Bound decoded values to prevent a pointer fan-out DoS (STF-1488) - #479
Bound decoded values to prevent a pointer fan-out DoS (STF-1488)#479oschwald wants to merge 8 commits into
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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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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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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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| if (++pool->length > MAXIMUM_DATA_STRUCTURE_VALUES) { | ||
| DEBUG_MSG("reached the maximum number of data structure values"); | ||
| return MMDB_INVALID_DATA_ERROR; | ||
| } |
| if (entry_data_list->entry_data.type == MMDB_DATA_TYPE_UTF8_STRING || | ||
| entry_data_list->entry_data.type == MMDB_DATA_TYPE_BYTES) { | ||
| if (entry_data_list->entry_data.data_size > | ||
| MAXIMUM_DATA_STRUCTURE_BYTES - pool->bytes) { | ||
| DEBUG_MSG("reached the maximum data structure size"); | ||
| return MMDB_INVALID_DATA_ERROR; | ||
| } | ||
| pool->bytes += entry_data_list->entry_data.data_size; | ||
| } |
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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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| records MaxMind produces decode a few hundred values. This matches the reader | ||
| resource limits recommended by a proposed update to the MaxMind DB | ||
| specification. See GHSA-hj94-g986-h9r7. | ||
| - Fixed a related payload-amplification denial of service. A crafted database |
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📐 Maintainability & Code Quality | 🟡 Minor | ⚡ Quick win
Hyphenate “denial-of-service.”
Change payload-amplification denial of service to payload-amplification denial-of-service issue for consistent compound-word usage.
Proposed wording
-- Fixed a related payload-amplification denial of service.
+- Fixed a related payload-amplification denial-of-service issue.📝 Committable suggestion
‼️ IMPORTANT
Carefully review the code before committing. Ensure that it accurately replaces the highlighted code, contains no missing lines, and has no issues with indentation. Thoroughly test & benchmark the code to ensure it meets the requirements.
| - Fixed a related payload-amplification denial of service. A crafted database | |
| - Fixed a related payload-amplification denial-of-service issue. A crafted database |
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[grammar] ~11-~11: Use a hyphen to join words.
Context: ...d a related payload-amplification denial of service. A crafted database can point ...
(QB_NEW_EN_HYPHEN)
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instructions embedded in them. Verify each finding against current code. Fix
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In `@Changes.md` at 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.
Source: Linters/SAST tools
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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;
| MMDB_s dos; | ||
| if (MMDB_open(dos_file, MMDB_MODE_MMAP, &dos) == MMDB_SUCCESS) { | ||
| int gai, err; | ||
| MMDB_lookup_result_s result = | ||
| MMDB_lookup_string(&dos, "1.1.1.1", &gai, &err); | ||
| if (result.found_entry) { | ||
| MMDB_entry_data_list_s *first = NULL; |
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A crafted data section could nest pointers to shared targets so that MMDB_get_entry_data_list decoded one entry with exponential time and memory from a small file (GHSA-hj94-g986-h9r7). The existing depth limit did not stop this: the blow-up comes from width (a shared graph re-walked), not from a single deep path. The decoder now counts the values it decodes for one entry and returns MMDB_INVALID_DATA_ERROR once the count exceeds 65,536, far above the few hundred values the largest records MaxMind produces decode. This matches the reader resource limits recommended by a proposed update to the MaxMind DB specification. Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
The value-count limit bounds how many nodes MMDB_get_entry_data_list produces, but not how many bytes they reference. libmaxminddb borrows payload bytes rather than copying them, so the node count alone bounds the library's own memory. A crafted database can still point many times at one large value, producing a bounded node list that together references far more bytes than the file holds. A caller that copies each node into a string then materializes that amplified total, for example about 512 MiB from an 82 KiB file. Charge the total string and bytes payload decoded for a single entry against a per-entry byte budget and return MMDB_INVALID_DATA_ERROR when it exceeds MAXIMUM_DATA_STRUCTURE_BYTES (2 MiB, overridable at build time with -DMAXIMUM_DATA_STRUCTURE_BYTES=<bytes>). The budget is a uint64 and the value-count limit caps how many payloads are charged, so the running total cannot overflow. Integers are size-validated and tiny, floats are fixed width, and container sizes are element counts, so only string and bytes payloads are charged. This also rejects a rare format-valid record whose own string and bytes fields exceed the limit. The largest records MaxMind produces hold about a kilobyte of payload, so the limit leaves a wide margin while stopping the amplification for every caller of the API. See GHSA-hj94-g986-h9r7. Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Bump t/maxmind-db to the coordinated MaxMind-DB branch commit that adds the pointer fan-out and payload amplification fixtures, so the new regression tests can use them. This pins a branch commit while both changes are in review and will move to the merged commit once the MaxMind-DB change lands. Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Exercise MMDB_get_entry_data_list against the coordinated fixtures. The value-count fan-out, the payload amplification, and its worst case under the value-count limit are each rejected with MMDB_INVALID_DATA_ERROR and leave a NULL output list. A normal record still decodes, confirming no false rejection, and a rejected decode does not affect a later one, confirming the counters are per call. Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
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| MMDB_s dos; | ||
| if (MMDB_open(dos_file, MMDB_MODE_MMAP, &dos) == MMDB_SUCCESS) { | ||
| int gai, err; | ||
| MMDB_lookup_result_s result = | ||
| MMDB_lookup_string(&dos, "1.1.1.1", &gai, &err); | ||
| if (result.found_entry) { | ||
| MMDB_entry_data_list_s *first = OUTPUT_SENTINEL; |
| for my $definition ( | ||
| '-DMAXIMUM_DATA_STRUCTURE_VALUES=1000000', | ||
| '-DMAXIMUM_DATA_STRUCTURE_BYTES=1<<31', | ||
| '-DMAXIMUM_DATA_STRUCTURE_BYTES=2*1024*1024*1024', | ||
| ) { |
| size_t const maximum_values = (size_t)MAXIMUM_DATA_STRUCTURE_VALUES; | ||
| if (decode_state->values >= maximum_values) { | ||
| DEBUG_MSG("reached the maximum number of data structure values"); | ||
| return MMDB_DECODER_LIMIT_ERROR; | ||
| } |
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 counts the values it decodes and returnsMMDB_INVALID_DATA_ERRORonce the count exceeds 65,536. The counter is a field on the per-call data pool, so it starts at zero for each top-level call and cannot be corrupted by concurrent callers. Every array element, map key, map value, and pointer follow routes through the single recursive function where the count is charged, so no decode path escapes the bound. The largest real records decode a few hundred values.This also caps a flat oversized container that uses no pointers. The lazy lookup path (
MMDB_get_value) is unchanged, since it skips values without following pointers and is already bounded by the depth limit.This matches the reader resource limits now recommended by the MaxMind DB specification (maxmind/MaxMind-DB#282).
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