diff --git a/Security-audit.md b/Security-audit.md
index ed35bfe..e77bb46 100644
--- a/Security-audit.md
+++ b/Security-audit.md
@@ -151,13 +151,17 @@ These findings are prioritized below. Severity describes the potential security
 
 ### SA-010: Avatar validation trusts the client MIME type
 
-**Severity:** Medium  
-**Evidence:** `upload_avatar()` in `backend/app/api/user_config.py` selects the extension from `UploadFile.content_type` and writes the bytes without decoding or inspecting the image.  
+**Severity:** Medium, remediated in current worktree
+**Evidence before remediation:** `upload_avatar()` in `backend/app/api/user_config.py` selected the extension from `UploadFile.content_type` and wrote the bytes without decoding or inspecting the image.
 **Impact:** A user can upload arbitrary content while labeling it as an image. Public serving may cause unexpected content handling, bandwidth consumption, or browser-side exposure. The current random user-ID filename reduces path traversal risk, but it does not establish that the content is a safe image.
 
-**Recommendation:** Decode images with a hardened image library, enforce pixel and dimension limits, re-encode to a safe format, strip metadata, and serve with a fixed safe `Content-Type` and `X-Content-Type-Options: nosniff`. Consider a separate media origin and a stricter content security policy.
+**Current state:** Avatar bytes are limited to 2 MB, decoded and verified with Pillow, checked against a 25-megapixel limit, fully loaded, converted to RGBA, and re-encoded as server-generated PNG. The client MIME type is used only as an initial allow-list check; invalid image content is rejected. Static serving uses the generated `.png` extension and therefore returns `image/png`.
 
-**Priority:** Medium.
+**Residual impact:** Add `X-Content-Type-Options: nosniff` at the application or reverse-proxy layer and consider a separate media origin for stronger isolation.
+
+**Recommendation:** Keep Pillow current, monitor decompression-bomb and upload failures, and preserve fixed image content types and dimensions. Add a stricter media-origin policy if avatars become a higher-risk feature.
+
+**Priority:** Completed in code; response-header and media-isolation hardening remain.
 
 ### SA-011: Error details can disclose infrastructure information
 
@@ -250,7 +254,7 @@ Before production exposure:
 - [x] Disable direct host publication of the application port in production.
 - [ ] Configure HTTPS, HSTS, CSP, Referrer-Policy, frame protections, `nosniff`, and trusted hosts.
 - [ ] Define a restrictive CORS policy or leave CORS disabled.
-- [ ] Add global request-size limits and hardened image decoding/re-encoding.
+- [x] Add global request-size limits and hardened image decoding/re-encoding.
 - [x] Add OTP recovery codes and a protected recovery workflow.
 - [ ] Remove or implement refresh-token behavior.
 - [ ] Add security audit events and centralized redacted logging.
diff --git a/VIBE/CHAT_LOG.md b/VIBE/CHAT_LOG.md
index 67812ec..b3e8fee 100644
--- a/VIBE/CHAT_LOG.md
+++ b/VIBE/CHAT_LOG.md
@@ -1,5 +1,11 @@
 # Chat Log
 
+### User
+Solve SA-010: Avatar validation trusts the client MIME type.
+
+### Assistant outcome
+Completed avatar hardening with Pillow: uploads are size-limited, decoded and verified rather than trusted by MIME type, bounded to 25 megapixels, fully loaded, normalized to PNG, and served under a fixed `.png` path. Tests now verify PNG normalization and rejection of fake image bytes. SA-010 is marked remediated with `nosniff` and media-origin isolation retained as deployment hardening.
+
 ### User
 Run full backend and frontend test suites.
 
diff --git a/VIBE/PROMPTS.md b/VIBE/PROMPTS.md
index e7a64ce..10ffdde 100644
--- a/VIBE/PROMPTS.md
+++ b/VIBE/PROMPTS.md
@@ -188,6 +188,7 @@
 183. Fix SA-009: TOTP enrollment has no recovery codes or reset workflow.
 183. Address SA-007 and use linklog.example.com as the default LINKLOG_PUBLIC_URL.
 184. Run full backend and frontend test suites.
+185. Solve SA-010: Avatar validation trusts the client MIME type.
 
 ## Future entries
 
diff --git a/backend/app/api/user_config.py b/backend/app/api/user_config.py
index 5c18690..af7b245 100644
--- a/backend/app/api/user_config.py
+++ b/backend/app/api/user_config.py
@@ -2,10 +2,13 @@
 ## SPDX-License-Identifier: GPL-3.0-or-later
 
 import json
+import warnings
+from io import BytesIO
 from datetime import datetime, timedelta, timezone
 from uuid import uuid4
 
 from fastapi import APIRouter, Depends, File, HTTPException, UploadFile
+from PIL import Image, UnidentifiedImageError
 from pydantic import BaseModel
 
 from backend.app.api.dependencies import get_current_user
@@ -19,6 +22,9 @@ from backend.app.services.secret_store import decrypt_secret, encrypt_secret
 
 router = APIRouter()
 
+MAX_AVATAR_BYTES = 2 * 1024 * 1024
+MAX_AVATAR_PIXELS = 25_000_000
+
 
 class UserConfigUpdate(BaseModel):
     bio: str | None = None
@@ -320,25 +326,33 @@ async def upload_avatar(
     avatar: UploadFile = File(...),
     user: dict = Depends(get_current_user),
 ):
-    allowed_types = {
-        'image/gif': '.gif',
-        'image/jpeg': '.jpg',
-        'image/png': '.png',
-        'image/webp': '.webp',
-    }
-    suffix = allowed_types.get(avatar.content_type or '')
-    if suffix is None:
+    if avatar.content_type not in {'image/gif', 'image/jpeg', 'image/png', 'image/webp'}:
         raise HTTPException(status_code=415, detail='Avatar must be a PNG, JPEG, GIF, or WebP image')
 
-    contents = await avatar.read(2 * 1024 * 1024 + 1)
-    if len(contents) > 2 * 1024 * 1024:
+    contents = await avatar.read(MAX_AVATAR_BYTES + 1)
+    if len(contents) > MAX_AVATAR_BYTES:
         raise HTTPException(status_code=413, detail='Avatar must be 2 MB or smaller')
 
-    avatar_path = AVATARS_DIR / f'{user["id"]}{suffix}'
+    try:
+        with warnings.catch_warnings():
+            warnings.simplefilter('error', Image.DecompressionBombWarning)
+            with Image.open(BytesIO(contents)) as image:
+                if image.width * image.height > MAX_AVATAR_PIXELS:
+                    raise HTTPException(status_code=413, detail='Avatar dimensions are too large')
+                image.verify()
+            with Image.open(BytesIO(contents)) as image:
+                image.load()
+                normalized = image.convert('RGBA')
+    except HTTPException:
+        raise
+    except (Image.DecompressionBombError, Image.DecompressionBombWarning, UnidentifiedImageError, OSError, ValueError) as error:
+        raise HTTPException(status_code=415, detail='Avatar content is not a valid image') from error
+
+    avatar_path = AVATARS_DIR / f'{user["id"]}.png'
+    normalized.save(avatar_path, format='PNG', optimize=True)
     for existing_path in AVATARS_DIR.glob(f'{user["id"]}.*'):
         if existing_path != avatar_path:
             existing_path.unlink(missing_ok=True)
-    avatar_path.write_bytes(contents)
     avatar_url = f'/media/{avatar_path.name}'
 
     with get_connection() as conn:
diff --git a/backend/requirements.txt b/backend/requirements.txt
index 172e798..17360c1 100644
--- a/backend/requirements.txt
+++ b/backend/requirements.txt
@@ -3,6 +3,7 @@ uvicorn==0.52.4
 pydantic==2.13.4
 jinja2==3.1.6
 python-multipart==0.0.20
+Pillow==11.3.0
 pytest==9.1.1
 httpx==0.28.1
 httpx2==2.12.0
diff --git a/backend/tests/test_user_config.py b/backend/tests/test_user_config.py
index a311ed0..0ee415b 100644
--- a/backend/tests/test_user_config.py
+++ b/backend/tests/test_user_config.py
@@ -1,7 +1,10 @@
 ## Copyright © 2026 Olaf Kolkman
 ## SPDX-License-Identifier: GPL-3.0-or-later
 
+from io import BytesIO
+
 from fastapi.testclient import TestClient
+from PIL import Image
 from unittest.mock import patch
 
 from backend.app.main import app
@@ -68,15 +71,29 @@ def test_user_config_api_and_profile_page():
         'new_password': 'secret123',
     }, headers=bob_headers).status_code == 200
 
+    image_buffer = BytesIO()
+    Image.new('RGB', (2, 2), 'red').save(image_buffer, format='JPEG')
     upload_response = client.post(
         '/api/user/avatar',
         headers=headers,
-        files={'avatar': ('avatar.png', b'fake-png-data', 'image/png')},
+        files={'avatar': ('avatar.jpg', image_buffer.getvalue(), 'image/jpeg')},
     )
     assert upload_response.status_code == 200
     avatar_url = upload_response.json()['avatar_url']
     assert avatar_url.startswith('/media/user-1.png')
-    assert client.get(avatar_url).content == b'fake-png-data'
+    stored_avatar = client.get(avatar_url)
+    assert stored_avatar.status_code == 200
+    assert stored_avatar.headers['content-type'] == 'image/png'
+    with Image.open(BytesIO(stored_avatar.content)) as image:
+        assert image.format == 'PNG'
+        assert image.size == (2, 2)
+
+    rejected_upload = client.post(
+        '/api/user/avatar',
+        headers=headers,
+        files={'avatar': ('avatar.png', b'fake-png-data', 'image/png')},
+    )
+    assert rejected_upload.status_code == 415
 
     updated_profile = client.get('/api/user/me', headers=headers).json()
     assert updated_profile['avatar_url'] == avatar_url
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