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CVE-2026-82760 – Unauthenticated Denial of Service – Ash Authentication (ash_authentication) 4.8.0–4

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Summary

CVE-2026-82760 is an inefficient algorithmic complexity (CWE-407) vulnerability in the ash_authentication Elixir library used by applications built on the Ash Framework to implement authentication, including the API Key sign-in strategy. The flaw allows an unauthenticated attacker to submit a crafted, oversized API key value that forces the server to perform extremely expensive decoding work, exhausting CPU and memory before any credential check takes place. The vulnerability carries a CVSS score of 8.2 (also reported as 7.5 by the GitHub Security Advisory using CVSS v3.1), reflecting a high availability impact with no confidentiality or integrity loss.

Technical details

  • Root cause: The AshAuthentication.Base.decode62/1 function decodes a base62-encoded string by folding over each character and recomputing Integer.pow(62, index) at every position rather than using an incremental (Horner’s-method-style) accumulation, giving it roughly cubic time complexity relative to input length. The companion bindecode62/1 function is quadratic due to its use of Integer.undigits/2 and Integer.digits/2. Independent analysis measured the combined complexity at approximately O(n^2.6).
  • Trigger conditions: Neither decode62/1 nor bindecode62/1 validates the byte size of its input before processing. AshAuthentication.Strategy.ApiKey.SignInPreparation splits a submitted API key into underscore-separated segments and passes them directly into these decoding functions — before any database lookup or authentication check occurs.
  • Attack vector: Network-based, unauthenticated. An attacker sends an API key sign-in request containing an oversized base62-encoded segment; no valid credentials or prior session are required.
  • Impact: CPU and memory exhaustion leading to denial of service. Reported measurements show roughly 60–630ms of CPU cost per request in default configurations; deployments that expose the API key sign-in strategy via request bodies (allowing much larger inputs) can see costs balloon to tens of seconds or longer for larger payloads, effectively allowing a single request to degrade or crash the service.
  • Exception-handling (rescue) clauses in the surrounding code do not mitigate this issue, since they catch raised errors, not excessive CPU/memory consumption.

Affected software

  • ash_authentication versions 4.8.0 through 4.14.2 (inclusive)
  • ash_authentication versions 5.0.0-rc.0 through 5.0.0-rc.13 (inclusive)
  • Applies to deployments that use the API Key sign-in strategy provided by Ash Authentication

Severity

  • CVSS Score: 8.2 (High) — CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N
  • The vendor advisory additionally rates the issue 7.5 (High) under CVSS v3.1: AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
  • Both scorings agree the impact is limited to availability, with no confidentiality or integrity loss, and require no authentication or user interaction.

Mitigation and recommended actions

  • Immediate: Upgrade ash_authentication to version 4.15.0 (for the 4.x line) or 5.0.0-rc.14 (for the 5.0 release candidate line), both of which add byte-size guards before decoding and re-encoding validation to eliminate the expensive decode path.
  • If patching is not immediately possible:
    • Enforce a maximum length on submitted API key values at the reverse proxy, load balancer, or WAF layer — legitimate API keys are typically well under 80 characters.
    • Audit whether your application exposes the API key sign-in strategy through request bodies (as opposed to headers), since body-based exposure allows much larger attacker-controlled payloads and dramatically worsens the impact.
    • Confirm web server/proxy header and body size limits remain at safe defaults to constrain the maximum size of attacker-supplied input reaching the vulnerable decoding functions.

References

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