vLLM: image EXIF Rotation & PNG tRNS Transparency Not Normalized, Causing Mismatch Between Model Input and Expectations
## Summary
Issue 1: EXIF orientation not normalized → The image orientation processed by the model differs from how humans view it, introducing interpretation bias.
Issue 2: PNG tRNS not explicitly flattened before converting to RGB → After conversion, transparent/semi-transparent pixels are rendered unexpectedly, making otherwise subtle overlay elements visible and distorting the input content. (This attack is similar to AlphaDog: RGBA handling is already correct in vLLM, but since tRNS permits RGB images, the correct processing path isn’t taken.)
Issue 3 : Pillow only loads the first frame when loading APNG or GIF files.
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## Root Cause
* **Rotation**: After opening an image, `ImageOps.exif_transpose` is not called to normalize EXIF orientation.
* **Transparency**: Only **RGBA→RGB** is flattened with a background; PNGs carrying **`tRNS`** in **`P`/`L`/`RGB + tRNS`** and other non-RGBA modes take the `image.convert("RGB")` path, which implicitly discards/remaps transparency semantics.
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## Affected Code
https://github.com/vllm-project/vllm/blob/16b37f3119918c1e5a39f303e0d0892c65c07a90/vllm/multimodal/image.py#L77-L84
https://github.com/vllm-project/vllm/blob/16b37f3119918c1e5a39f303e0d0892c65c07a90/vllm/multimodal/image.py#L37-L43
https://github.com/vllm-project/vllm/blob/16b37f3119918c1e5a39f303e0d0892c65c07a90/vllm/multimodal/image.py#L26-L34
> Current state: `ImageOps.exif_transpose` is not used. (Although the `rescale_image_size` function (
https://github.com/vllm-project/vllm/blob/main/vllm/multimodal/image.py#L14 https://github.com/vllm-project/vllm/blob/main/vllm/multimodal/image.py#L14) exists and includes a `transpose` parameter, I’ve found that it doesn’t seem to be called anywhere outside the `test` directory.）
> **Call order**: `_convert_image_mode` runs first; if the conditions are met, `convert_image_mode` is called.
>
> **Issue**: Only the “RGBA → RGB” path is explicitly flattened. `P`, `L`, or `RGB` with `tRNS` all fall back to `image.convert("RGB")`. For PNGs that include `tRNS`, `convert("RGB")` directly produces 24-bit RGB, leading to:
>
> * **`P` mode**: The transparent index becomes an actual RGB color (often black, white, or an undefined background), so transparency is lost.
> * **`L/LA` and `RGB + tRNS`**: `convert("RGB")` doesn’t composite against a chosen background first, so elements that relied on transparency to be hidden or softened become solid.
## Impact & Scope
* **Impact**: Pixels the model sees can diverge from operator expectations (due to orientation or transparency handling), potentially altering downstream reasoning.
* **Scope**: The image I/O and mode-conversion paths in `vllm/multimodal/image.py`. The existing **RGBA→RGB** flattening is correct; the issues center on **missing EXIF normalization** and **non-RGBA `tRNS` not being explicitly composited**.
## Case
EXIF：
http://qiniu.funxingzuo.top/exif_orient_180.jpg
tRNS:
http://qiniu.funxingzuo.top/hello.png
## Fix
A fix for this vulnerability was merged here:
https://github.com/vllm-project/vllm/pull/44974
