Light leaks happen when the body of a camera isn’t light-tight and some light leaks in and overexposes the film (or sensor). It’s often used along with vintage effects to give a sense of warmth and a reminiscense of analog cameras.
Create a new texture in your world, call it “leaks” or something descriptive. Delete the default checker node and add a clouds node. Set it to “Greyscale” and “Hard”, “Depth” to 0 and “Size” to 0.0001.
Add a Texture node ( input > texture ) and select our leaks texture. Set the scale on X to something big like 0.5 and the Y scale to about 0.1 (ignore Z). Now make a new colorRamp node (color > colorRamp) and plug the color output of our texture into it. Flip the positions of the black and white colors by clicking the icon with two arrows, this will invert the colors of our texture. Ctrl+click on this node to add a viewer to it and activate the backdrop option. Start moving the sliders together in the colorRamp to crush the contrast in our texture. Once you’re happy with it, add a Blur node. Set it to Fast Gaussian and it’s values to something like 150.
Add a mix node, set it to add and mix your image with a bright orange color. Use the output of the Blur as your factor.