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CVE-2026-59880 — immutable

Fix: immutable-js/immutable-js@3dd7e56

CVE-2026-59880 is a CWE-407 vulnerability in immutable. A fix is available for immutable — see the affected versions and patch details below.

Immutable.js: Hash-collision algorithmic complexity denial of service in Immutable.Map/Set

Also known asGHSA-xvcm-6775-5m9r
Published
Updated
Affected
3 pkgs
Patched
3 / 3
Exploits
None indexed
Exploitation data as of Oct 7, 2026 · OSV.dev, NVD, FIRST.org (EPSS)

Exploitation Status

Proof-of-concept exploit code exists

  • CISA’s SSVC triage found public proof-of-concept exploit code for this CVE, though no confirmed active exploitation.
  • CISA assesses this as automatable — exploitation doesn’t require manual, per-target effort, which raises the odds of mass scanning and opportunistic attacks.

Exploitation and automatability from CISA’s SSVC triage for CVE-2026-59880.

EPSS Exploitation Probability

via FIRST.org ↗
0.7%probability of exploitation in next 30 days
Lower Risk+0.23%
Lower risk than most CVEs50th percentile — riskier than 50% of all scored CVEsHighest risk
0.00%0.39%0.78%1.17%0.4%0.7%Aug 26Oct 26

Probability of exploitation in the next 30 days, from FIRST.org EPSS.

Real-World Exposure

3 pkgs affected

How broadly this vulnerability is actually deployed: weekly install volume shows current usage, and reverse-dependency count shows how many other packages break if it stays unpatched.

8Kother npm packages depend on this — each one inherits the vulnerability until it's patched upstream
immutablenpm
51.2Mdownloads / week

Description

Summary

Immutable.Map and Immutable.Set keep keys that share the same 32-bit hash in a collision bucket that is scanned linearly. The string hash is public and deterministic, so an attacker who controls the keys inserted into a Map can craft many keys that all collide, degrading insertion and lookup from amortized O(1) to O(n) per operation — and O(n²) to build or read the whole set. A small, attacker-shaped payload can therefore consume disproportionate CPU and, on a single-threaded runtime such as Node.js, stall the event loop and deny service.

Details

The string hash uses the JVM-style polynomial hashed = (31 * hashed + charCode) | 0. Strings such as "Aa" and "BB" hash to the same value (65*31+97 == 66*31+66 == 2112), and concatenating such blocks yields 2^n distinct strings sharing one hash (40 characters ⇒ >1,000,000 colliding keys). All such keys route to a single HashCollisionNode, whose get/update walk the entire bucket testing is(). There is no per-process salt, so the colliding set is fully precomputable from the open-source algorithm.

Proof of concept

Inserting N colliding keys (e.g. via Immutable.Map(obj) / Immutable.fromJS(obj)) is O(N²). Measured on one machine, ~8,000 colliding keys take ~0.7 s to build and ~0.6 s to read, scaling ×4 per doubling; ~16,000 keys exceed several seconds.

Impact

CPU-bound denial of service in applications that ingest attacker-controlled object keys into Immutable structures, e.g. Immutable.Map(req.body), Immutable.fromJS(req.body), state.merge(userObject) / mergeDeep(...). Applications that only store attacker input as values under fixed keys are not affected.

Affected versions

  • = 4.0.0-beta.1, < 4.3.9

  • = 5.0.0-beta.1, < 5.1.8

  • < 3.8.4

Patches

Fixed in 5.1.8 (adjust to the actual release): large collision buckets are indexed by a per-process seeded secondary hash, restoring near-linear behavior for the affected paths. The public hash() is unchanged (no breaking change), and is() remains the sole authority on key equality. Also fixed in 4.3.9 and 3.8.4.

Workarounds

Before passing untrusted data to Immutable.js: cap request body size, limit object key count/length, and reject high-cardinality payloads; avoid building Maps directly from untrusted object keys.

References

  • CWE-407 (Inefficient Algorithmic Complexity), CWE-400 (Uncontrolled Resource Consumption)
  • OWASP API4:2023 (Unrestricted Resource Consumption)

Affected Packages

3 total 3 fixed
EcosystemPackageVulnerable rangeFix
📦npmimmutable≥ 4.0.0-beta.1&&< 4.3.94.3.9npm install immutable@4.3.9
📦npmimmutable≥ 5.0.0-beta.1&&< 5.1.85.1.8npm install immutable@5.1.8
📦npmimmutableall versions3.8.4npm install immutable@3.8.4

Affected Products

1 product · 2 configurations
Application
immutableimmutable-js
≥ 5.0.0 && < 5.1.8
range

Detection & mitigation playbook

Open-source dependency

  1. Detect

    Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for immutable, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.

  2. Fix

    Update immutable to 4.3.9 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2026-59880 is resolved across your whole dependency graph.

  3. Workarounds

    Cap what an attacker can consume: apply request size, rate and timeout limits in front of the affected component, and run it with memory and CPU limits so exhaustion degrades one worker rather than the whole service.

Frequently Asked Questions

## Summary `Immutable.Map` and `Immutable.Set` keep keys that share the same 32-bit hash in a collision bucket that is scanned linearly. The string hash is public and deterministic, so an attacker who controls the **keys** inserted into a Map can craft many keys that all collide, degrading insertion and lookup from amortized O(1) to O(n) per operation — and O(n²) to build or read the whole set. A small, attacker-shaped payload can therefore consume disproportionate CPU and, on a single-threaded runtime such as Node.js, stall the event loop and deny service. ## Details The string hash uses t
O3 Security · Impact-Aware SCA

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CVE-2026-59880: immutable DoS — Fixed in 4.3.9