Bound decoder resource use to prevent denial of service (STF-1488) - #439
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Included review availability: Your plan provides up to 2 included reviews per hour; 1 remains after this review. 📝 WalkthroughWalkthroughThe 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 ChangesDecoder safety limits
Estimated code review effort: 3 (Moderate) | ~20 minutes Merge Risk: ⚪ Minimal · up to 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. 🚥 Pre-merge checks | ✅ 4 | ❌ 1❌ Failed checks (1 warning)
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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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HISTORY.rstmaxminddb/decoder.pytests/decoder_test.py
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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
RecursionErrorduring decoding intoInvalidDatabaseErrorto 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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HISTORY.rst:7
HISTORY.rstentries below include a release date in the heading (e.g.,3.1.1 (2026-03-05)), but3.2.0does not. For consistency (and to avoid ambiguity in packaged artifacts), the3.2.0heading should include a date in the same format once known (or follow whatever convention the project uses for unreleased entries).
3.2.0
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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
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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(
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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
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A ctrl byte, size, or pointer read that ran off the end of the buffer escaped from get() and open_database() as IndexError or struct.error. Convert both to InvalidDatabaseError at the decode root, where the RecursionError fallback already lives, so callers see one error type for corrupt data. Invalid UTF-8 keeps raising UnicodeDecodeError. Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
Most values store their size in the low five bits of the ctrl byte, and a pointer's size bits are not a size at all. The decoder still called _size_from_ctrl_byte for every value to find that out, so each value paid for a method call that returned its arguments unchanged. Read the size bits inline and call the helper only for size codes 29 to 31, which are followed by size bytes. On GeoLite2-City-Test.mmdb in MODE_MEMORY this offsets the cost of the decoder resource limits: about 47 us per lookup with the limits alone versus 43 us on main, and about 44 us with this change. Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
A pointer is about a third of the values in a City record. Decoding one built a new bytes object by concatenation and then unpacked it with struct, which made _decode_pointer the second most expensive function in a lookup profile. Read the pointer bytes once with int.from_bytes and add the ctrl-byte bits and the fixed size offsets arithmetically. A slice that is shorter than the declared pointer size is truncated data and is rejected, which struct.unpack used to do implicitly. GeoLite2-City.mmdb lookups in MODE_MEMORY: 47.13 us to 43.97 us. Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
typing.cast is a real function call at runtime, and _decode_map made one per entry, about a million calls in a 20,000-lookup profile, to satisfy the type checker. Index the dict directly and tell mypy to ignore the Record-typed key instead. Both container loops also looked up the bound _decode method on every iteration; bind it once per container. GeoLite2-City.mmdb lookups in MODE_MEMORY: 43.98 us to 43.47 us. Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
Move every type, including strings, into match cases. Put common types first and document the order. Remove the dispatch table and its callback type alias. Strings still decode through _decode_utf8_string.
Keep string decoding inside the first case to avoid a method call for the most common value type. Preserve payload accounting and remove the unused _decode_utf8_string method. Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
_read_node built a bytes or bytearray object for every node it read and unpacked it with struct, and it fetched node_byte_size through a property call each time. A lookup reads about 18 nodes, so this was the largest cost outside the decoder. Compute each record with int.from_bytes and bit arithmetic on one slice, and cache the record size on the reader when the database is opened. struct.unpack raised on a short read; int.from_bytes does not, so the reader now rejects a database whose search tree extends past the end of the file when it is opened. That keeps every node read inside the buffer without a length check per read. GeoLite2-City.mmdb lookups in MODE_MEMORY: 41.8 us to 39.8 us. Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
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🟢 Approval recommended
The implementation and tests consistently cover the stated resource limits, malformed-input handling, reader bounds checks, and concurrent decoder reuse.
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LGTM. I left some Claude nits.
| (value, _) = self.decode(pointer) | ||
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| # The value at the pointer's position was charged by its containing | ||
| # array or map, so the target costs nothing more. Only the depth changes. |
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Pointer chains bypass the value budget entirely — a 322 KB file costs ~8.8 s per lookup and passes every limit.
The premise in this comment holds only when a pointer resolves to a non-pointer. When the target is another pointer, each hop is a full _decode call charged nothing against budget[0]. Depth is restored on the way out, so every element of a wide container can re-walk the same chain.
Reproduced against this branch — an array of 65,535 pointers into a shared chain terminating in a boolean:
| chain length | data section | decode time | result |
|---|---|---|---|
| 1 | 321 KB | 0.08 s | ok |
| 300 | 321 KB | 6.46 s | ok |
| 400 | 322 KB | 8.84 s | ok — all three limits passed |
Cost is linear in chain length at essentially constant file size.
This is also a divergence from the C extension, not just a DoS: the vendored libmaxminddb rejects the construct outright at extension/libmaxminddb/src/maxminddb.c:1466 and :1820 — "pointer points to another pointer" / "Pointers to pointers are illegal under the spec". So the same file is refused by the extension and accepted here.
Cheapest fix, and the one that matches libmaxminddb: in _decode_pointer, reject a target whose ctrl byte is itself a pointer. Charging budget[0] -= 1 per follow also caps it, but would require resizing the at-limit fixtures in tests/decoder_test.py.
No test covers a pointer whose target is a pointer — _pointer_chain in tests/decoder_test.py chains arrays holding pointers, which are charged normally.
🤖 Comment by Claude (Claude Code) on behalf of Will.
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Codex, responding on Greg’s behalf.
Fixed in 2d1bea5. A pointer whose target is another pointer now raises InvalidDatabaseError. Tests cover shared chains, direct cycles, and valid pointers to containers. The check uses the existing control-byte read and preserves the flat value budget. Passing the internal flag positionally reduced the measured lookup overhead to about 0.4–1.3% relative to the reviewed HEAD on CPython 3.14.
| DecoderFunc = Callable[["Decoder", int, int], tuple[Record, int]] | ||
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| # Per-lookup limit on the number of values decoded, recommended by the MaxMind | ||
| # DB specification. It stops a pointer fan-out, where nested pointers to shared |
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This comment is the primary documentation of the security model, and it currently promises more than the accounting delivers: it stops the container fan-out, but not the pointer-hop multiplier (see my comment on _decode_pointer). Worst case is _MAX_VALUES * _MAX_DEPTH decode operations, not _MAX_VALUES.
Either correct the claim or — better — make it true via the pointer-to-pointer rejection, after which it reads accurately as written.
Minor, while you're here: "each array and map charges its declared children" is loose for maps, whose declared size is the pair count. The doubling is explained at line 171, 150 lines away. Something like "each array charges its declared size, and each map twice its declared size, since a key and a value are separate values" is self-contained.
🤖 Comment by Claude (Claude Code) on behalf of Will.
| 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) |
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This line and the & 0x0FFFFFFF below have no effective test coverage.
I confirmed the rewrite is bit-exact — I ran the old struct.unpack/bytearray version against this one over all edge byte patterns and 20k random buffers per record size, 0 mismatches. So this is a test gap, not a live bug. But three mutations survive the entire suite:
((record & 0xF0) << 20)→<< 24: 375 passed((record & 0xF0) << 20)→((record & 0x0F) << 24): 375 passedreturn record & 0x0FFFFFFF→return record: 375 passed
The reason: across all 25 28-bit fixtures the largest record value is 34,645 (0x8755). Byte 3's nibbles are always zero, so the left record's << 20 term is always 0 and the right record's mask never removes anything.
A synthetic 7-byte node fixes this without needing a database — e.g. AA BB CC DE FF 00 11 asserting left == 0xDAABBCC and right == 0xEFF0011, table-driven over the three record sizes.
🤖 Comment by Claude (Claude Code) on behalf of Will.
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Codex, responding on Greg’s behalf.
Added synthetic 28-bit nodes in 20853e9. Both child records have nonzero high nibbles, and a second node also checks the base-offset calculation. These assertions exercise both the left shift and the right mask.
| reader.get(self.ipf("2001:220::")) | ||
| reader.close() | ||
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| def test_search_tree_past_end_of_file(self) -> None: |
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This test does not pin the new check. I replaced if tree_end > self._buffer_size: in reader.py with if False: and the whole suite stayed green.
The fixture opens fine without the check (node_count 100000, tree_size 700000, file 22876 bytes) and the subsequent get() raises InvalidDatabaseError: Invalid node in search tree — a different error that the bare assertRaises happily accepts.
Three things compound: no message regex, open and lookup bundled under one assertRaises so it can't tell which raised, and the self.close() on the reject path is never verified.
For the pure-Python reader, assert assertRaisesRegex(InvalidDatabaseError, "search tree extends past the end of the file") on open_database alone. Keep the current loose form only for the extension, which fails at open with a different message.
🤖 Comment by Claude (Claude Code) on behalf of Will.
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Codex, responding on Greg’s behalf.
Strengthened this test in 86d82c6. For the Python reader, it now requires the search-tree error during open, with no lookup inside the assertion. The extension keeps its existing open-or-lookup contract. Initialization failures now close the buffer through the common cleanup path added in da62052, with file and mmap cleanup tests.
| if size > _MAX_UINT_BYTES: | ||
| raise InvalidDatabaseError(_BAD_DATA) | ||
| new_offset = offset + size | ||
| uint_bytes = self._buffer[offset:new_offset] |
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Truncated payloads still return silently wrong values. Python slicing past the end never raises, so this and three sibling sites decode short data to a plausible wrong value with no error. Measured on this branch:
| input | returns | should be |
|---|---|---|
| uint32 declaring 4 bytes, 2 present | 65535 |
error |
| uint128 declaring 16 bytes, 2 present | 65535 |
error |
| string declaring 5 bytes, 2 present | "ab" |
error |
| bytes declaring 5 bytes, 2 present | b"ab" |
error |
| int32 declaring 3 bytes, 1 present | 255 |
error |
Sites: here, line 121 (_decode_bytes), line 286 (inline string), and _decode_int32 — where rjust(4, b"\x00") pads a short slice into a valid struct.unpack input, so only size < 4 is silent.
Most of this predates the PR (_decode_uint already used int.from_bytes on main). Flagging it because the PR now advertises the opposite in HISTORY.rst and in the comment at line 255, and because a plausible wrong integer is the worst failure mode for a parser being hardened against hostile input.
_decode_pointer at line 198 already has exactly the right pattern. Applying it here is three lines:
new_offset = offset + size
uint_bytes = self._buffer[offset:new_offset]
if len(uint_bytes) != size:
raise InvalidDatabaseError(_BAD_DATA)
return int.from_bytes(uint_bytes, "big"), new_offset🤖 Comment by Claude (Claude Code) on behalf of Will.
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Codex, responding on Greg’s behalf.
Confirmed that these short-payload cases also occur on main. They are tracked in ENG-5393. Preliminary payload-length checks added about 2% to memory-mode decoding, so Greg prefers to assess them separately. The changelog already limits its claim to reads that previously raised IndexError or struct.error, and the exception-handler comment now describes only indexing and unpacking failures.
| # declared size past that is malformed and could copy attacker-controlled bytes. | ||
| _MAX_UINT_BYTES = 16 | ||
| _MAX_INT32_BYTES = 4 | ||
| # Added to a pointer value, by pointer size. A 4-byte pointer adds nothing. |
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This comment describes the one case that never indexes the tuple: line 201 guards with if pointer_size < 4, so a 4-byte pointer never reaches the lookup. Meanwhile index 0 is unused padding (unstated, and "by pointer size" invites the reader to expect it to mean something), and the entry that is in the tuple and adds nothing — pointer size 1 — goes unmentioned.
Suggest: "Indexed by pointer size; index 0 is unused. A 1-byte pointer adds nothing."
🤖 Comment by Claude (Claude Code) on behalf of Will.
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Codex, responding on Greg’s behalf.
Kept the short description of the offset rule. The four-byte case is handled by the adjacent pointer_size < 4 guard, so it does add no offset without indexing this tuple. Greg prefers to avoid expanding this comment into a description of every tuple entry.
| ) | ||
| _TOO_DEEP = "The MaxMind DB file's data section exceeds the maximum depth" | ||
| _TOO_LARGE = "The MaxMind DB file's data section exceeds the maximum payload size" | ||
| _BAD_DATA = ( |
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_BAD_DATA is now the message for eight distinct failures: unknown type number, truncated ctrl byte, truncated size code, short pointer, over-wide uint, over-wide int32, bad float size, bad double size, and any struct.error from anywhere in the recursion. It carries no offset, no type number, no declared size — and decode at line 256 discards the original exception's own message too.
For a library whose users report "my database is corrupt" with nothing else, the offset is the single most useful fact. Including it at the except would cost one f-string.
Separately: decode is also used for the metadata section (reader.py:87), so a truncated metadata block reports "The MaxMind DB file's data section contains bad data", pointing the user at the wrong part of the file.
🤖 Comment by Claude (Claude Code) on behalf of Will.
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Codex, responding on Greg’s behalf.
Tracked section and offset diagnostics in ENG-5393. The original exception is preserved through raise … from ex, so its message remains available in the chained traceback. More precise corruption messages, including distinguishing metadata from the data section, can be assessed with the broader error contract.
| limits to the C extension. | ||
| * Truncated reads that previously raised ``IndexError`` or ``struct.error`` | ||
| now raise ``InvalidDatabaseError``. | ||
| * Improved pure Python lookup performance. |
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Worth one more bullet: Reader now raises InvalidDatabaseError at open time for a database whose search tree extends past the end of the file. Previously such a file opened successfully and failed at the first lookup with a different message.
Confirmed against GeoIP2-City-Test-Invalid-Node-Count.mmdb: on main it opens and get() raises "Invalid node in search tree"; on this branch the constructor raises. That's a user-visible move of a failure from lookup to open, and anyone catching around get() but not around open_database() will notice.
🤖 Comment by Claude (Claude Code) on behalf of Will.
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Codex, responding on Greg’s behalf.
Added a short changelog bullet in 53622e7 stating that the Python reader rejects invalid search-tree sizes when opening a database.
| "MaxMind-DB-test-decoder-payload-limit-over.mmdb" | ||
| ) | ||
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| def test_metadata_payload_limit_is_enforced_on_open(self) -> None: |
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This test leaks the file handle — it emits a ResourceWarning for an unclosed MaxMind-DB-test-metadata-payload-limit.mmdb, because the metadata decode raises before close() is reachable in Reader.__init__.
Nothing asserts on it, so it won't fail; but the new test surfaced a real leak in the constructor's error paths (see my comment on reader.py line 102).
Unrelated nit on this file: _bounded() is applied inconsistently — this test and test_value_count_boundary decode hostile fixtures without the alarm or the address-space cap, while similar tests get both. Either is defensible; the inconsistency will puzzle the next reader.
🤖 Comment by Claude (Claude Code) on behalf of Will.
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Codex, responding on Greg’s behalf.
The leak is fixed in da62052, with explicit file and mmap cleanup assertions. The metadata resource-limit test already uses _bounded(), and 2983246 adds it to the new cleanup cases too. Kept the value-count boundary tests uncapped because their fixtures contain bounded scalar records rather than recursive fan-out or payload amplification.
| _TOO_DEEP = "^The MaxMind DB file's data section exceeds the maximum depth$" | ||
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| class _HeaderOnlyBuffer(bytes): |
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Worth calling out as a strength: _HeaderOnlyBuffer proves a check runs before the decoder touches any child or payload bytes, rather than only that the check eventually fires. That's a meaningfully stronger assertion than the usual assertRaises, and it turns a missed check into an AssertionError rather than a silent pass. Nice.
One follow-on: the tests using it (lines ~441, ~449) still use bare assertRaises, so they'd pass if the wrong InvalidDatabaseError fired. Adding the message regex would close that, cheaply.
🤖 Comment by Claude (Claude Code) on behalf of Will.
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Codex, responding on Greg’s behalf.
Kept these assertions focused on rejecting oversized integers before reading their payload. _HeaderOnlyBuffer enforces that order. The generic contains bad data message is shared by integer validation failures, so asserting it would not identify which integer check fired.
Replace numeric indices with named fields while preserving the shared, call-local counters and resource limits. GeoLite2-City memory-mode lookups were about 1.4% faster on Python 3.14 and 3.9% slower on 3.10. Python 3.11-3.13 showed no slowdown. On 3.14, mmap lookups were about 1.7% faster and file-mode lookups were roughly even. Tests pass on Python 3.10-3.14. The older interpreters ran the pure Python tests, with extension tests skipped. Type, lint, and formatting checks pass with the existing CPY001 exclusion.
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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:
Exceeding a limit raises
InvalidDatabaseError. The budget is a call-local list threaded through the recursion, so a sharedDecoderstays 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
get()oropen_database()asIndexErrororstruct.errornow raisesInvalidDatabaseError. Invalid UTF-8 keeps raisingUnicodeDecodeError.extension/libmaxminddbsubmodule moves to 1.14.0, which adds the same limits to the C extension (Bound decoded values to prevent a pointer fan-out DoS (STF-1568) libmaxminddb#479).structon concatenated bytes, drop a runtimetyping.castand 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 1.14.0.
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 raisesInvalidDatabaseError.Minor version bump (3.2.0).
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