• Vulnerability : A missing authorization check on an internal API allowed any user to control critical database migration processes
• Business Impact : High risk of data corruption and denial-of-service (DoS),
  risking system-wide outages and erosion of customer trust
• Discovery : Identified by the DepthFirst platform, which analyzes application
  context to find exploitable vulnerabilities that are invisible to traditional scanners
• Status : Patched same-day as reported. Tracked as CVE-2025-59305
• Key Insight : Authorization flaws are a systemic blind spot for traditional
  security tools, and the rapid adoption of AI code assistants is now scaling this
  hidden risk across applications

The Hidden Threat in Web Applications

Some of the most powerful and sensitive operations in a modern web application happen in the background: data migrations, report generation, internal maintenance. Because these processes run behind the scenes, their access controls are often overlooked.

We recently discovered a textbook example of this in Langfuse, a leading
open-source LLM engineering platform with 16k stars on Github. A subtle flaw in its background job controls allowed any authenticated user to access highly sensitive administrative functions, creating a significant business risk.

The Langfuse team remediated the threat the same day it was reported. This case study breaks down the vulnerability to illustrate a crucial lesson for every engineering leader: the simplest oversight can sometimes create the biggest threat.

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Anatomy of the Vulnerability

A single missing authorization check created two severe, high-impact business risks.

Potential Impact: Data Corruption and Denial of Service

While the code flaw was small, its potential impact was serious. An attacker with a standard, low-privilege account could cause:
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• Data Corruption : By restarting a data migration while it was already in
  progress, an attacker could create a race condition. This could leave
  the database in a partially-migrated, inconsistent state, leading to silent data loss, incorrect analytics, and an erosion of customer trust.

• Denial of Service (DoS) : An attacker could repeatedly trigger multiple
  resource-intensive migration jobs at once. This would consume significant
  database and worker resources, overwhelming the system and leading to a platform-wide outage for all customers. For any SaaS business, this means SLA violations, reputational damage, and engineering time diverted to firefighting.

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Technical Root Cause

The vulnerability was located in the tRPC router responsible for background
migrations. The endpoints to retry, list, and view the status of migrations were secured using tRPC’s protectedProcedure.
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This middleware correctly performs authentication, ensuring a user has a valid session. However, it does not perform authorization. It fails to check if that user has administrative privileges. The DepthFirst system identified that, because Langfuse allows self-serve sign-ups, any user could register and gain immediate access to these sensitive administrative controls.
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The vulnerable code in background-migrations-router.ts:
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// The endpoint for retrying a migration job
// This checks for a valid session (authentication)
retry: protectedProcedure
// but not for an admin role (authorization).
.input(z.object({ name: z.string() }))
.mutation(async ({ input, ctx }) => {
// Logic to restart a sensitive migration
}),

Exploit Scenario

An attacker could have exploited this with minimal effort.

1. Register : Sign up for a standard account on the target Langfuse instance.
2. Discover : Call the backgroundMigrations.all API endpoint to list all ongoing and available migration jobs.
3. Attack : Call the backgroundMigrations.retry endpoint on a critical job,
   restarting the process and triggering the data corruption or DoS conditions
   described above.
   ‍

curl -s -X POST 'https://<tenant>/api/trpc/backgroundMigrations.retry' \
-H 'Content-Type: application/json' \
-b '<AUTH_COOKIES>' \
--data '{"input":{"name":"<migration_name>"}}'

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The Broader Lesson: The AuthN vs. AuthZ Blind Spot

This vulnerability is a textbook example of a common blind spot in modern
development: confusing Authentication (AuthN) with Authorization (AuthZ).
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• Authentication asks : "Is this user logged in?"
• Authorization asks : "Is this logged-in user allowed to do this specific thing?"

This authorization flaw is an easy oversight for a human developer to make, and an even easier one for an AI assistant. LLMs write code by repeating the most common patterns they have learned. It sees that most API endpoints require a user to be logged in, so it correctly adds an authentication check. However, the LLM lacks the business context to know this specific API controls a sensitive process and requires a stricter authorization check. By repeating the common pattern, it inadvertently introduces a critical vulnerability.

Security teams are blind to the risk from these vulnerabilities because traditional
SAST scanners are unable to identify these flaws. They are pattern-matchers that can verify authentication is present, but they are architecturally incapable of
understanding the business logic that makes an admin API different from a user API.

At DepthFirst, we specialize in finding these exact kinds of contextual business logic flaws: the ones that are invisible to traditional tools but create the most significant business risk.

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The Fix & Coordinated Disclosure

We privately reported the vulnerability to the Langfuse team in accordance with their security policy. They responded promptly, validating the report and shipping a comprehensive fix the same day, demonstrating their commitment to security.

The solution was to implement a new adminProcedure that enforces role-based authorization, protecting users before the vulnerability could be exploited. In recognition of the report and our collaboration, the Langfuse team has acknowledged DepthFirst in their security Hall of Fame.

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Disclosure Timeline

• September 9, 2025 : Vulnerability privately reported to the Langfuse team
• September 9, 2025 : A comprehensive patch was deployed
• September 15, 2025 : Publicly disclosure of CVE-2025-59305

Is Your Code Exposed to a Similar Risk?

This case study highlights how a single, overlooked authorization check can create a critical security risk. These flaws, invisible to traditional scanners, may be hiding in your own applications.