EUVD-2026-51915Severity by source
CVSS:4.0/AV:L/AC:L/AT:P/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:H/E:X/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/MVI:X/MVA:X/MSC:X/MSI:X/MSA:X/S:X/AU:X/R:X/V:X/RE:X/U:X
Network-delivered via HTTP but requires application to route user input to Guardian key functions (AC:H); BEAM node crash affects all co-hosted apps, changing scope (S:C); no confidentiality or integrity impact.
Primary rating from Vendor (EEF).
CVSS VectorVendor: EEF
Lifecycle Timeline
2DescriptionCVE.org
Allocation of Resources Without Limits or Throttling in ueberauth guardian allows denial of service via unbounded atom creation from attacker-influenced binary input.
Guardian.Plug.Keys derives connection and session namespace keys by passing arbitrary binaries to String.to_atom/1. base_key/1 in lib/guardian/plug/keys.ex converts any binary into the atom :"guardian_<input>", and the derived helpers claims_key/1, resource_key/1, and token_key/1 create a second atom on top of that. key_from_other/1 likewise converts a regex-captured binary through String.to_atom/1. The public specs advertise String.t() as a valid argument, so passing a string is documented usage, and higher-level entry points such as Guardian.Plug.current_token(conn, key: key) thread the caller-supplied key straight into these functions.
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 routes attacker-influenced data (a tenant identifier, header, or other request input) into a Guardian key 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, taking down every application running on it.
This issue affects guardian: from 0.1.0 before 2.4.1.
AnalysisAI
Unbounded atom creation in Guardian.Plug.Keys allows any attacker who can route arbitrary string values through Guardian's key derivation functions to permanently exhaust the BEAM virtual machine's atom table, crashing the entire BEAM node and denying service to every application co-hosted on it. All guardian releases from 0.1.0 through 2.4.0 are affected whenever an application passes attacker-influenced data - a tenant identifier, HTTP header, or similar request value - as the key parameter to Guardian.Plug.current_token/2, current_claims/2, current_resource/2, or directly to Guardian.Plug.Keys functions. No public exploit code has been identified at time of analysis, though the attack mechanism is mechanically trivial given that it requires only a stream of distinct HTTP requests to a single vulnerable endpoint.
Technical ContextAI
The root cause is CWE-770 (Allocation of Resources Without Limits or Throttling) in the BEAM virtual machine's atom interning model. In Elixir and Erlang, atoms are interned strings stored in a process-global table with a default hard cap of approximately 1,048,576 entries; once allocated, atoms are never garbage collected. Guardian.Plug.Keys (lib/guardian/plug/keys.ex) used String.to_atom/1 unconditionally in base_key/1, claims_key/1, resource_key/1, token_key/1, and key_from_other/1 to convert caller-supplied strings into the atom-keyed identifiers required by Plug.Conn private storage. Because the public API declared String.t() as a valid argument type, passing a string - including one derived from request data - was documented, intended usage. Higher-level entry points such as Guardian.Plug.current_token(conn, key: key) threaded the caller-supplied key directly into these atom-creating functions without any validation. The fix at commit 2952657e replaces String.to_atom/1 with String.to_existing_atom/1 on all read paths (returning nil for unknown atoms rather than interning new ones), introduces bang variants (token_key!/1, claims_key!/1, resource_key!/1) that intern atoms only from developer-controlled write paths, and switches session and cookie name derivation to pure string helpers that require no atom at all. Affected CPE: cpe:2.3:a:ueberauth:guardian:*:*:*:*:*:*:*:*.
RemediationAI
Upgrade guardian to version 2.4.1 or later; the fix is delivered at commit 2952657e42e6341a67e6aaad09d8f0b40ae917cb and replaces all unsafe String.to_atom/1 calls on read paths with String.to_existing_atom/1. If an immediate upgrade is not feasible, audit every call site of Guardian.Plug.current_token/2, current_claims/2, current_resource/2, and any direct invocations of Guardian.Plug.Keys functions: ensure the key argument is always a hard-coded atom literal (e.g., :default, :admin) and never a variable derived from request data. As a targeted compensating control, add an explicit allowlist validation at application boundaries that restricts Guardian key values to a finite set of known atoms before they reach any Guardian function - for example, pattern-matching on an enumerated set of tenant identifiers and rejecting unknown values with a 400 or 404 response. This prevents atom creation without requiring an application rewrite. Note that this control must be applied at every entry point consistently; missing a single call site restores exposure. Vendor advisory: https://github.com/ueberauth/guardian/security/advisories/GHSA-xqch-c77q-rgh5.
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External POC / Exploit Code
Leaving vuln.today