Bound decoder work to prevent a pointer fan-out DoS (STF-1488) - #355
Bound decoder work to prevent a pointer fan-out DoS (STF-1488)#355oschwald wants to merge 4 commits into
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Included review availability: Your plan provides up to 2 included reviews per hour; 0 remain after this review. 📝 WalkthroughWalkthroughThe decoder now enforces per-lookup nesting-depth and decoded-value limits. Nested pointers, maps, arrays, dictionaries, objects, map keys, and skipped values propagate decode state. Tests cover fan-out, oversized maps, depth limits, and pointer cycles. Release notes document the behavior. ChangesDecoder resource limits
Estimated code review effort: 4 (Complex) | ~45 minutes Merge Risk: 🟡 Moderate · up to The decoder’s new resource limits may not apply when skipping unknown fields, which could let a crafted database still trigger excessive CPU or memory use. This bounded denial-of-service risk should be resolved or explicitly accepted before merging. Sequence Diagram(s)sequenceDiagram
participant DatabaseLookup
participant Decoder
participant NestedValue
participant ExceptionHandling
DatabaseLookup->>Decoder: start lookup with decode limits
Decoder->>NestedValue: decode or skip pointer, map, array, dictionary, or object
NestedValue-->>Decoder: propagate depth and value budget
Decoder->>ExceptionHandling: throw InvalidDatabaseException on limit violation
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Pull request overview
This PR hardens the MaxMind DB decoder against crafted databases that can trigger denial-of-service via pointer fan-out (exponential decode work) and pointer cycles (stack overflow risk), aligning behavior with recommended MaxMind DB resource limits.
Changes:
- Add per-lookup decode guards in
Decoder(depth limit + decoded-values budget) and throwInvalidDatabaseExceptionwhen limits are exceeded. - Thread guard state through decode calls to preserve
Decoderconcurrency safety. - Add targeted xUnit tests covering pointer fan-out bounding and cyclic-pointer rejection; document the fix in release notes.
Reviewed changes
Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.
| File | Description |
|---|---|
| releasenotes.md | Adds a 5.2.0 release note entry describing the decoder DoS fix and the GHSA reference. |
| MaxMind.Db/Decoder.cs | Introduces depth/value-budget guards and plumbs them through decode paths (containers/pointers). |
| MaxMind.Db.Test/DecoderTest.cs | Adds regression tests for pointer fan-out and cyclic-pointer behavior (including cyclic map-key pointer). |
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Inline comments:
In `@MaxMind.Db.Test/DecoderTest.cs`:
- Around line 51-74: Expand the decoder tests around TestPointerFanOutIsBounded
to cover both sides of the 65,536-value and 512-depth limits: accept exactly at
each boundary and reject one value beyond it. Exercise both map and array
declarations, ensuring declared sizes are validated before allocation and
preserving the existing fan-out rejection coverage.
In `@MaxMind.Db/Decoder.cs`:
- Around line 466-467: Update DecodeMapToType’s unknown-field skip path to pass
the current depth and budget into NextValueOffset, and ensure each skipped
nested container is charged through the existing container-limit logic. Add a
regression test using a KeyOnlyModel record with an ignored array containing
65,537 elements, verifying the per-lookup value limit is enforced.
In `@releasenotes.md`:
- Around line 26-32: Update the decoder denial-of-service release-note entry to
include its author and GitHub issue number using the required release-note
metadata format; retain the existing feature description and GHSA reference.
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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 InvalidDatabaseException. 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 raising an uncatchable StackOverflowException. The depth and remaining-value budget are threaded as method parameters rather than stored on the shared Decoder, so they add no thread-local access and keep the decoder 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>
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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 (1)
Previously missed (1) — in code that hasn't changed since the last review.
releasenotes.md:32
- The PR description mentions a minor version bump to 5.2.0, but the project files still appear to be set to 5.1.0 (e.g., MaxMind.Db/MaxMind.Db.csproj has 5.1.0). If the version bump is intended as part of this PR, the package/version metadata should be updated to match the 5.2.0 release notes section to avoid publishing an incorrectly-versioned build.
- Fixed a denial-of-service issue in the decoder. A crafted database could nest
data-section pointers to shared targets so that decoding one record cost
exponential time and memory from a small file. The decoder now limits the
number of values it decodes for a single record and rejects a database that
exceeds it, along with pointer cycles and over-deep data, with an
`InvalidDatabaseException`. This matches the reader resource limits now
recommended by the MaxMind DB specification. See GHSA-hj94-g986-h9r7.
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 counts the values it decodes per lookup and rejects a database that exceeds 65,536, along with pointer cycles and over-deep data (depth limit 512), with an
InvalidDatabaseException. AStackOverflowExceptionis not catchable in .NET, so the explicit depth limit is required.Guarding is done at container boundaries: each map and array charges its declared size against the budget and checks the depth, and each pointer follow checks the depth. Charging the declared size also rejects an oversized declared size before it is used as an allocation hint. The
Decoderis shared across concurrent lookups, so the depth and value budget are threaded as method parameters rather than stored on the decoder, which keeps the decoder safe for concurrent reads with no thread-local access. The two guard helpers are aggressively inlined, so the normal-decode overhead stays near zero. The largest real records decode a few hundred values.This matches the reader resource limits now recommended by the MaxMind DB specification (maxmind/MaxMind-DB#282).
Minor version bump (5.2.0).
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Summary by CodeRabbit
Bug Fixes
InvalidDatabaseException.Documentation