CVE-2026-55518: Avo: Missing Authorization in Avo Association Attach Endpoint Allows Unauthorized Relationship Manipulation and Privilege Escalation

Should you act?

Critical, preconditions apply

Severe if exploited, but exploitation needs conditions that may not apply to you. This signal is computed from CISA KEV, EPSS and CVSS data; it is not an editorial verdict.

• Not in CISA KEV, but CISA’s SSVC assessment records a public proof-of-concept, so attack code exists.
• EPSS 0.4%, percentile 38: FIRST’s model estimates a 0.4% chance of exploitation activity in the next 30 days, higher than 38% of all scored CVEs (low). EPSS is a probability, not a verdict.
• Needs an account (low (any account)). Exposure depends on who can log in.
• Only applies if your dependency tree includes avo (RubyGems) <= 3.32.0, fixed in 3.32.1; avo (RubyGems) >= 4.0.0.beta.1, < 4.0.0.beta.51, fixed in 4.0.0.beta.51. Check your lockfile, including transitive dependencies.

What it is

Avo is a framework to create admin panels for Ruby on Rails apps. Prior to 3.32.1 and 4.0.0.beta.51, Avo's association attach workflow checks attach ? in the UI and GET /resources/:resource/:id/:related/new path, but the actual write endpoint, POST /resources/:resource/:id/:related, does not run the same authorization check before mutating the association through Avo::AssociationsController#create. An authenticated low-privileged Avo user can bypass hidden or disabled attach controls and directly attach related records to a parent record by sending a crafted POST request, which can lead to privilege escalation and cross-tenant data exposure where associations represent authorization-bearing relationships. This issue is fixed in versions 3.32.1 and 4.0.0.beta.51.

From the GitHub advisory

Summary

A critical missing authorization flaw exists in Avo's association attach workflow. The UI and GET /resources/:resource/:id/:related/new path can check attach ?, but the actual write endpoint, POST /resources/:resource/:id/:related, does not run the same authorization check before mutating the association.

As a result, an authenticated low-privileged Avo user can bypass hidden/disabled attach controls and directly attach related records to a parent record by sending a crafted POST request. In applications where associations represent teams, tenants, roles, projects, users, memberships, ownership, or other authorization-bearing relationships, this can lead to privilege escalation and cross-tenant data exposure.

Details

The association attach route writes relationships through Avo::AssociationsController#create:

Weakness type: CWE-639, CWE-862, CWE-863.

Product: avo (RubyGems) · Assigned by: GitHub_M · Published: 2026-07-17

Affected and fixed versions

Severity scores

• Attack vector: network (remotely reachable)
• Attack complexity: low
• Privileges required: low (any account)
• User interaction: none
• Scope: changed (can affect other components)

CVSS measures how bad a flaw is if exploited, not how likely exploitation is. Read it next to the exploitation data below.

Exploitation and urgency

• CISA KEV: not listed (no confirmed in-the-wild exploitation known to CISA).
• CISA SSVC assessment: exploitation poc, automatable no, technical impact total.
• FIRST EPSS (model of 2026-10-21): 0.45% probability of exploitation activity in the next 30 days; percentile 38.

Timeline

• 2026-06-17 — GitHub-reviewed advisory GHSA-8fq9-273g-6mrg published (critical)
• 2026-07-17 — CVE record published by GitHub_M
• 2026-07-20 — CVE record last updated

Sources and references

Original sources first. Version and fix details should always be confirmed against the vendor or project advisory.

Vulnerability records

1. GitHub Advisory GHSA-8fq9-273g-6mrg github.com
2. CVE.org record CVE-2026-55518 cve.org
3. NVD entry CVE-2026-55518 nvd.nist.gov

Other references from the CVE record and advisory

1. github.com github.com
2. github.com github.com
3. github.com github.com
4. github.com github.com
5. github.com github.com

Critical RubyGems advisories · Today’s Security Digest