GHSA-8jr5-v98p-w75m vLLM: image EXIF Rotation & PNG tRNS Transparency Not Normalized, Causing Mismatch Between Model Input and Expectations

• PyPI vllm: >= 0.11.0, < 0.24.0 → fixed in 0.24.0

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. --- 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. --- 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) 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