Bound decoded values to prevent a pointer fan-out DoS (STF-1568) - #479
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📝 WalkthroughWalkthroughMMDB entry decoding now limits each entry to 65,536 values and 2 MiB of cumulative string and bytes payload by default. The payload limit is configurable at build time. Exceeding either limit returns ChangesEntry decoding limits
Estimated code review effort: 2 (Simple) | ~15 minutes Merge Risk: ⚪ Minimal · up to The change is limited to wording in the changelog and has no user or production behavior impact; no actionable merge-blocking risk remains. Poem
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Pull request overview
This PR hardens MMDB_get_entry_data_list() against a data-section pointer fan-out denial-of-service by introducing a per-call cap on total decode work, preventing crafted databases from triggering exponential decode behavior.
Changes:
- Added a maximum decoded-value budget (
MAXIMUM_DATA_STRUCTURE_VALUES) and enforcement inget_entry_data_list(). - Extended the per-call data pool struct to track a running decode counter.
- Documented the security fix and behavior change in
Changes.md.
Reviewed changes
Copilot reviewed 3 out of 3 changed files in this pull request and generated 2 comments.
| File | Description |
|---|---|
| src/maxminddb.c | Adds a decoded-value budget constant and enforces it during recursive entry decoding. |
| src/data-pool.h | Adds a per-call counter field to track decode work across the pool lifetime. |
| Changes.md | Documents the DoS fix and the new decode limit behavior for the next release. |
Suppressed comments (1)
src/maxminddb.c:1736
- This debug message triggers when the count exceeds the maximum (because the condition is
> MAXIMUM_DATA_STRUCTURE_VALUES), so "reached" is misleading. Either change the message to "exceeded" or change the condition to>=if you want it to fire when reaching the limit.
if (++pool->length > MAXIMUM_DATA_STRUCTURE_VALUES) {
DEBUG_MSG("reached the maximum number of data structure values");
return MMDB_INVALID_DATA_ERROR;
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Actionable comments posted: 2
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Inline comments:
In `@Changes.md`:
- Around line 3-9: Update the Changes.md description of
MMDB_get_entry_data_list() to state that MMDB_INVALID_DATA_ERROR is returned
when an entry exceeds the 65,536 decoded-value limit, rather than implying the
limit applies to the entire database.
In `@src/maxminddb.c`:
- Around line 1734-1737: Add regression tests for the decoder’s
MAXIMUM_DATA_STRUCTURE_VALUES limit, covering shared-pointer fan-out and flat
arrays/maps at the exact limit and one value beyond it. Assert MMDB_SUCCESS at
the allowed boundary and MMDB_INVALID_DATA_ERROR above it, and verify concurrent
top-level decode calls keep independent counters.
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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)
src/data-pool.h:39
- The new
lengthfield is used as a per-call decoded-value budget, but the comment currently describes it as the number of list elements decoded across blocks. Becauseget_entry_data_list()can increment this counter multiple times for the same list node when following pointers, the current wording is misleading; please clarify that it counts decoded values/work for the top-level call.
// Total number of list elements decoded so far, across all blocks. Used to
// bound the work done for a single entry (see
// MAXIMUM_DATA_STRUCTURE_VALUES).
size_t length;
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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)
src/data-pool.h:49
- The new
lengthfield is incremented once per decoded value (including pointer targets), but the comment describes it as “list elements decoded … across all blocks”. This is misleading because pointer decoding can incrementlengthwithout allocating a new list element. Update the comment to reflect what’s actually being counted.
// Total number of list elements decoded so far, across all blocks. Used to
// bound the work done for a single entry (see
// MAXIMUM_DATA_STRUCTURE_VALUES).
size_t length;
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Actionable comments posted: 1
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Inline comments:
In `@src/maxminddb.c`:
- Around line 1858-1863: Update the payload accounting in the UTF8_STRING/BYTES
branch of the entry-data processing flow to check whether data_size exceeds
SIZE_MAX minus pool->bytes before adding it. Return MMDB_INVALID_DATA_ERROR on
overflow, while preserving the existing maximum-budget check and successful
accumulation behavior.
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Inline comments:
In `@Changes.md`:
- Line 11: Update the wording in the changelog sentence beginning “Fixed a
related payload-amplification” to hyphenate “denial-of-service” and include
“issue” as requested.
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Pull request overview
Copilot reviewed 7 out of 7 changed files in this pull request and generated 1 comment.
Suppressed comments (1)
t/pointer_dos_t.c:115
- The second half of test_per_call_state() (the "good" database) also conditionally skips assertions if MMDB_open() or MMDB_lookup_string() fails. Since this is meant to verify rejected decodes don’t poison later decodes, it should explicitly assert that opening and lookup succeed so the test can’t pass without executing the intended path.
MMDB_s good;
if (MMDB_open(ok_file, MMDB_MODE_MMAP, &good) == MMDB_SUCCESS) {
int gai, err;
MMDB_lookup_result_s result =
MMDB_lookup_string(&good, "81.2.69.142", &gai, &err);
if (result.found_entry) {
MMDB_entry_data_list_s *list = NULL;
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🔵 Needs a closer look
It introduces security-critical behavioral changes in the core decoder and error semantics, which warrants final human validation despite strong test coverage.
Review details
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- Comments generated: 0 new
- Review effort level: Lite
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🟡 Changes recommended
The new t/decoder_limits_t.pl test uses a backtick shell invocation of $CC --version, which is avoidable and can execute a shell with an environment-controlled command.
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| my $cc = $ENV{CC} || 'cc'; | ||
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| # The checks below rebuild the library with -Werror. Only gcc and clang are | ||
| # known to compile it cleanly with the flags used here, so skip elsewhere | ||
| # instead of failing on a missing compiler or an unrelated warning. | ||
| my $cc_version = `$cc --version 2>&1`; | ||
| if ( $? != 0 || $cc_version !~ /gcc|clang|Free Software Foundation/ ) { | ||
| plan( skip_all => "decoder limit override tests need gcc or clang" ); | ||
| } |
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Fixed in f5fd27d. Compiler detection now uses array-form run3(), captures both output streams, and preserves the skip behavior when the compiler cannot be launched. This keeps detection consistent with the later compiler invocations without treating CC as shell syntax.
— Codex, on behalf of Greg.
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🔵 Needs a closer look
It changes security-sensitive core decoding behavior and public error semantics in C, so it warrants final human review despite strong test coverage.
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🔵 Needs a closer look
The new default limit macros use 1U << n shifts that can be undefined on platforms where unsigned int is narrower than the shift width, so the defaults should be expressed with fixed-width unsigned literal macros (e.g., UINT32_C/UINT64_C).
Review details
Suppressed comments (2)
src/maxminddb.c:55
- The default value uses
(1U << 16), which is undefined behavior on targets whereunsigned intis narrower than 17 bits (the left shift is performed inunsigned int). Using a fixed-width literal macro avoids UB while keeping this a preprocessor-friendly integer constant for the#ifchecks.
#ifndef MAXIMUM_DATA_STRUCTURE_VALUES
#define MAXIMUM_DATA_STRUCTURE_VALUES (1U << 16)
#endif
src/maxminddb.c:75
- The default byte-limit macro uses
(1U << 21), which has the same left-shift portability hazard as the value-count macro on platforms whereunsigned intis narrower than 22 bits. Prefer a fixed-width unsigned literal macro so the constant expression is well-defined in all supported C environments.
#ifndef MAXIMUM_DATA_STRUCTURE_BYTES
#define MAXIMUM_DATA_STRUCTURE_BYTES (1U << 21)
#endif
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| `NULL`. | ||
| work and the caller-visible payload. Each call decodes at most 65,536 list | ||
| values and 2 MiB of UTF-8 string and bytes payload. A structure exactly at | ||
| either limit is accepted. The existing recursive-decoder depth limit of 512 also |
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"The existing" seems weird in this context.
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Regarding Copilot’s suppressed note about — Codex, on behalf of Greg. |
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🔵 Needs a closer look
It modifies core decoder behavior and error semantics in a security-sensitive C codepath, so a final human review is warranted even with the added tests and documentation.
Review details
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- Comments generated: 0 new
- Review effort level: Lite
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I deleted all the comments from my latest Claude review as they seemed not very important. If you're interested that review is at https://gist.github.com/horgh/667d09757a2cb5969511f1823d820345. I'm going to look at the changes one more time myself but otherwise I plan to merge. |
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>
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. The existing depth limit does not stop this, because the blow-up comes from width, not depth.
Change
MMDB_get_entry_data_listnow applies two per-call limits and returns the newMMDB_DECODER_LIMIT_ERRORwhen a record exceeds either one, leaving the output list set toNULL:Both counters live in call-local state, so they start at zero for each top-level call and cannot be corrupted by concurrent callers. The data pool stops doubling its blocks past the value limit, which halves the memory a record at the limit reserves.
MMDB_opendecodes thelanguagesanddescriptionmetadata as complete lists and reports an over-limit structure asMMDB_INVALID_METADATA_ERROR.MMDB_get_valueandMMDB_aget_valueare unchanged: they skip values without following pointers, copy nothing, and are already bounded by the depth limit. They remain the way to read a field from an otherwise over-limit record with a packaged library.Both limits can be raised when rebuilding the library with
-DMAXIMUM_DATA_STRUCTURE_VALUES=<values>and-DMAXIMUM_DATA_STRUCTURE_BYTES=<bytes>.This follows the Reader Resource Limits section of the MaxMind DB specification (maxmind/MaxMind-DB#282). The regression tests use the fixtures that change added.
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