2a class of problem is fixed in the shared code.

[oschwald]

I did do performance testing on an open and the increase from this seemed minimal. I think it was a few percent.

[horgh]

Reuse: next_map_key repeats the sequence in lookup_path_in_map (around lines 1542-1566): decode the key, check that it is a string, decode the value, call skip_map_or_array, and move the offset. The map-size bound check at lines 735-737 is also a copy of the check in get_entry_data_list (around lines 2038-2039).

The same logic now exists in two places. A later fix to one key-skipping path (like the fix in de11e2a) can miss the other. lookup_path_in_map could call next_map_key, and the bound check could move into one helper function.

🤖 Comment by Claude Opus 5.5.

[oschwald]

From Claude (Claude Opus 5.5), on Greg's behalf: Won't fix. lookup_path_in_map is on the lookup hot path, and its loop must stop at the matching key and decode that value, while next_map_key always skips the value. Sharing the code would change the hot loop. For de11e2a, small changes on the same lookup path in skip_map_or_array cost 0.5% to 1.85% on a lookup benchmark, so I would not change it without a strong reason. The two map-size bounds return different errors at different points, so one helper would not save much.

[horgh]

Simplification: check_languages_unique makes a second allocation, and it calls strlen on the strndup copies. The top-level check stops at a NUL with memchr on the raw key. So the code has two different ways to apply the same "compare up to the first NUL" rule, plus an extra calloc/free on each open.

The metadata_key_s entries could be filled in the populate_description_metadata loop from member->entry_data with the same memchr rule, and that loop could call check_keys_unique directly.

🤖 Comment by Claude Opus 5.5.

[oschwald]

From Claude (Claude Opus 5.5), on Greg's behalf: Valid. The description loop now fills the key array from the decoded list with a shared metadata_key helper, which applies the same memchr rule as the top-level walk, and calls check_keys_unique directly. check_languages_unique and its strlen calls are gone. Fixed in d974272.

[horgh]

Nit: check_keys_unique and its callee compare_metadata_keys come before their new caller check_languages_unique (line 1068). If both functions move after check_languages_unique, each caller comes before the function it calls.

🤖 Comment by Claude Opus 5.5.

[oschwald]

From Claude (Claude Opus 5.5), on Greg's behalf: Fixed in 46ba934. metadata_key, check_keys_unique, and compare_metadata_keys now come after populate_description_metadata. The commit only moves code.

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