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 3 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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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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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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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
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>
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The existing resource-limit tests force MODE_MEMORY, so they cover only the pure Python decoder. The C extension decodes through libmaxminddb, which has its own copy of the limits, and nothing asserted that path rejects the DoS fixtures. Add extension-path checks that decode each DoS fixture through MODE_MMAP_EXT and assert an InvalidDatabaseError. The limits live in libmaxminddb, so 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 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>
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