Strip metadata from processed images (closes #82)

Every output is exported with govips' StripMetadata, so it carries no
EXIF (GPS, serial numbers, embedded thumbnails), XMP, IPTC or ICC
profile, the orig format included: it is always re-encoded, and pixa
never serves the source bytes. The image is turned upright with
AutoRotate right after decoding, so dropping the orientation tag does
not leave it rotated, and a requested size applies to the upright
image. An image with an ICC profile is converted to sRGB before export.
No setting turns this off. README.md documents it.

Model: opus-5-5

README.md

@@ -111,6 +111,21 @@ with a private address can choose the address it is counted by through its own
 its own address is trusted too. Setting `trusted_proxies` to the proxy's own
 address closes this.
 
+### Image Metadata
+
+pixa decodes and re-encodes every image it serves, and removes all metadata from
+the output: EXIF (GPS position, camera make, model and serial number, capture
+time, embedded thumbnail), XMP, IPTC and the ICC colour profile. This cannot be
+turned off.
+
+- The `orig` format means the source's own format, not the source's bytes: an
+  `orig` image is re-encoded and stripped like any other.
+- An image with an EXIF orientation is turned upright first, so it displays the
+  same without the tag; a requested size applies to the upright image.
+- An image with an ICC profile is converted to sRGB first, since clients show an
+  image with no profile as sRGB. Colours outside sRGB, such as the most
+  saturated ones in a Display P3 photo, are clipped.
+
 ### Source Hosts
 
 Source hosts may be allowlisted in the configuration. Non-allowlisted

TODO.md

@@ -30,6 +30,12 @@ exhaustion
 
 # Completed Steps
 
+- 2026-09-28 strip metadata from processed images (closes #82): every output is
+  exported with govips' `StripMetadata`, so it carries no EXIF, XMP, IPTC or ICC
+  profile; the image is first turned upright with `AutoRotate` (before sizes are
+  worked out) and, when it has an ICC profile, converted to sRGB; the `orig`
+  format is re-encoded and stripped like any other, as pixa never serves the
+  source bytes; there is no setting to keep metadata; documented in `README.md`.
 - 2026-09-28 rate limit the login form (closes #66): `POST /` is limited to 5
   attempts per minute per client address, and an attempt over the limit is
   refused with 429 and a `Retry-After` header; the address is the one
@@ -251,7 +257,6 @@ exhaustion
 
 # Future Steps
 
-- P1: strip EXIF and other metadata from processed images (privacy)
 - P2: security
     - referer blacklist
     - per-IP rate limiting on the image routes

internal/imageprocessor/imageprocessor.go

@@ -161,6 +161,13 @@ func (p *ImageProcessor) Process(
 	}
 	defer img.Close()
 
+	// Turn the image upright now: encode strips the EXIF orientation tag,
+	// and sizes below must be worked out on the upright image.
+	err = img.AutoRotate()
+	if err != nil {
+		return nil, fmt.Errorf("failed to auto-rotate: %w", err)
+	}
+
 	// Get original dimensions
 	origWidth := img.Width()
 	origHeight := img.Height()
@@ -404,6 +411,21 @@ func (p *ImageProcessor) encode(
 		return nil, fmt.Errorf("%w: %s", ErrUnsupportedOutputFormat, format)
 	}
 
+	// Stripping drops the ICC profile as well, and clients show an image
+	// with no profile as sRGB, so convert to sRGB first. "srgb" names
+	// libvips' built-in profile; govips' own sRGB path variable is set on
+	// first use but read without a lock, so concurrent requests race on it.
+	if img.HasICCProfile() {
+		err := img.TransformICCProfileWithFallback("srgb", "srgb")
+		if err != nil {
+			return nil, fmt.Errorf("failed to convert to sRGB: %w", err)
+		}
+	}
+
+	// Drop EXIF, XMP, IPTC and the ICC profile. govips ignores this for
+	// GIF, which carries none of them.
+	params.StripMetadata = true
+
 	output, _, err := img.Export(&params)
 	if err != nil {
 		return nil, err

internal/imageprocessor/imageprocessor_internal_test.go

@@ -9,7 +9,9 @@ import (
 	"image/jpeg"
 	"image/png"
 	"io"
+	"math"
 	"os"
+	"slices"
 	"testing"
 
 	"github.com/davidbyttow/govips/v2/vips"
@@ -561,3 +563,152 @@ func TestImageProcessor_EncodeAVIF(t *testing.T) {
 
 	encodeAndCheck(t, FormatAVIF, 85, mimeAVIF)
 }
+
+// processAndDecode runs input through Process and decodes the output with
+// vips, so a test can inspect the image a client would receive.
+func processAndDecode(t *testing.T, input []byte, req *Request) *vips.ImageRef {
+	t.Helper()
+
+	result, err := New(Params{}).Process(
+		context.Background(), bytes.NewReader(input), req,
+	)
+	if err != nil {
+		t.Fatalf("Process() error = %v", err)
+	}
+
+	defer func() { _ = result.Content.Close() }()
+
+	data, err := io.ReadAll(result.Content)
+	if err != nil {
+		t.Fatalf("failed to read result: %v", err)
+	}
+
+	output, err := vips.NewImageFromBuffer(data)
+	if err != nil {
+		t.Fatalf("failed to decode output: %v", err)
+	}
+
+	t.Cleanup(output.Close)
+
+	return output
+}
+
+func TestImageProcessor_StripsEXIF(t *testing.T) {
+	t.Parallel()
+
+	// gps-exif.jpg carries GPS coordinates, a camera make, model and serial
+	// number, and a capture time.
+	input, err := os.ReadFile("testdata/gps-exif.jpg")
+	if err != nil {
+		t.Fatalf("failed to read test JPEG: %v", err)
+	}
+
+	fixture, err := vips.NewImageFromBuffer(input)
+	if err != nil {
+		t.Fatalf("failed to decode test JPEG: %v", err)
+	}
+
+	t.Cleanup(fixture.Close)
+
+	if !slices.Contains(fixture.GetFields(), "exif-ifd3-GPSLatitude") {
+		t.Fatal("testdata/gps-exif.jpg has no GPS latitude")
+	}
+
+	formats := []Format{
+		FormatJPEG, FormatPNG, FormatWebP, FormatAVIF, FormatGIF, FormatOriginal,
+	}
+
+	for _, format := range formats {
+		t.Run(string(format), func(t *testing.T) {
+			t.Parallel()
+
+			output := processAndDecode(t, input, &Request{Format: format})
+
+			if output.HasExif() {
+				t.Errorf("output has EXIF: %v", output.GetExif())
+			}
+		})
+	}
+}
+
+func TestImageProcessor_AppliesEXIFOrientation(t *testing.T) {
+	t.Parallel()
+
+	// orientation-6.jpg is stored 16x8, red on the left and blue on the
+	// right, with EXIF orientation 6 (turn 90 degrees clockwise to view).
+	// Upright it is 8x16, red on top and blue below.
+	input, err := os.ReadFile("testdata/orientation-6.jpg")
+	if err != nil {
+		t.Fatalf("failed to read test JPEG: %v", err)
+	}
+
+	tests := []struct {
+		name  string
+		size  Size
+		wantW int
+		wantH int
+	}{
+		{name: "original size", size: Size{}, wantW: 8, wantH: 16},
+		{name: "width only", size: Size{Width: 4}, wantW: 4, wantH: 8},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			t.Parallel()
+
+			output := processAndDecode(t, input, &Request{
+				Size:   tt.size,
+				Format: FormatPNG,
+			})
+
+			if output.Width() != tt.wantW || output.Height() != tt.wantH {
+				t.Fatalf("output is %dx%d, want %dx%d",
+					output.Width(), output.Height(), tt.wantW, tt.wantH)
+			}
+
+			top, err := output.GetPoint(tt.wantW/2, 0)
+			if err != nil {
+				t.Fatalf("GetPoint() error = %v", err)
+			}
+
+			bottom, err := output.GetPoint(tt.wantW/2, tt.wantH-1)
+			if err != nil {
+				t.Fatalf("GetPoint() error = %v", err)
+			}
+
+			if top[0] <= top[2] || bottom[2] <= bottom[0] {
+				t.Errorf("top pixel = %v, bottom pixel = %v, want red above blue",
+					top, bottom)
+			}
+		})
+	}
+}
+
+func TestImageProcessor_ConvertsWideGamutToSRGB(t *testing.T) {
+	t.Parallel()
+
+	// display-p3.jpg is a flat 8x8 image with the Display P3 profile
+	// embedded, filled with Display P3 (234, 51, 35), which is sRGB red.
+	input, err := os.ReadFile("testdata/display-p3.jpg")
+	if err != nil {
+		t.Fatalf("failed to read test JPEG: %v", err)
+	}
+
+	output := processAndDecode(t, input, &Request{Format: FormatPNG})
+
+	if output.HasICCProfile() {
+		t.Error("output has an ICC profile")
+	}
+
+	pixel, err := output.GetPoint(4, 4)
+	if err != nil {
+		t.Fatalf("GetPoint() error = %v", err)
+	}
+
+	want := []float64{255, 0, 0}
+	for i := range want {
+		if math.Abs(pixel[i]-want[i]) > 5 {
+			t.Fatalf("pixel = %v, want within 5 of %v", pixel, want)
+		}
+	}
+}