PYSEC-2026-565 
## Summary

**A chain of vulnerabilities in vLLM allow Remote Code Execution (RCE):**

1. **Info Leak** - PIL error messages expose memory addresses, bypassing ASLR
2. **Heap Overflow** - JPEG2000 decoder in OpenCV/FFmpeg has a heap overflow that lets us hijack code execution

**Result:** Send a malicious video URL to vLLM Completions or Invocations **for a video model** -> Execute arbitrary commands on the server

Completely default vLLM instance directly from pip, or docker, does not have authentication so "None" privileges are required, but even with non-default api-key enabled configuration this exploit is feasible through invocations route that allows payload to execute pre-auth. 

Example heap target is provided, other heap targets can be exploited as well to achieve rce. Leak allows for simple ASLR bypass. Leak + heap overflow achieves RCE on versions prior to 0.14.1. 

 Deployments not serving a video model are not affected.

---


## 1. Vulnerability Overview

### 1.1 The Bug: JPEG2000 cdef Box Heap Overflow
The JPEG2000 decoder used by OpenCV (cv2) honors a `cdef` box that can remap color channels. When Y (luma) is mapped into the U (chroma) plane buffer, the decoder writes a large Y plane into the smaller U buffer, causing a heap overflow.

**Root Cause**
- `cdef` allows channel remapping (e.g., Y→U, U→Y).
- Y plane size: `W×H`; U plane size: `(W/2)×(H/2)`.
 - Overflow size = `W×H - (W/2×H/2)` = `0.75 × W × H` bytes.

**Example (150×64)**
 - Y plane: 150×64 = 9,600 bytes  
- U plane: 75×32 = 2,400 bytes  
- Overflow: 7,200 bytes past the U buffer

### 1.2 Malicious cdef Box
```
Offset  Size  Field           Value
0       4     Box Length      0x00000016 (22 bytes)
4       4     Box Type        'cdef'
8       2     N (channels)    0x0003
10      2     Channel 0 Cn    0x0000 (Y channel)
12      2     Channel 0 Typ   0x0000 (color)
14      2     Channel 0 Asoc  0x0002 (→ maps Y into U plane)
16      2     Channel 1 Cn    0x0001 (U channel)
18      2     Channel 1 Typ   0x0000 (color)
20      2     Channel 1 Asoc  0x0001 (→ maps U into Y plane)
22      2     Channel 2 Cn    0x0002 (V channel)
24      2     Channel 2 Typ   0x0000 (color)
26      2     Channel 2 Asoc  0x0003 (→ maps V plane)
```
Key control: `Asoc=2` for channel 0 forces Y data into the U buffer, triggering the overflow.

---

## Vulnerable Code Chain

### 1) Entry: vLLM accepts a remote `video_url` and downloads raw bytes

vLLM’s OpenAI-compatible API supports a `video_url` content part:
 
```python
class VideoURL(TypedDict, total=False):
    url: Required[str]

 class ChatCompletionContentPartVideoParam(TypedDict, total=False):
    video_url: Required[VideoURL]
    type: Required[Literal["video_url"]]
```

Source: `src/vllm/entrypoints/chat_utils.py`.
 
When the URL is HTTP(S), vLLM downloads it as **raw bytes** and passes the bytes into the modality loader:

```python
if url_spec.scheme.startswith("http"):
    data = connection.get_bytes(url, timeout=fetch_timeout, allow_redirects=...)
    return media_io.load_bytes(data)
```

Source: `src/vllm/multimodal/utils.py` (`MediaConnector.load_from_url`).

---

### 2) Decode: vLLM uses OpenCV (cv2) VideoCapture on an in-memory byte stream

The default video backend is OpenCV, and it constructs `cv2.VideoCapture` over a `BytesIO` buffer containing the downloaded bytes:

```python
backend = cls().get_cv2_video_api()
cap = cv2.VideoCapture(BytesIO(data), backend, [])
if not cap.isOpened():
    raise ValueError("Could not open video stream")
```

Source: `src/vllm/multimodal/video.py` (`OpenCVVideoBackend.load_bytes`).
 
The backend is selected from OpenCV’s stream-buffered backends registry:

```python
 import cv2.videoio_registry as vr
for backend in vr.getStreamBufferedBackends():
    if vr.hasBackend(backend) and ...:
        api_pref = backend
        break
 return api_pref
```

Source: `src/vllm/multimodal/video.py` (`OpenCVVideoBackend.get_cv2_video_api`).
 
**Implication**: vLLM is delegating container parsing + codec decode to OpenCV’s Video I/O stack (which, in typical builds, is backed by FFmpeg for MOV/MP4 and codecs like JPEG2000).

---

### 3) The actual overflow: Y (full-res) written into U (quarter-res)

When the decoder honors the remap and writes Y into the U-plane buffer, it writes **too many bytes**:

- Y plane bytes: \(W \times H\)
- U plane bytes: \((W/2) \times (H/2)\)
- Overflow bytes: \(W \times H - (W/2 \times H/2) = 0.75 \times W \times H\)

Concrete example tried (150×64):
 
- **Y**: \(150 \times 64 = 9600\) bytes  
- **U**: \(75 \times 32 = 2400\ ) bytes  
- **Overflow**: \(9600 - 2400 = 7200\) bytes past the end of the U allocation

This is a **heap buffer overflow** into whatever allocations follow the U-plane buffer in the decoder’s heap layout (structures, metadata, other buffers, etc.). The exact victims depend on build + runtime allocator layout.

---

 ## The Exploit Chain 

### Vuln 1: PIL BytesIO Address Leak (ASLR Bypass)

 When you send an **invalid image** to vLLM's multimodal endpoint, PIL throws an error like:

```
cannot identify image file <_io.BytesIO object at 0x7a95e299e750>
                                                   ^^^^^^^^^^^^^^^^
                                                   LEAKED ADDRESS!
```

vLLM returns this error to the client, **leaking a heap address**. This address is ~10.33 GB before `libc` in memory. With this leak, we reduce ASLR from **4 billion guesses to ~8 guesses**.

### Vuln 2: JPEG2000 cdef Heap Overflow (RCE)

vLLM uses **OpenCV (cv2)** to decode videos. OpenCV bundles **FFmpeg 5.1.x** which has a heap overflow in the JPEG2000 decoder. The OpenCV is used for video decoding so if we build a video from JPEG2000 frames it will reach the vuln:

```
vLLM API Request to Completions/Invocation
     ↓
OpenCV cv2.VideoCapture()
     ↓
FFmpeg 5.1 (bundled in OpenCV)
     ↓
JPEG2000 decoder (libopenjp2)
     ↓
HEAP OVERFLOW via malicious "cdef" box
     ↓
Overwrite function pointer → RCE!
```

**How the overflow works:**
 - JPEG2000 has a `cdef` box that remaps color channels
- We remap Y (luma) into the U (chroma) buffer
- Y plane = 9,600 bytes, U plane = 2,400 bytes
- On small geometry like 150x64 pixel image we get **7,200 bytes overflow** past the U buffer. We can grow that exponentially by making bigger images. 
- This overwrites an `AVBuffer` structure containing a `free()` function pointer. This could be any function pointer or other targets. 
- We set `free = system()` and `opaque = "command string" `
- When the buffer is freed → `system("our command")` executes

---

## vLLM Attack Surface

### Affected Endpoints

Both multimodal endpoints are vulnerable:
 
```
POST /v1/chat/completions     (with video_url in content)
POST /v1/invocations          (with video_url in content)
```

### Request Flow

```
1. Attacker sends request with video_url pointing to malicious .mov file
2. vLLM fetches the video from the URL
3. vLLM passes video bytes to cv2.VideoCapture()
4. OpenCV's bundled FFmpeg decodes JPEG2000 frames
5. Malicious cdef box triggers heap overflow
 6. AVBuffer.free pointer overwritten with system()
7. When buffer is released → system("attacker command") executes
```

---

## Versions Affected

| Component | Version | Notes |
|-----------|---------|-------|
| vLLM | >= 0.8.3, < 0.14.1 | Default config vulnerable when serving a video model |
| OpenCV (cv2) | 4.x with FFmpeg bundle | Bundled FFmpeg is vulnerable |
| FFmpeg | 5.1.x (bundled) | JPEG2000 cdef overflow |
| libopenjp2 | 2.x | Honors malicious cdef box |

---

## Fixes
 
* https://github.com/vllm-project/vllm/pull/31987
* https://github.com/vllm-project/vllm/pull/32319
 * https://github.com/vllm-project/vllm/pull/32668
CVSS v3.1 score: 9.8