diff --git a/src/lib/__tests__/gif-fixture.ts b/src/lib/__tests__/gif-fixture.ts
new file mode 100644
index 000000000..c166e5e7a
--- /dev/null
+++ b/src/lib/__tests__/gif-fixture.ts
@@ -0,0 +1,35 @@
+import { Buffer } from "node:buffer";
+
+const u16 = (n: number) => [n & 0xff, (n >> 8) & 0xff];
+
+/**
+ * A GIF with the given logical screen and frames. Each frame carries one
+ * byte of image data, which is enough for a header walk and for a decoder.
+ */
+export function gif(
+  screen: { width: number; height: number; colourTable?: boolean },
+  frames: Array<{
+    width: number;
+    height: number;
+    left?: number;
+    top?: number;
+    extension?: boolean;
+  }>,
+): Buffer {
+  const out: number[] = [...Buffer.from("GIF89a", "latin1")];
+  out.push(...u16(screen.width), ...u16(screen.height));
+  out.push(screen.colourTable ? 0x80 : 0x00, 0, 0);
+  if (screen.colourTable) out.push(0, 0, 0, 255, 255, 255);
+  for (const frame of frames) {
+    if (frame.extension) {
+      // A graphic control extension.
+      out.push(0x21, 0xf9, 4, 0, 0, 0, 0, 0);
+    }
+    out.push(0x2c, ...u16(frame.left ?? 0), ...u16(frame.top ?? 0));
+    out.push(...u16(frame.width), ...u16(frame.height), 0x80);
+    out.push(0, 0, 0, 255, 255, 255); // local colour table, two entries
+    out.push(2, 2, 0x44, 0x01, 0); // LZW size, one sub-block, terminator
+  }
+  out.push(0x3b);
+  return Buffer.from(out);
+}
diff --git a/src/lib/__tests__/image-dimensions.test.ts b/src/lib/__tests__/image-dimensions.test.ts
index 9145b1304..76cdc39ce 100644
--- a/src/lib/__tests__/image-dimensions.test.ts
+++ b/src/lib/__tests__/image-dimensions.test.ts
@@ -4,6 +4,7 @@ import { createCanvas } from "@napi-rs/canvas";
 import { describe, expect, it } from "vitest";
 
 import { readImageDimensions } from "../image-dimensions";
+import { gif } from "./gif-fixture";
 
 /**
  * The header walk both the avatar upload and the document thumbnail trust to
@@ -36,9 +37,58 @@ describe("readImageDimensions", () => {
     );
   });
 
-  it("reads a GIF's logical screen", () => {
-    const gif = Buffer.from("GIF89a\x25\x00\x17\x00\x00\x00\x00", "latin1");
-    expect(readImageDimensions(gif, "image/gif")).toEqual(SIZE);
+  it("reads a GIF's logical screen when its frames fit inside it", () => {
+    expect(
+      readImageDimensions(
+        gif({ width: 37, height: 23 }, [{ width: 37, height: 23 }]),
+        "image/gif",
+      ),
+    ).toEqual(SIZE);
+  });
+
+  it("sizes a GIF by its largest frame, not only its logical screen", () => {
+    // The decoder allocates for the frame: 10 x 10 on the screen, 20000 x
+    // 20000 in the descriptor was a 1.6 GB decode behind a 100-pixel check.
+    expect(
+      readImageDimensions(
+        gif({ width: 10, height: 10 }, [{ width: 20_000, height: 20_000 }]),
+        "image/gif",
+      ),
+    ).toEqual({ width: 20_000, height: 20_000 });
+    // A later frame, an offset, a global colour table and an extension in
+    // between are all walked.
+    expect(
+      readImageDimensions(
+        gif({ width: 10, height: 10, colourTable: true }, [
+          { width: 4, height: 4 },
+          { left: 60_000, top: 5, width: 5_000, height: 3, extension: true },
+        ]),
+        "image/gif",
+      ),
+    ).toEqual({ width: 65_000, height: 10 });
+  });
+
+  it("refuses a GIF it cannot walk to the end of a frame descriptor", () => {
+    const whole = gif({ width: 10, height: 10 }, [{ width: 10, height: 10 }]);
+    // Logical screen only: no frame, no size.
+    expect(readImageDimensions(whole.subarray(0, 13), "image/gif")).toBeNull();
+    // Cut inside the image descriptor.
+    expect(readImageDimensions(whole.subarray(0, 18), "image/gif")).toBeNull();
+    // A block no decoder knows.
+    const unknown = Buffer.from(whole);
+    unknown[13] = 0x99;
+    expect(readImageDimensions(unknown, "image/gif")).toBeNull();
+    // A zero-sized frame.
+    expect(
+      readImageDimensions(
+        gif({ width: 10, height: 10 }, [{ width: 0, height: 10 }]),
+        "image/gif",
+      ),
+    ).toBeNull();
+    // Not a GIF signature a decoder accepts.
+    const signature = Buffer.from(whole);
+    signature.write("GIF80a", 0, "latin1");
+    expect(readImageDimensions(signature, "image/gif")).toBeNull();
   });
 
   it("skips fill bytes, RSTn and TEM before the frame header", () => {
diff --git a/src/lib/documents/__tests__/thumbnail.test.ts b/src/lib/documents/__tests__/thumbnail.test.ts
index 68fc8cc6d..7620df07e 100644
--- a/src/lib/documents/__tests__/thumbnail.test.ts
+++ b/src/lib/documents/__tests__/thumbnail.test.ts
@@ -19,6 +19,7 @@ vi.mock("@napi-rs/canvas", async (importActual) => {
   };
 });
 
+import { gif } from "../../__tests__/gif-fixture";
 import { generateThumbnail, THUMB_LONG_EDGE } from "../thumbnail";
 
 /**
@@ -214,6 +215,32 @@ describe("generateThumbnail", () => {
     expect(result.ok).toBe(true);
   });
 
+  it("refuses a GIF whose frame is far larger than its logical screen, before decoding", async () => {
+    loadImageMock.mockClear();
+    // A 10 x 10 screen around a 20000 x 20000 frame: the decoder sizes its
+    // buffer from the frame.
+    const bomb = gif({ width: 10, height: 10 }, [
+      { width: 20_000, height: 20_000 },
+    ]);
+    expect(bomb.length).toBeLessThan(64);
+    await expect(generateThumbnail(bomb, "image/gif")).resolves.toEqual({
+      ok: false,
+      reason: "pixel-cap",
+    });
+    expect(loadImageMock).not.toHaveBeenCalled();
+  });
+
+  it("still previews an ordinary GIF", async () => {
+    const result = await generateThumbnail(
+      gif({ width: 1, height: 1 }, [{ width: 1, height: 1 }]),
+      "image/gif",
+    );
+    expect(result).toMatchObject({
+      ok: true,
+      thumbnail: { width: 1, height: 1 },
+    });
+  });
+
   it("refuses an image whose size cannot be read instead of decoding it", async () => {
     loadImageMock.mockClear();
     // Start of image, then straight into a scan: no frame header to read.
diff --git a/src/lib/image-dimensions.ts b/src/lib/image-dimensions.ts
index 64346a025..806c5f6b1 100644
--- a/src/lib/image-dimensions.ts
+++ b/src/lib/image-dimensions.ts
@@ -64,12 +64,74 @@ function readPng(bytes: Buffer): ImageDimensions | null {
   return { width: bytes.readUInt32BE(16), height: bytes.readUInt32BE(20) };
 }
 
+/**
+ * The area a GIF decoder has to allocate: the logical screen, grown to cover
+ * every frame's image descriptor.
+ *
+ * The logical screen alone is not that size. A decoder sizes its buffer from
+ * the frame it decodes, so a file declaring a 10 x 10 screen and a
+ * 20000 x 20000 frame passed a screen-only check and allocated 1.6 GB. Every
+ * block is walked (GIF89a, section 17 onwards): extensions and image data are
+ * skipped sub-block by sub-block, each image descriptor is read, and anything
+ * that is not a block a decoder accepts makes the file unreadable. The walk
+ * is linear in the file's length.
+ *
+ * A file that ends inside a frame's image data is still read up to there,
+ * since decoders draw what they got; one that ends inside a header or a
+ * descriptor, or has no frame at all, has no size.
+ */
 function readGif(bytes: Buffer): ImageDimensions | null {
-  // "GIF87a" / "GIF89a", then the logical screen: width, height (LE).
-  if (bytes.length < 10 || bytes.toString("ascii", 0, 4) !== "GIF8") {
-    return null;
+  // "GIF87a" / "GIF89a", then the logical screen descriptor (7 bytes).
+  if (bytes.length < 13) return null;
+  const signature = bytes.toString("ascii", 0, 6);
+  if (signature !== "GIF87a" && signature !== "GIF89a") return null;
+  let width = bytes.readUInt16LE(6);
+  let height = bytes.readUInt16LE(8);
+  const screenFlags = bytes[10]!;
+  let offset = 13;
+  if (screenFlags & 0x80) offset += 3 << ((screenFlags & 0x07) + 1);
+
+  /** Past a run of data sub-blocks; `bytes.length` when the file ends in it. */
+  const skipSubBlocks = (from: number): number => {
+    let at = from;
+    while (at < bytes.length) {
+      const size = bytes[at]!;
+      at += 1;
+      if (size === 0) return at;
+      at += size;
+    }
+    return bytes.length;
+  };
+
+  let frames = 0;
+  while (offset < bytes.length) {
+    const introducer = bytes[offset]!;
+    if (introducer === 0x3b) break; // trailer
+    if (introducer === 0x21) {
+      // Extension: introducer, label, then data sub-blocks.
+      if (offset + 2 > bytes.length) return null;
+      offset = skipSubBlocks(offset + 2);
+      continue;
+    }
+    if (introducer !== 0x2c) return null;
+    // Image descriptor: left, top, width, height (LE), flags.
+    if (offset + 10 > bytes.length) return null;
+    const left = bytes.readUInt16LE(offset + 1);
+    const top = bytes.readUInt16LE(offset + 3);
+    const frameWidth = bytes.readUInt16LE(offset + 5);
+    const frameHeight = bytes.readUInt16LE(offset + 7);
+    const frameFlags = bytes[offset + 9]!;
+    if (frameWidth === 0 || frameHeight === 0) return null;
+    width = Math.max(width, left + frameWidth);
+    height = Math.max(height, top + frameHeight);
+    frames += 1;
+    offset += 10;
+    if (frameFlags & 0x80) offset += 3 << ((frameFlags & 0x07) + 1);
+    // The LZW minimum code size, then the image data sub-blocks.
+    if (offset >= bytes.length) break;
+    offset = skipSubBlocks(offset + 1);
   }
-  return { width: bytes.readUInt16LE(6), height: bytes.readUInt16LE(8) };
+  return frames > 0 ? { width, height } : null;
 }
 
 /**