CVE-2026-55733 Atom-table exhaustion denial of service in Guardian permissions AtomEncoding via unbounded atom creation
Attacker can crash the BEAM node by exhausting the atom table via unbounded atom creation in Guardian permissions, requiring non-default AtomEncoding configuration.
- Exploited in the wild
- Not known
- Login needed
- Not needed
- Attackable
- Local or same network only
- Victim action needed
- Not needed
- Fix
- Available
What is CVE-2026-55733?
CVE-2026-55733 is a medium-severity denial of service vulnerability in ueberauth guardian, published on 1 Aug 2026. It can only be exploited from the same machine or local network, with no action from the victim.
Official description
Allocation of Resources Without Limits or Throttling in ueberauth guardian allows denial of service via unbounded atom creation from attacker-controlled binary input.
Guardian.Permissions.AtomEncoding encodes permission scopes by passing arbitrary binaries to String.to_atom/1. When encode/3 in lib/guardian/permissions/atom_encoding.ex is called with a list, each binary entry is handled by the encode_value/3 binary clause, which calls String.to_atom(value) with no allow-list check. The perm_set argument (the application's small, finite set of legitimate permission names) is discarded, so any external string flows straight into atom creation. This encoder is selected with use Guardian.Permissions, encoding: Guardian.Permissions.AtomEncoding and reached through the imported encode/3 entry point.
String.to_atom/1 creates a brand-new atom for every previously unseen binary, atoms are never garbage collected, and the BEAM atom table is fixed at roughly 1,048,576 entries by default. An application that funnels attacker-influenced permission scopes (from a request body, a JWT claim, or other external input) into encode/3 therefore mints one permanent atom per distinct value. A modest stream of varied, unauthenticated input permanently consumes the atom table and crashes the BEAM node with system_limit, taking down every application running on it.
The default encoder is Guardian.Permissions.BitwiseEncoding, which is not affected.
This issue affects guardian: from 2.0.0 before 2.4.1.
Weakness: CWE-770
Which versions are affected by CVE-2026-55733?
1 open source package ship an affected version, including guardian. It affects 1 product.
| Package | Affected | Fixed in |
|---|---|---|
guardian Hex | From 2.0.0 | 2.4.1 |
Products include Ueberauth Guardian.
How do I fix CVE-2026-55733?
Upgrade guardian to 2.4.1 or later.
Upgrade, then put limits on request size, rate and nesting depth so one client cannot exhaust the service. More about Denial of service.
Is CVE-2026-55733 being exploited?
It is not confirmed as exploited in the wild, but public exploit code for CVE-2026-55733 exists.
Other vulnerabilities in ueberauth guardian
- CVE-2026-55735 Guardian.revoke/3 acts on unverified token claims, allowing forged-token session revocation
- CVE-2026-54894 Atom-table exhaustion denial of service in Guardian via unbounded atom creation from binary keys
- CVE-2026-55734 guardian atom exhaustion in Guardian.Permissions.encode_permissions!/1
Sources
Also known as EEF-CVE-2026-55733, GHSA-fjr5-7xrc-hmpj.