Suspected breaking change in libmaxminddb v1.13.x

@vimt

First of all thanks for your efforts on this package, it has proven valuable over the last few years. 👍

After we've recently updated libmaxminddb from v1.12.x to v1.13.x on our environments, we encountered a suspected breaking change: MMDB-files generated with this package are no longer readable, possibly due to recently introduced stricter validation in release v1.13.x.

Tested with libmaxminddb v1.13.2:

OSX 15.7.4
Fedora 43

writer = MMDBWriter()
writer.insert_network(IPSet(['1.1.0.0/24', '1.1.1.0/24']), {'country': 'COUNTRY', 'isp': 'ISP'})
writer.to_db_file('test.mmdb')

$ mmdblookup --file test.mmdb --ip 1.1.0.0
Can't open test.mmdb - The MaxMind DB file's data section contains bad data (unknown data type or corrupt data)

fix: compatibility with libmaxminddb v1.13.x stricter validation

Thanks for reporting this! I've identified the root cause and submitted a fix in #17.

Root Cause

libmaxminddb v1.13.0 added a new validation check in get_entry_data_list() (release notes):

if (offset >= mmdb->data_section_size ||
    size > (mmdb->data_section_size - offset) / 2) {
    return MMDB_INVALID_DATA_ERROR;
}

This check is triggered during MMDB_open → read_metadata → populate_description_metadata, which calls MMDB_get_entry_data_list to parse the metadata.

In our metadata encoding, description was the last field. When description is an empty dict {} (the default when no languages are specified), the metadata section ends with \xe0 (MAP with 0 entries). The offset_to_next after this byte exactly equals metadata_section_size, causing offset >= data_section_size to evaluate to true — hence the "bad data" error.

Fix

Reorder metadata fields so that build_epoch (a scalar UINT64, not subject to the MAP/ARRAY validation) is always the last entry. This ensures empty description or languages values are never at the tail of the metadata section.

The fix is a one-line change in _build_meta() — just swapping the order of description and build_epoch.

release: v0.2.6

Fix validation of empty maps/arrays at end of metadata

@vimt

Hello @vimt,
Brilliant, thanks for the quick follow up and RCA, much appreciated!
Confirmed: Everything operational again after upgrading this packages to v0.2.6. 👍

merge: sync fixes from master

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Suspected breaking change in libmaxminddb v1.13.x #16

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Suspected breaking change in libmaxminddb v1.13.x #16

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vimt commented on Mar 3, 2026

Root Cause

Fix

plivius commented on Mar 4, 2026

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