Bound decoder resource use to prevent denial of service (STF-1488)

[oschwald]

Fixes the data-section denial-of-service issues reported in GHSA-hj94-g986-h9r7 in the pure Python decoder, and tests the C extension against the libmaxminddb fix.

A crafted database can nest pointers to shared targets so that decoding one record costs exponential time and memory (pointer fan-out). It can also point many times at one large string or bytes value so that a record with few values materializes far more data than the file holds (payload amplification). A recursion depth limit alone stops neither, because the blow-up comes from width, not depth.

Change

The decoder follows the Reader Resource Limits section of the MaxMind DB specification. Every caller-requested decode, which is one record lookup or the metadata read when a database is opened, gets a fresh budget:

• 65,536 decoded values, the count the specification recommends, using its flat accounting rule. The root is one value. Each array or map reserves its declared children before it reads any of them, so an oversized header is rejected before the first child. A pointer costs nothing beyond the value it resolves to.
• 512 levels of nesting, the depth the specification recommends. Entering an array or map, or following a pointer, is one level. This also stops pointer cycles. Under CPython's default recursion limit the interpreter can reject a record before 512 levels; that failure is converted to the same error.
• 2 MiB of string and bytes payload. The specification leaves the payload strategy and limit to the reader; 2 MiB matches libmaxminddb. Each string or bytes value is charged its encoded length where it is decoded, before the bytes are read, so a shared target reached through another pointer is charged again, including a value stored inline in a pointed-to container. An oversized variable-length integer is rejected by a size check.

Exceeding a limit raises InvalidDatabaseError. The budget is a call-local list threaded through the recursion, so a shared Decoder stays safe for repeated and concurrent lookups.

The limits are fixed. Python never preallocates from a declared size, never skips values, and has no path-selection API, so the specification's capacity-hint, skip, and decode-path concerns do not apply.

Also in this PR

• A truncated data section that previously escaped from get() or open_database() as IndexError or struct.error now raises InvalidDatabaseError. Invalid UTF-8 keeps raising UnicodeDecodeError.
• The extension/libmaxminddb submodule moves to the main-branch commit that adds the same limits (Bound decoded values to prevent a pointer fan-out DoS (STF-1568) libmaxminddb#479). It will be re-pinned to the 1.14.0 tag once that is released.
• Five decoder and reader optimizations, one per commit, that more than offset the cost of the limits: skip the size call when the ctrl byte holds the size, decode pointers and search tree nodes with integer arithmetic instead of struct on concatenated bytes, drop a runtime typing.cast and per-iteration method lookups in the container loops, and decode strings inline in _decode.

Performance

GeoLite2-City.mmdb (production database), 50,000 random IPv4 addresses that all have a record, medians of three fresh interpreters each, CPython 3.14, identical result checksums. The extension row compares the vendored libmaxminddb 1.13.3 on main with the pinned main-branch commit.

Mode	main	this PR
MODE_MEMORY (pure Python)	47.2 us	39.7 us
MODE_MMAP (pure Python)	48.2 us	41.0 us
MODE_MMAP_EXT (C extension)	3.76 us	3.83 us

The limits alone cost about 8% on this workload. Each optimization commit records its own before-and-after measurement.

Tests

Shared fixtures from maxmind/MaxMind-DB (submodule at 363086b), run in every pure Python mode (MODE_MEMORY, MODE_FILE, MODE_MMAP, MODE_FD) and through the C extension (a system libmaxminddb without the fix skips those): IPv4 and IPv6 pointer fan-out, bytes and string payload amplification, the worst case at exactly the value limit, the value-count and payload boundaries one unit either side of the limits, and amplified metadata rejected at open. Hostile lookups run under an address-space and wall-clock cap so a regression fails instead of hanging or exhausting memory.

Decoder unit tests pin the invariants: a header-only oversized array or map and an over-limit string, bytes, or integer are rejected through a buffer that fails any read past the header; 512 nesting levels succeed and 513 fail, with and without intervening pointers, independently of sys.setrecursionlimit; a pointer cycle is rejected; a wrapped inline payload and a pointer-backed map key are charged; the at-limit value count decodes repeatedly and from eight threads at once; truncated data raises InvalidDatabaseError.

Minor version bump (3.2.0).

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📝 Walkthrough

Walkthrough

The pure Python decoder now enforces per-lookup limits for values, structural depth, string/bytes payload, and variable-length integers. It rejects malformed or oversized data with InvalidDatabaseError. Tests and the 3.2.0 changelog document the protections.

Changes

Decoder safety limits

Layer / File(s)	Summary
Shared decode budget and recursion handling maxminddb/decoder.py	Decoder callbacks share per-lookup value and depth budgets. Arrays, maps, and pointers consume the budgets. Excessive limits and Python recursion failures raise InvalidDatabaseError.
Payload and integer allocation limits maxminddb/decoder.py	String and bytes values consume a shared 2 MiB payload budget before copying. Oversized unsigned integers and int32 values are rejected before copying.
Regression coverage and release record tests/decoder_test.py, tests/data, HISTORY.rst	Tests cover pointer fan-out, cycles, nesting depth, oversized values, payload boundaries, metadata decoding, and normal records. The fixture reference and 3.2.0 changelog are updated.

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

Merge Risk: ⚪ Minimal · up to cc1fb

The decoder now bounds work for attacker-controlled database structures and rejects excessive decoding instead of allowing pointer fan-out to cause unbounded resource use. No actionable merge-blocking risk remains beyond normal checks and review.

Poem

A rabbit counts each value in flight
Payload limits keep the bytes tight
Cyclic pointers meet a bound
Deep containers stop before they round
Safe records hop through guarded ground

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 5.41% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 37 functions across 2 files. (2 skipped: 2…	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly and concisely describes the main change: limiting decoder resource use to prevent denial-of-service attacks. The issue identifier adds useful context without obscuring the change.

Full details: Docstring Coverage

Explanation

Docstring coverage is 5.41% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 37 functions across 2 files. (2 skipped: 2 unsupported.)

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Inline comments:
In `@maxminddb/decoder.py`:
- Around line 135-141: Update the map decoding logic around _decode so each
entry consumes budget for both its key and value, rather than subtracting only
the entry count. Enforce the 65,536-value limit before decoding children and add
a regression covering a map with more than 32,768 entries.

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[Copilot]

Pull request overview

This PR hardens the pure-Python MaxMind DB data-section decoder against pointer fan-out denial-of-service inputs by bounding per-lookup decode work and normalizing cyclic/over-deep pointer failures into InvalidDatabaseError.

Changes:

• Add a per-lookup decode budget to the pure-Python decoder to cap work and reject pathological pointer fan-out structures.
• Convert RecursionError during decoding into InvalidDatabaseError to make pointer cycles/over-deep structures catchable.
• Add regression tests for pointer fan-out and cyclic pointers; document the fix in HISTORY.rst.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 2 comments.

File	Description
tests/decoder_test.py	Adds regression tests covering pointer fan-out bounding and cyclic pointer handling.
maxminddb/decoder.py	Introduces per-lookup decode budget plumbing and RecursionError-to-InvalidDatabaseError conversion.
HISTORY.rst	Adds a 3.2.0 changelog entry describing the DoS fix.

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[Copilot]

Pull request overview

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

Suppressed comments (1)

HISTORY.rst:7

• HISTORY.rst entries below include a release date in the heading (e.g., 3.1.1 (2026-03-05)), but 3.2.0 does not. For consistency (and to avoid ambiguity in packaged artifacts), the 3.2.0 heading should include a date in the same format once known (or follow whatever convention the project uses for unreleased entries).

3.2.0
+++++

[Copilot]

Pull request overview

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

Suppressed comments (1)

HISTORY.rst:7

• This changelog entry introduces 3.2.0 without a date, while the existing entries in this file use the X.Y.Z (YYYY-MM-DD) format. Consider either adding the release date (when known) or explicitly marking it as unreleased to keep formatting consistent.

3.2.0
+++++

[Copilot]

Pull request overview

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

Suppressed comments (1)

tests/decoder_test.py:283

• As above, setting the process recursion limit to 10,000 is higher than needed for this assertion and can be unsafe on some runtimes. A smaller value still above the decoder’s internal depth limit (512) is sufficient to demonstrate that the decoder’s call-local limit is what triggers the error.

        old_recursion_limit = sys.getrecursionlimit()
        try:
            sys.setrecursionlimit(10_000)
            Decoder(at_limit, pointer_base=0).decode(0)
            with self.assertRaisesRegex(

[Copilot]

Pull request overview

Copilot reviewed 3 out of 3 changed files in this pull request and generated no new comments.

Suppressed comments (2)

Previously missed (2) — in code that hasn't changed since the last review.

maxminddb/decoder.py:242

• The budget[1] counter is described as tracking "structural depth", but it is also incremented when following pointers (_decode_pointer). This makes the comment slightly misleading and harder to reason about when diagnosing depth-limit failures involving pointer chains/cycles.

        # memory. ``budget`` carries the remaining value count and current
        # structural depth so both are shared across the recursion. It is
        # call-local, which keeps the decoder safe for concurrent reads. The
        # explicit depth limit is independent of Python's process-wide recursion

HISTORY.rst:11

• Grammar: the sentence uses "could" earlier but then switches to "cost". Consider changing to "could cost" for consistent modality.

  cost exponential time and memory from a small file. The decoder now limits the

[Copilot]

Pull request overview

Copilot reviewed 3 out of 3 changed files in this pull request and generated no new comments.

[Copilot]

Copilot was unable to review this pull request because the user who requested the review has reached their quota limit.

[Copilot]

Copilot was unable to review this pull request because the user who requested the review has reached their quota limit.

[Copilot]

🔵 Needs a closer look

It changes core decoder behavior in a security-sensitive code path (resource-limiting, recursion/depth semantics), so it warrants final human review despite strong test coverage.

Review details

• Files reviewed: 4/4 changed files
• Comments generated: 0 new
• Review effort level: Lite

[Copilot]

🟡 Changes recommended

The decoder can still silently accept truncated bytes/utf-8/integer payloads due to non-raising slice semantics, which undermines the intended “bad data” handling and should be fixed before merging.

Once you've addressed the issues Copilot identified, you can request another Copilot review.

Review details

Suppressed comments (3)

maxminddb/decoder.py:231

• _decode_uint uses int.from_bytes on a slice that may be shorter than size when the data section is truncated; this can return an incorrect value without raising and bypass the top-level truncated-data handling. Add an explicit length check before converting.

        # Reject a declared size past the widest defined unsigned integer before
        # copying, so a crafted size cannot force a large allocation.
        if size > _MAX_UINT_BYTES:
            raise InvalidDatabaseError(_BAD_DATA)
        new_offset = offset + size
        uint_bytes = self._buffer[offset:new_offset]
        return int.from_bytes(uint_bytes, "big"), new_offset

maxminddb/decoder.py:160

• _decode_int32 pads packed_bytes with zeros when size != 4. If the buffer is truncated (fewer than size bytes available), slicing can return a shorter byte string and the padding will hide the truncation, producing a value instead of raising InvalidDatabaseError. Check the slice length against size before padding/unpacking.

        if size > _MAX_INT32_BYTES:
            raise InvalidDatabaseError(_BAD_DATA)
        if size == 0:
            return 0, offset
        new_offset = offset + size
        packed_bytes = self._buffer[offset:new_offset]

        if size != 4:
            packed_bytes = packed_bytes.rjust(4, b"\x00")
        (value,) = struct.unpack(b"!i", packed_bytes)

maxminddb/decoder.py:245

• _decode_utf8_string can silently accept truncated payloads because slicing past EOF may return fewer than size bytes without raising (especially for bytes/FileBuffer). This should be treated as corrupt data and raised as InvalidDatabaseError rather than returning a shortened string.

        budget[2] -= size
        if budget[2] < 0:
            raise InvalidDatabaseError(_TOO_LARGE)
        new_offset = offset + size
        return self._buffer[offset:new_offset].decode("utf-8"), new_offset

• Files reviewed: 6/6 changed files
• Comments generated: 2
• Review effort level: Lite

[Copilot]

🟡 Changes recommended

The new int.from_bytes/slicing code paths can silently accept truncated data (search tree nodes and several data-section value payloads) because short reads don’t raise, undermining the intended “truncation => InvalidDatabaseError” behavior.

Once you've addressed the issues Copilot identified, you can request another Copilot review.

Review details

Suppressed comments (4)

maxminddb/decoder.py:300

• String decoding slices the payload without verifying that size bytes were actually available. For bytes, mmap, and FileBuffer, slicing past EOF returns fewer bytes without raising, so a truncated string can be silently accepted and the returned offset can advance past the buffer end. Add an explicit length check and raise InvalidDatabaseError(_BAD_DATA) when the payload is short.

            end = new_offset + size
            return self._buffer[new_offset:end].decode("utf-8"), end

maxminddb/decoder.py:125

• _decode_bytes slices the declared payload size without verifying the slice length. Because slicing past EOF returns a shorter bytes without raising, truncated data can be accepted and the offset can move beyond the buffer end. Check len(payload) == size and raise InvalidDatabaseError(_BAD_DATA) on short reads.

        new_offset = offset + size
        return self._buffer[offset:new_offset], new_offset

maxminddb/decoder.py:237

• _decode_uint uses int.from_bytes on a slice without validating that the declared number of bytes was present. For truncated data, slicing past EOF returns fewer bytes and int.from_bytes will still succeed, potentially accepting malformed databases. Add a short-read check and raise InvalidDatabaseError(_BAD_DATA) when len(uint_bytes) != size.

        new_offset = offset + size
        uint_bytes = self._buffer[offset:new_offset]
        return int.from_bytes(uint_bytes, "big"), new_offset

maxminddb/decoder.py:162

• _decode_int32 can silently accept truncated payloads when size < 4: slicing past EOF returns fewer bytes, then rjust(4, ...) pads and struct.unpack succeeds. This defeats the intent of treating truncation as invalid database data. Validate len(packed_bytes) == size before padding/unpacking and raise InvalidDatabaseError(_BAD_DATA) on short reads.

This issue also appears in the following locations of the same file:

• line 235
• line 299

        new_offset = offset + size
        packed_bytes = self._buffer[offset:new_offset]

• Files reviewed: 7/7 changed files
• Comments generated: 1
• Review effort level: Lite

[Copilot]

🟡 Changes recommended

The new int.from_bytes-based decoding paths can silently accept truncated/corrupt payloads and search-tree nodes unless explicit slice-length validation is added.

Once you've addressed the issues Copilot identified, you can request another Copilot review.

Review details

Suppressed comments (5)

maxminddb/decoder.py:300

• String decoding slices self._buffer[new_offset:end] without verifying that the buffer actually contains size bytes. For bytes-like buffers and FileBuffer, slicing past EOF returns fewer bytes (no IndexError), so truncated string payloads can be silently accepted and decoded, contradicting the intent to treat truncation as InvalidDatabaseError.

            end = new_offset + size
            return self._buffer[new_offset:end].decode("utf-8"), end

maxminddb/decoder.py:125

• _decode_bytes returns self._buffer[offset:new_offset] without checking that size bytes were actually read. For bytes, mmap, and FileBuffer, an out-of-range slice can return fewer bytes, so truncated bytes payloads may be accepted instead of raising InvalidDatabaseError.

        new_offset = offset + size
        return self._buffer[offset:new_offset], new_offset

maxminddb/decoder.py:165

• _decode_int32 (and similarly _decode_uint) uses int.from_bytes / rjust on a slice that can be shorter than the declared size without raising, which can silently treat truncated integer payloads as valid (padding missing bytes with zeros). If truncation should be reported as InvalidDatabaseError, the slice length needs to be validated.

This issue also appears on line 235 of the same file.

        new_offset = offset + size
        packed_bytes = self._buffer[offset:new_offset]

        if size != 4:
            packed_bytes = packed_bytes.rjust(4, b"\x00")

maxminddb/decoder.py:237

• _decode_uint uses int.from_bytes on a slice that may be shorter than size without raising (e.g., truncated buffer), which can silently accept corrupt data. If truncation should raise InvalidDatabaseError, the slice length should be validated before converting.

        new_offset = offset + size
        uint_bytes = self._buffer[offset:new_offset]
        return int.from_bytes(uint_bytes, "big"), new_offset

maxminddb/reader.py:231

• Reader._read_node now uses int.from_bytes on slices of the search tree. Unlike struct.unpack, int.from_bytes will happily accept short (or empty) slices, which can silently treat a truncated/corrupt search tree as valid and potentially return incorrect results rather than an InvalidDatabaseError. It should validate that the slice length matches the expected node record width before decoding.

                record = int.from_bytes(self._buffer[offset : offset + 4], "big")
                return record & 0x0FFFFFFF
            record = int.from_bytes(self._buffer[base_offset : base_offset + 4], "big")
            return (record >> 8) | ((record & 0xF0) << 20)

• Files reviewed: 7/7 changed files
• Comments generated: 1
• Review effort level: Lite

[Copilot]

🔵 Needs a closer look

It changes core security-sensitive decoding and reader behavior (including new limits and error mapping), so a final human review is recommended despite only minor issues found.

Review details

• Files reviewed: 8/8 changed files
• Comments generated: 1
• Review effort level: Lite