Bound decoder work to prevent a pointer fan-out DoS (STF-1488) - #439
Bound decoder work to prevent a pointer fan-out DoS (STF-1488)#439oschwald wants to merge 5 commits into
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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. Poem
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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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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.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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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
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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(
| try: | ||
| sys.setrecursionlimit(10_000) | ||
| with self.assertRaisesRegex( | ||
| InvalidDatabaseError, |
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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
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🔵 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
A crafted data section could nest pointers to shared targets so that decoding one record cost exponential time and memory from a small file (GHSA-hj94-g986-h9r7). The decoder now limits the number of values it decodes for a single record and rejects a database that exceeds the limit with an InvalidDatabaseError. The limit is 65,536, far above the few hundred values the largest real records decode. Pointer cycles and over-deep data are rejected the same way rather than exhausting the stack. The limit state is call-local, so the decoder stays safe for concurrent reads. This matches the reader resource limits now recommended by the MaxMind DB specification. Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
A crafted database could aim many data-section pointers at one large string or bytes value. The value count stayed low, but the pure Python decoder copied each target, so a small file could materialize gigabytes. Add a call-local 2 MiB budget for the total string and bytes payload a single decode produces. Each value is charged its length wherever it is decoded, so re-decoding a shared target through another pointer recharges the budget, which stops the amplification. Also reject a variable-length integer whose declared size exceeds its type before the bytes are copied. The metadata read when a database is opened uses the same decoder, so the limit covers it too. Bump the test-data submodule to the fixtures for these cases. See GHSA-hj94-g986-h9r7. Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
The existing resource-limit tests force the pure Python modes, so they cover only the pure Python decoder. The C extension decodes through the vendored libmaxminddb, and nothing asserted that path rejects the DoS fixtures. Move the libmaxminddb submodule to the main-branch commit that adds the decoder resource limits (maxmind/libmaxminddb#479), ahead of the 1.14.0 release. Add extension-path checks that decode each DoS fixture through MODE_MMAP_EXT and assert an InvalidDatabaseError, and check that the amplified metadata fixture is rejected when the database is opened. The checks first probe a fixture one byte over the 2 MiB payload limit, which is small and safe to decode. A libmaxminddb with the fix rejects it with the decoder-limit message and the checks run; an older one, such as a system library selected with MAXMINDDB_USE_SYSTEM_LIBMAXMINDDB, decodes it and the checks skip rather than run the large DoS fixtures through a decoder that would exhaust memory. See GHSA-hj94-g986-h9r7. Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com> Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
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>
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🟡 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.
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Review details
Suppressed comments (3)
maxminddb/decoder.py:231
_decode_uintusesint.from_byteson a slice that may be shorter thansizewhen 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_int32padspacked_byteswith zeros whensize != 4. If the buffer is truncated (fewer thansizebytes available), slicing can return a shorter byte string and the padding will hide the truncation, producing a value instead of raisingInvalidDatabaseError. Check the slice length againstsizebefore 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_stringcan silently accept truncated payloads because slicing past EOF may return fewer thansizebytes without raising (especially forbytes/FileBuffer). This should be treated as corrupt data and raised asInvalidDatabaseErrorrather 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
| new_offset = offset + size | ||
| return self._buffer[offset:new_offset], new_offset |
| The extension decodes through libmaxminddb, so these limits live in that | ||
| library, not in the pure-Python decoder that TestDecoderResourceLimits | ||
| covers. The checks run only when the linked libmaxminddb enforces the | ||
| limits and skip otherwise; see setUp. |
Fixes the data-section pointer fan-out denial of service (GHSA-hj94-g986-h9r7). A crafted database can nest pointers to shared targets so that decoding one record costs exponential time and memory from a small file. A recursion depth limit alone does not stop this, because the blow-up comes from width, not depth.
Change
The decoder bounds the work per lookup. It counts the values it decodes and rejects a database that exceeds 65,536 with an
InvalidDatabaseError. Each array and map subtracts its declared size before iterating, so a re-decoded (fanned-out) container drains the budget and an oversized declared size is rejected before any element is read. The largest real records decode a few hundred values.The budget is call-local, so concurrent reads stay thread-safe. A pointer cycle exhausts the interpreter recursion limit before the value limit, so
RecursionErroris converted toInvalidDatabaseError.This matches the reader resource limits now recommended by the MaxMind DB specification (maxmind/MaxMind-DB#282).
This change covers the pure-Python decoder. The C extension decodes through libmaxminddb, which is fixed separately.
Minor version bump (3.2.0).
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Summary by CodeRabbit
Bug Fixes
InvalidDatabaseErrorresponses for invalid content.Documentation