Attack Chain

1. An unauthenticated attacker crafts a small HTTP request, embedding a numeric string (e.g., 1073741824) in the range [2 ** 30, 2 ** 31) into a request body, URL, or key-path.
2. The vulnerable application, using immutable.js versions < 4.3.9 or >= 5.0.0-beta.1, < 5.1.8, receives this request.
3. The application processes the untrusted input, passing the crafted numeric string as an index to List#set, List#setSize, List#setIn, or List#updateIn.
4. Within immutable.js, the setListBounds() function attempts to handle the index using signed 32-bit bitwise arithmetic.
5. Due to a bug where JavaScript's shift count is taken modulo 32, the level-raising loop condition newTailOffset >= 1 << (newLevel + SHIFT) becomes perpetually true.
6. If the List is empty, this results in an uncatchable infinite CPU spin, freezing the worker process.
7. If the List is populated (e.g., from an array with >= 32 elements), each iteration of the loop allocates a new VNode without bound, rapidly exhausting the process heap.
8. Heap exhaustion leads to the application process aborting (e.g., SIGABRT, exit 134, or kernel OOM-kill 137), causing a Denial of Service.
9. Alternatively, if setSize is called with a large value like 2 ** 31 or 2 ** 32 + 5, silent wraparound due to ToInt32 coercion can corrupt the List's size to 0 or 5 respectively, leading to data corruption without crashing.

Impact

This vulnerability primarily impacts the availability of applications using vulnerable versions of immutable.js. A single, small, unauthenticated HTTP request can trigger an unrecoverable Denial of Service (DoS) by either causing a worker process to enter an infinite CPU spin, effectively hanging, or by forcing the process to consume all available memory, leading to an application crash (e.g., SIGABRT or OOM-kill). This can affect common application patterns such as configuration stores, document/collection editors, Redux-Immutable reducers, or JSON-Patch endpoints that route untrusted input into List operations. There is no observed impact on confidentiality or integrity, nor does it lead to Remote Code Execution (RCE). The companion silent data corruption issue can lead to applications operating with incorrect state, potentially causing further malfunctions, but without an immediate crash.

Recommendation

• Upgrade immutable.js: Immediately upgrade immutable.js to version 4.3.9 or higher, or to 5.1.8 or higher for the v5 beta releases, to remediate CVE-2026-59879.
• Implement Input Validation: For systems that cannot be immediately upgraded, validate and clamp any externally supplied List index or setIn/updateIn key-path segments. Reject numeric path segments greater than or equal to 2 ** 30 before passing them to immutable.js methods.
• Containerize and Resource Limit Applications: Run request handling processes in isolated worker environments with resource limits (e.g., cap heap size using --max-old-space-size) to contain any potential process aborts, allowing for faster recovery or restart of affected workers.