Player heads can use any texture from any minecraft.net site.

I'm marking this as private because I don't want people to use this to show adult content to children before it is fixed, and I also don't know if this has already been reported privately and therefore do not wish to interfere.

What I expected to happen:
When I gave myself a player skull with a texture hosted on the Minecraft blog or the Minecraft feedback site, it would not render that texture.

What actually happened was:
I was able to give myself a player skull with a texture hosted on the Minecraft blog, and another one hosted on the Minecraft feedback site, and both rendered. The latter would allow any user to upload an arbitrary image and render it to the skull.

In fact, the textures can be rendered in full HD, and if you size the images properly, avoid the texture layer that replaces transparency in the upper left 64x64 with black.

The reason this is problematic is because I am also able to upload whatever images I want to the feedback.minecraft.net without actually submitting feedback, and use them on player skulls. This could be used to show children adult content, for example.

Steps to reproduce arbitrary feedback.minecraft.net images:
1. Begin to submit new feedback in any category.
2. Use the "embed image" button and choose an image from your computer.
3. Right click the image and copy its link.
4. Regardless of whether you submit your feedback, this image link is persistent.

Steps to reproduce the in-game bug:
1. Copy an image link from the blog or feedback site (instructions for the latter is shown above, the former is trivial when you right click an image).
2. Use this string as reference for step 3:

{"textures":{"SKIN":{"url":""}}}

3. Inside of the "url" property's value string, paste the URL.
4. Encode this JSON into base 64.
5. Generate a random UUID.
6. Use this string as reference for steps 7 and 8:

give @p minecraft:player_head{SkullOwner:{Id:"",Properties:{textures:[{Value:""}]}}}

7. Inside of the Id property's string value, paste the random UUID.
8. Inside of the Value property's string, paste the base 64 string.
9. Run this command in a command block. Your chosen image will used for the faces of the head, in perfect HD quality.

Here is a working example using the MINECON Earth blog post thumbnail:

give @p minecraft:player_head{SkullOwner:{Id:"fef7d32c-4c9c-443f-8679-4c76a999fb79",Properties:{textures:[{Value:"eyJ0ZXh0dXJlcyI6eyJTS0lOIjp7InVybCI6Imh0dHBzOi8vY29tbXVuaXR5LWNvbnRlbnQtYXNzZXRzLm1pbmVjcmFmdC5uZXQvdXBsb2FkLzQ3MWIzNTdiZTJmYzlhODRiYTc0M2VjZmU3Y2Y1MTMzLVRpbGVUd29CeVR3by5qcGcifX19"}]}}}

Here is a working example using an image uploaded to the feedback site:

give @p minecraft:player_head{SkullOwner:{Id:"b73aecc1-dc60-4473-9c64-ad5106b38a9b",Properties:{textures:[{Value:"eyJ0ZXh0dXJlcyI6eyJTS0lOIjp7InVybCI6Imh0dHBzOi8vZmVlZGJhY2subWluZWNyYWZ0Lm5ldC9oYy91c2VyX2ltYWdlcy91NVhEOWg5dUpvZlRUUi1kakVzU1NnLnBuZyJ9fX0="}]}}}

A screenshot of both placed in-game on 1.13.2-pre1 is attached.