diff --git a/apps/sim/executor/utils/file-tool-processor.ts b/apps/sim/executor/utils/file-tool-processor.ts
index 1fa86ffc731..eabac4b35c8 100644
--- a/apps/sim/executor/utils/file-tool-processor.ts
+++ b/apps/sim/executor/utils/file-tool-processor.ts
@@ -138,7 +138,7 @@ export class FileToolProcessor {
       }
 
       if (!buffer && data.url) {
-        buffer = await downloadFileFromUrl(data.url)
+        buffer = await downloadFileFromUrl(data.url, { userId: context.userId })
       }
 
       if (buffer) {
diff --git a/apps/sim/lib/execution/files.ts b/apps/sim/lib/execution/files.ts
index c0b69446fcf..e23582aba18 100644
--- a/apps/sim/lib/execution/files.ts
+++ b/apps/sim/lib/execution/files.ts
@@ -57,7 +57,7 @@ export async function processExecutionFile(
 
   if (file.type === 'url' && file.data) {
     const { downloadFileFromUrl } = await import('@/lib/uploads/utils/file-utils.server')
-    const buffer = await downloadFileFromUrl(file.data)
+    const buffer = await downloadFileFromUrl(file.data, { userId })
 
     if (buffer.length > MAX_FILE_SIZE) {
       const fileSizeMB = (buffer.length / (1024 * 1024)).toFixed(2)
diff --git a/apps/sim/lib/knowledge/documents/document-processor.ts b/apps/sim/lib/knowledge/documents/document-processor.ts
index 33ee06eb11b..63f0bb7eae4 100644
--- a/apps/sim/lib/knowledge/documents/document-processor.ts
+++ b/apps/sim/lib/knowledge/documents/document-processor.ts
@@ -295,7 +295,7 @@ async function parseDocument(
   if (isPDF && (hasAzureMistralOCR || hasMistralOCR)) {
     if (hasAzureMistralOCR) {
       logger.info(`Using Azure Mistral OCR: ${filename}`)
-      return parseWithAzureMistralOCR(fileUrl, filename, mimeType)
+      return parseWithAzureMistralOCR(fileUrl, filename, mimeType, userId)
     }
 
     if (hasMistralOCR) {
@@ -305,7 +305,7 @@ async function parseDocument(
   }
 
   logger.info(`Using file parser: ${filename}`)
-  return parseWithFileParser(fileUrl, filename, mimeType)
+  return parseWithFileParser(fileUrl, filename, mimeType, userId)
 }
 
 async function handleFileForOCR(
@@ -321,7 +321,7 @@ async function handleFileForOCR(
     if (mimeType === 'application/pdf') {
       logger.info(`handleFileForOCR: Downloading external PDF to check page count`)
       try {
-        const buffer = await downloadFileWithTimeout(fileUrl)
+        const buffer = await downloadFileWithTimeout(fileUrl, userId)
         logger.info(`handleFileForOCR: Downloaded external PDF: ${buffer.length} bytes`)
         return { httpsUrl: fileUrl, buffer }
       } catch (error) {
@@ -340,7 +340,7 @@ async function handleFileForOCR(
 
   logger.info(`Uploading "${filename}" to cloud storage for OCR`)
 
-  const buffer = await downloadFileWithTimeout(fileUrl)
+  const buffer = await downloadFileWithTimeout(fileUrl, userId)
 
   logger.info(`Downloaded ${filename}: ${buffer.length} bytes`)
 
@@ -380,11 +380,11 @@ async function handleFileForOCR(
   }
 }
 
-async function downloadFileWithTimeout(fileUrl: string): Promise<Buffer> {
-  return downloadFileFromUrl(fileUrl, TIMEOUTS.FILE_DOWNLOAD)
+async function downloadFileWithTimeout(fileUrl: string, userId?: string): Promise<Buffer> {
+  return downloadFileFromUrl(fileUrl, { timeoutMs: TIMEOUTS.FILE_DOWNLOAD, userId })
 }
 
-async function downloadFileForBase64(fileUrl: string): Promise<Buffer> {
+async function downloadFileForBase64(fileUrl: string, userId?: string): Promise<Buffer> {
   if (/^data:/i.test(fileUrl)) {
     const [, base64Data] = fileUrl.split(',')
     if (!base64Data) {
@@ -393,7 +393,7 @@ async function downloadFileForBase64(fileUrl: string): Promise<Buffer> {
     return Buffer.from(base64Data, 'base64')
   }
   if (/^https?:\/\//i.test(fileUrl)) {
-    return downloadFileWithTimeout(fileUrl)
+    return downloadFileWithTimeout(fileUrl, userId)
   }
   throw new Error('Unsupported fileUrl scheme: only data: URIs and http(s):// URLs are allowed')
 }
@@ -468,7 +468,12 @@ async function makeOCRRequest(
   }
 }
 
-async function parseWithAzureMistralOCR(fileUrl: string, filename: string, mimeType: string) {
+async function parseWithAzureMistralOCR(
+  fileUrl: string,
+  filename: string,
+  mimeType: string,
+  userId?: string
+) {
   validateOCRConfig(
     env.OCR_AZURE_API_KEY,
     env.OCR_AZURE_ENDPOINT,
@@ -476,7 +481,7 @@ async function parseWithAzureMistralOCR(fileUrl: string, filename: string, mimeT
     'Azure Mistral OCR'
   )
 
-  const fileBuffer = await downloadFileForBase64(fileUrl)
+  const fileBuffer = await downloadFileForBase64(fileUrl, userId)
 
   if (mimeType === 'application/pdf') {
     const pageCount = await getPdfPageCount(fileBuffer)
@@ -485,7 +490,7 @@ async function parseWithAzureMistralOCR(fileUrl: string, filename: string, mimeT
         `PDF has ${pageCount} pages, exceeds Azure OCR limit of ${MISTRAL_MAX_PAGES}. ` +
           `Falling back to file parser.`
       )
-      return parseWithFileParser(fileUrl, filename, mimeType)
+      return parseWithFileParser(fileUrl, filename, mimeType, userId)
     }
     logger.info(`Azure Mistral OCR: PDF page count for ${filename}: ${pageCount}`)
   }
@@ -529,7 +534,7 @@ async function parseWithAzureMistralOCR(fileUrl: string, filename: string, mimeT
     })
 
     logger.info(`Falling back to file parser: ${filename}`)
-    return parseWithFileParser(fileUrl, filename, mimeType)
+    return parseWithFileParser(fileUrl, filename, mimeType, userId)
   }
 }
 
@@ -589,7 +594,7 @@ async function parseWithMistralOCR(
     })
 
     logger.info(`Falling back to file parser: ${filename}`)
-    return parseWithFileParser(fileUrl, filename, mimeType)
+    return parseWithFileParser(fileUrl, filename, mimeType, userId)
   }
 }
 
@@ -773,7 +778,12 @@ async function processMistralOCRInBatches(
   }
 }
 
-async function parseWithFileParser(fileUrl: string, filename: string, mimeType: string) {
+async function parseWithFileParser(
+  fileUrl: string,
+  filename: string,
+  mimeType: string,
+  userId?: string
+) {
   try {
     let content: string
     let metadata: FileParseMetadata = {}
@@ -781,7 +791,7 @@ async function parseWithFileParser(fileUrl: string, filename: string, mimeType:
     if (/^data:/i.test(fileUrl)) {
       content = await parseDataURI(fileUrl, filename, mimeType)
     } else if (/^https?:\/\//i.test(fileUrl)) {
-      const result = await parseHttpFile(fileUrl, filename, mimeType)
+      const result = await parseHttpFile(fileUrl, filename, mimeType, userId)
       content = result.content
       metadata = result.metadata || {}
     } else {
@@ -820,9 +830,10 @@ async function parseDataURI(fileUrl: string, filename: string, mimeType: string)
 async function parseHttpFile(
   fileUrl: string,
   filename: string,
-  mimeType?: string
+  mimeType?: string,
+  userId?: string
 ): Promise<{ content: string; metadata?: FileParseMetadata }> {
-  const buffer = await downloadFileWithTimeout(fileUrl)
+  const buffer = await downloadFileWithTimeout(fileUrl, userId)
 
   const extension = resolveParserExtension(filename, mimeType)
   const result = await parseBuffer(buffer, extension)
diff --git a/apps/sim/lib/knowledge/documents/service.ts b/apps/sim/lib/knowledge/documents/service.ts
index 307db69bc7c..bdfe3e5d557 100644
--- a/apps/sim/lib/knowledge/documents/service.ts
+++ b/apps/sim/lib/knowledge/documents/service.ts
@@ -515,7 +515,6 @@ export async function processDocumentAsync(
     // KB config + workspace billing + doc tags in one JOIN (was 3 SELECTs).
     const contextRows = await db
       .select({
-        userId: knowledgeBase.userId,
         workspaceId: knowledgeBase.workspaceId,
         chunkingConfig: knowledgeBase.chunkingConfig,
         embeddingModel: knowledgeBase.embeddingModel,
@@ -644,7 +643,12 @@ export async function processDocumentAsync(
           kbConfig.maxSize,
           kbConfig.overlap,
           kbConfig.minSize,
-          ctx.userId,
+          // Authorize the source file (and run OCR/processing) as the billed
+          // actor — the uploader when known, else the workspace billed account —
+          // the same principal embeddings are billed to. Using the KB owner here
+          // would authorize an attacker-supplied internal fileUrl against the
+          // owner, letting a KB write-member ingest a file only the owner can read.
+          billingUserId,
           ctx.workspaceId,
           rawConfig?.strategy,
           rawConfig?.strategyOptions
diff --git a/apps/sim/lib/uploads/utils/file-utils.server.ts b/apps/sim/lib/uploads/utils/file-utils.server.ts
index f0fb7a606a6..b5460c8440a 100644
--- a/apps/sim/lib/uploads/utils/file-utils.server.ts
+++ b/apps/sim/lib/uploads/utils/file-utils.server.ts
@@ -1,6 +1,6 @@
 'use server'
 
-import type { Logger } from '@sim/logger'
+import { createLogger, type Logger } from '@sim/logger'
 import { getErrorMessage } from '@sim/utils/errors'
 import { getMaxExecutionTimeout } from '@/lib/core/execution-limits'
 import {
@@ -25,6 +25,8 @@ import {
 import { verifyFileAccess } from '@/app/api/files/authorization'
 import type { UserFile } from '@/executor/types'
 
+const logger = createLogger('FileUtilsServer')
+
 /**
  * Result type for file input resolution
  */
@@ -138,19 +140,62 @@ export async function resolveFileInputToUrl(
 }
 
 /**
- * Download a file from a URL (internal or external)
- * For internal URLs, uses direct storage access (server-side only)
- * For external URLs, validates DNS/SSRF and uses secure fetch with IP pinning
+ * Options for {@link downloadFileFromUrl}.
+ */
+export interface DownloadFileFromUrlOptions {
+  /** Download timeout for external URLs. Defaults to the max execution timeout. */
+  timeoutMs?: number
+  /** Hard cap on the number of bytes read from the source. */
+  maxBytes?: number
+  /**
+   * Principal the download is performed on behalf of. Required to authorize
+   * internal (`/api/files/serve/...`) URLs: the resolved storage key is checked
+   * with {@link verifyFileAccess} before any bytes are read. Without it, internal
+   * URLs are rejected (fail closed) so a `/api/files/serve/` substring can never
+   * be treated as implicitly trusted.
+   */
+  userId?: string
+}
+
+/**
+ * Download a file from a URL (internal or external).
+ *
+ * For internal URLs, uses direct storage access (server-side only) after
+ * authorizing the resolved storage key against `userId`. Context is derived
+ * from the key via {@link inferContextFromKey}, never from a caller-controlled
+ * `?context=` query param — trusting the param would let a private key be
+ * labeled with a world-readable context (e.g. profile-pictures) so
+ * {@link verifyFileAccess} short-circuits to granted while the private object is
+ * still read. This mirrors how `/api/files/serve` resolves context.
+ *
+ * For external URLs, validates DNS/SSRF and uses secure fetch with IP pinning.
  */
 export async function downloadFileFromUrl(
   fileUrl: string,
-  timeoutMs = getMaxExecutionTimeout(),
-  maxBytes?: number
+  options: DownloadFileFromUrlOptions = {}
 ): Promise<Buffer> {
-  const { parseInternalFileUrl } = await import('./file-utils')
+  const { timeoutMs = getMaxExecutionTimeout(), maxBytes, userId } = options
 
   if (isInternalFileUrl(fileUrl)) {
-    const { key, context } = parseInternalFileUrl(fileUrl)
+    if (!userId) {
+      logger.warn('Internal file download denied: no userId provided', { fileUrl })
+      throw new Error('Access denied: internal file URL requires an authenticated user')
+    }
+
+    const key = extractStorageKey(fileUrl)
+    if (!key) {
+      logger.warn('Internal file download denied: could not resolve storage key', { fileUrl })
+      throw new Error('Access denied: could not resolve internal file key')
+    }
+
+    const context = inferContextFromKey(key)
+
+    const hasAccess = await verifyFileAccess(key, userId, undefined, context, false)
+    if (!hasAccess) {
+      logger.warn('Internal file download denied: access check failed', { key, context, userId })
+      throw new Error('Access denied: file not found or insufficient permissions')
+    }
+
     const { downloadFile } = await import('@/lib/uploads/core/storage-service')
     return downloadFile({ key, context, maxBytes })
   }
diff --git a/apps/sim/lib/uploads/utils/file-utils.test.ts b/apps/sim/lib/uploads/utils/file-utils.test.ts
index f5e3c45ebde..63793e0e144 100644
--- a/apps/sim/lib/uploads/utils/file-utils.test.ts
+++ b/apps/sim/lib/uploads/utils/file-utils.test.ts
@@ -2,7 +2,42 @@
  * @vitest-environment node
  */
 import { describe, expect, it } from 'vitest'
-import { isAbortError, isNetworkError } from '@/lib/uploads/utils/file-utils'
+import { isAbortError, isInternalFileUrl, isNetworkError } from '@/lib/uploads/utils/file-utils'
+
+describe('isInternalFileUrl', () => {
+  it('classifies relative serve paths as internal', () => {
+    expect(isInternalFileUrl('/api/files/serve/kb/123-file.pdf')).toBe(true)
+    expect(isInternalFileUrl('/api/files/serve/workspace/ws-1/file.txt?context=workspace')).toBe(
+      true
+    )
+  })
+
+  it('classifies absolute serve URLs as internal regardless of host', () => {
+    expect(isInternalFileUrl('https://www.sim.ai/api/files/serve/kb/x.pdf')).toBe(true)
+    expect(isInternalFileUrl('http://localhost:3000/api/files/serve/blob/kb/x')).toBe(true)
+    // Host is not used to gate (self-hosted/multi-domain); the storage sink authorizes.
+    expect(isInternalFileUrl('https://other-host/api/files/serve/workspace/v/x')).toBe(true)
+  })
+
+  it('does not match the marker outside the path (query/fragment)', () => {
+    expect(isInternalFileUrl('https://evil.com/x?next=/api/files/serve/secret')).toBe(false)
+    expect(isInternalFileUrl('https://evil.com/page#/api/files/serve/secret')).toBe(false)
+    expect(isInternalFileUrl('https://evil.com/redirect?u=/api/files/serve/kb/x')).toBe(false)
+  })
+
+  it('preserves traversal sequences so they survive downstream rejection', () => {
+    // Must stay internal (not normalized away) so the parse route applies its `..` check.
+    expect(isInternalFileUrl('https://attacker.com/api/files/serve/../../../etc/passwd')).toBe(true)
+    expect(isInternalFileUrl('/api/files/serve/../../app.js')).toBe(true)
+  })
+
+  it('returns false for non-internal and non-string inputs', () => {
+    expect(isInternalFileUrl('https://example.com/file.pdf')).toBe(false)
+    expect(isInternalFileUrl('data:text/plain;base64,abc')).toBe(false)
+    // @ts-expect-error verifying runtime guard
+    expect(isInternalFileUrl(undefined)).toBe(false)
+  })
+})
 
 describe('isAbortError', () => {
   it('returns true for AbortError-named errors', () => {
diff --git a/apps/sim/lib/uploads/utils/file-utils.ts b/apps/sim/lib/uploads/utils/file-utils.ts
index d11fdf1b70f..e99d83c0564 100644
--- a/apps/sim/lib/uploads/utils/file-utils.ts
+++ b/apps/sim/lib/uploads/utils/file-utils.ts
@@ -534,10 +534,33 @@ export function extractStorageKey(filePath: string): string {
 }
 
 /**
- * Check if a URL is an internal file serve URL
+ * Whether a URL targets the internal file-serve endpoint (`/api/files/serve/`).
+ *
+ * The marker is matched only in the URL's path component, so it cannot be
+ * smuggled through a query string or fragment (e.g.
+ * `https://evil.com/x?next=/api/files/serve/...`) to skip DNS/SSRF validation.
+ *
+ * The raw path is inspected without URL normalization on purpose: callers such
+ * as the files parse route rely on traversal sequences (`..`) surviving this
+ * check so they are rejected downstream rather than collapsed away. A path-only
+ * marker still classifies any host as internal (e.g.
+ * `https://other-host/api/files/serve/<key>`); cross-tenant reads are prevented
+ * at the storage sink by {@link verifyFileAccess}, not by host matching, which
+ * would break self-hosted and multi-domain deployments.
  */
 export function isInternalFileUrl(fileUrl: string): boolean {
-  return fileUrl.includes('/api/files/serve/')
+  if (typeof fileUrl !== 'string') {
+    return false
+  }
+
+  let path = fileUrl
+  const scheme = /^[a-z][a-z0-9+.-]*:\/\/[^/?#]*/i.exec(path)
+  if (scheme) {
+    path = path.slice(scheme[0].length)
+  }
+  path = path.split(/[?#]/, 1)[0]
+
+  return path.startsWith('/api/files/serve/')
 }
 
 /**