Invalid Percent Encoding in UTM Parameters: What Breaks and Why

"Our campaign URLs looked perfect. But Google Analytics showed garbled data like 'summer%2Gsale' and 'email%XYnewsletter.' Turns out our URL encoding tool was broken, generating invalid percent codes that browsers couldn't decode."

James Wilson, a marketing technologist at a SaaS company, spent two days debugging why 40% of their campaign traffic was showing corrupted parameter values. The culprit: malformed percent encoding.

What is Percent Encoding?

Percent encoding (also called URL encoding) is the method for representing special characters in URLs using the % symbol followed by two hexadecimal digits.

Valid percent encoding:

Space → %20
! → %21
" → %22
# → %23
$ → %24
% → %25

The rule: % followed by exactly two hexadecimal digits (0-9, A-F)

What Makes Percent Encoding Invalid?

Invalid Pattern 1: Non-Hexadecimal Characters

Valid hex digits: 0 1 2 3 4 5 6 7 8 9 A B C D E F

Invalid examples:

❌ %2G (G is not hex)
❌ %ZZ (Z is not hex)
❌ %XY (X and Y are not hex)
❌ %1K (K is not hex)
❌ %M5 (M is not hex)

What happens: Browser cannot decode, shows literal characters or throws error

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Invalid Pattern 2: Incomplete Encoding

Missing digits:

❌ %2 (only one digit instead of two)
❌ % (percent sign alone)
❌ %A (only one digit)

What happens: Browser treats as malformed, unpredictable behavior

Invalid Pattern 3: Truncated Sequences

Cut-off encoding at end of value:

❌ utm_campaign=summer-sale%2
   (Encoding started but not finished)

❌ utm_source=email%
   (Percent with no following digits)

Invalid Pattern 4: Wrong Case for Reserved Percent

Lowercase hex digits in some contexts:

⚠️ %2f vs %2F
Generally works but not technically valid per RFC 3986
Should use uppercase: %2F

Real-World Failure Examples

Case 1: The Malformed Comma Encoding

Intended campaign: "2024, Summer Sale"

Someone tried to encode the comma:

❌ WRONG:
utm_campaign=2024%2G summer sale
(G is not hexadecimal)

Browser sees: "2024%2G summer sale" (literal characters, not decoded)

Correct:

✅ RIGHT:
utm_campaign=2024-summer-sale
(No encoding needed, use hyphens)

Or if you must encode:
utm_campaign=2024%2C%20summer%20sale
(Comma = %2C, Space = %20, both valid hex)

Case 2: The Truncated Encoding Error

Generated URL (automated tool bug):

❌ BROKEN:
https://example.com?utm_source=email&utm_campaign=spring%2

What happened:
- Tool started encoding (%)
- Crashed or truncated before finishing
- Left incomplete %2 sequence

Browser behavior:

Chrome: Shows "spring%2" literally
Firefox: May show "spring%" or error
Safari: Unpredictable
Analytics: Records gibberish

Impact:

2,847 sessions tracked with malformed campaign name
Unable to attribute $12,600 in revenue
Campaign appears as multiple different names in reports

Case 3: The Copy-Paste Encoding Corruption

Original URL:

utm_campaign=black-friday-sale

After copy-paste through multiple tools:

❌ CORRUPTED:
utm_campaign=black%XYfriday%ZZsale

Why it happened:
1. Pasted into Word document (auto-formatting applied)
2. Copied from Word to email (encoding changed)
3. Email client "helped" by trying to encode
4. Result: Invalid percent sequences

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How to Detect Invalid Percent Encoding

Visual Inspection

Red flags to look for:

❌ %2G, %XY, %ZZ (non-hex characters)
❌ Trailing % at end of value
❌ %A, %2, %F (single digit after %)
❌ Multiple %% characters
❌ % followed by space

JavaScript Detection

function detectInvalidPercentEncoding(url) {
  const issues = [];
 
  try {
    const urlObj = new URL(url);
    const params = urlObj.searchParams;
 
    ['utm_source', 'utm_medium', 'utm_campaign', 'utm_content', 'utm_term'].forEach(param => {
      const value = params.get(param);
      if (!value) return;
 
      // Pattern 1: % followed by non-hex characters
      const invalidHex = value.match(/%[^0-9A-Fa-f]/g);
      if (invalidHex) {
        issues.push({
          parameter: param,
          issue: 'Invalid hex characters after %',
          examples: invalidHex,
          severity: 'CRITICAL'
        });
      }
 
      // Pattern 2: % followed by only one digit
      const incompleteEncoding = value.match(/%[0-9A-Fa-f](?![0-9A-Fa-f])/g);
      if (incompleteEncoding) {
        issues.push({
          parameter: param,
          issue: 'Incomplete percent encoding (single digit)',
          examples: incompleteEncoding,
          severity: 'CRITICAL'
        });
      }
 
      // Pattern 3: % at end of string
      if (value.endsWith('%')) {
        issues.push({
          parameter: param,
          issue: 'Percent sign at end of value',
          severity: 'CRITICAL'
        });
      }
 
      // Pattern 4: % followed by space
      if (value.includes('% ')) {
        issues.push({
          parameter: param,
          issue: 'Percent followed by space',
          severity: 'CRITICAL'
        });
      }
 
      // Pattern 5: Multiple consecutive %
      if (value.includes('%%')) {
        issues.push({
          parameter: param,
          issue: 'Multiple consecutive percent signs',
          severity: 'WARNING'
        });
      }
    });
 
  } catch (e) {
    issues.push({
      parameter: 'URL',
      issue: 'Malformed URL',
      error: e.message,
      severity: 'CRITICAL'
    });
  }
 
  return {
    valid: issues.length === 0,
    issues: issues
  };
}
 
// Usage
const testURLs = [
  'https://example.com?utm_campaign=sale%2Gspecial',
  'https://example.com?utm_source=email%',
  'https://example.com?utm_medium=social%2',
  'https://example.com?utm_campaign=summer-sale'  // This one is valid
];
 
testURLs.forEach(url => {
  const result = detectInvalidPercentEncoding(url);
  console.log(`URL: ${"{"}{"{"}url{"}"}{"}"}}`);
  console.log(`Valid: ${result.valid}`);
  if (!result.valid) {
    console.log('Issues:', result.issues);
  }
  console.log('---');
});

Browser Console Test

Quick manual check:

// Paste this in browser console with your URL
const testUrl = 'https://example.com?utm_campaign=sale%2Gspecial';
 
try {
  const urlObj = new URL(testUrl);
  const campaign = urlObj.searchParams.get('utm_campaign');
  console.log('Campaign value:', campaign);
 
  // Try to decode explicitly
  try {
    const decoded = decodeURIComponent(campaign);
    console.log('Decoded:', decoded);
  } catch (e) {
    console.error('Decoding failed:', e.message);
    console.log('⚠️ INVALID PERCENT ENCODING DETECTED');
  }
} catch (e) {
  console.error('Invalid URL:', e.message);
}

Valid Percent Encoding Reference

Common Characters Properly Encoded

Character	Name	Hex Value	Percent Encoded
Space	Space	20	%20
!	Exclamation	21	%21
"	Quote	22	%22
#	Hash	23	%23
$	Dollar	24	%24
%	Percent	25	%25
&	Ampersand	26	%26
'	Apostrophe	27	%27
(	Left Paren	28	%28
)	Right Paren	29	%29
*	Asterisk	2A	%2A
+	Plus	2B	%2B
,	Comma	2C	%2C
/	Slash	2F	%2F
:	Colon	3A	%3A
;	Semicolon	3B	%3B
=	Equals	3D	%3D
?	Question	3F	%3F
@	At	40	%40

Hexadecimal Reference

Valid hex digits (0-15):

0 1 2 3 4 5 6 7 8 9 A B C D E F
(Also lowercase: a b c d e f, though uppercase preferred)

Invalid (not hex):

G H I J K L M N O P Q R S T U V W X Y Z
(Any letter beyond F)

How to Fix Invalid Percent Encoding

Fix 1: Don't Encode at All

Best solution: Avoid characters that need encoding

❌ NEEDS ENCODING:
utm_campaign=Save 50%! Limited Time

✅ NO ENCODING NEEDED:
utm_campaign=save-50-percent-limited-time

Fix 2: Use Proper Encoding Function

JavaScript:

function properlyEncodeUTM(value) {
  // First, clean to safe characters (best approach)
  const cleaned = value
    .toLowerCase()
    .replace(/\s+/g, '-')
    .replace(/%/g, '-percent')
    .replace(/&/g, '-and-')
    .replace(/[^a-z0-9-_]/g, '');
 
  return cleaned;
}
 
// If you MUST preserve original and encode:
function encodeUTMValue(value) {
  return encodeURIComponent(value);
}
 
// Examples
console.log(properlyEncodeUTM('Save 50%! Limited Time'));
// Output: save-50-percent-limited-time
 
console.log(encodeURIComponent('Save 50%'));
// Output: Save%2050%25 (valid, but avoid this approach)

PHP:

<?php
// Don't use raw urlencode on entire UTM value
// Instead, clean the value first
 
function cleanUTMValue($value) {
    $cleaned = strtolower($value);
    $cleaned = str_replace(' ', '-', $cleaned);
    $cleaned = str_replace('%', '-percent', $cleaned);
    $cleaned = preg_replace('/[^a-z0-9-_]/', '', $cleaned);
    return $cleaned;
}
 
echo cleanUTMValue('Save 50%! Limited Time');
// Output: save-50-percent-limited-time
?>

Python:

import re
from urllib.parse import quote
 
def clean_utm_value(value):
    """Best approach: clean to safe characters"""
    cleaned = value.lower()
    cleaned = cleaned.replace(' ', '-')
    cleaned = cleaned.replace('%', '-percent')
    cleaned = re.sub(r'[^a-z0-9-_]', '', cleaned)
    return cleaned
 
# If you must encode:
def encode_utm_value(value):
    """Use only if absolutely necessary"""
    return quote(value, safe='')
 
print(clean_utm_value('Save 50%! Limited Time'))
# Output: save-50-percent-limited-time
 
print(encode_utm_value('Save 50%'))
# Output: Save%2050%25 (valid but not recommended)

Fix 3: Validate Before Use

function validateAndFixPercentEncoding(url) {
  try {
    const urlObj = new URL(url);
    let fixed = false;
 
    ['utm_source', 'utm_medium', 'utm_campaign', 'utm_content', 'utm_term'].forEach(param => {
      const value = urlObj.searchParams.get(param);
      if (!value) return;
 
      // Check for invalid encoding
      const hasInvalidEncoding =
        /%[^0-9A-Fa-f]/.test(value) ||  // Non-hex after %
        /%[0-9A-Fa-f](?![0-9A-Fa-f])/.test(value) ||  // Single digit after %
        value.endsWith('%');  // % at end
 
      if (hasInvalidEncoding) {
        // Clean the value
        const cleaned = value
          .replace(/%[^0-9A-Fa-f]{0,2}/g, '')  // Remove invalid sequences
          .replace(/%$/, '')  // Remove trailing %
          .toLowerCase()
          .replace(/\s+/g, '-')
          .replace(/[^a-z0-9-_]/g, '');
 
        urlObj.searchParams.set(param, cleaned);
        fixed = true;
      }
    });
 
    return {
      original: url,
      fixed: urlObj.toString(),
      wasFixed: fixed
    };
 
  } catch (e) {
    return {
      original: url,
      error: e.message
    };
  }
}
 
// Usage
const problematic = 'https://example.com?utm_campaign=sale%2Gspecial&utm_source=email%';
const result = validateAndFixPercentEncoding(problematic);
 
console.log('Original:', result.original);
console.log('Fixed:', result.fixed);
console.log('Was fixed:', result.wasFixed);

Prevention Strategies

1. Never Manual Encode

Don't try to encode by hand:

❌ MANUAL ENCODING (error-prone):
Thinking: "Space is %20, so Summer Sale = Summer%20Sale"
Result: Often leads to %2G, %XY type errors

✅ USE FUNCTIONS:
encodeURIComponent('Summer Sale')
Result: Always correct

2. Use Clean Values Only

Best practice: avoid encoding entirely

// URL Builder that prevents encoding issues
function buildCleanUTMUrl(baseUrl, params) {
  const url = new URL(baseUrl);
 
  Object.keys(params).forEach(key => {
    // Clean value to not need encoding
    const clean = params[key]
      .toLowerCase()
      .replace(/\s+/g, '-')
      .replace(/%/g, '-percent')
      .replace(/&/g, '-and-')
      .replace(/[^a-z0-9-_]/g, '');
 
    url.searchParams.set(key, clean);
  });
 
  return url.toString();
}
 
// Usage
const trackedUrl = buildCleanUTMUrl('https://example.com', {
  utm_source: 'Email Newsletter',
  utm_campaign: 'Save 50%! Limited Time'
});
 
console.log(trackedUrl);
// https://example.com?utm_source=email-newsletter&utm_campaign=save-50-percent-limited-time
// No percent encoding needed!

3. Validate After Any Tool

Test all URLs from automation:

// After generating URL from any tool
function validateNoInvalidEncoding(url) {
  const pattern = /%(?:[^0-9A-Fa-f]|[0-9A-Fa-f](?![0-9A-Fa-f])|$)/;
 
  if (pattern.test(url)) {
    throw new Error('Invalid percent encoding detected in URL: ' + url);
  }
 
  return true;
}
 
// Use in your workflow
const generated = urlBuilderTool.generate(params);
validateNoInvalidEncoding(generated);  // Throws if invalid
deployurl(generated);

4. Regular Audits

Check for encoding issues monthly:

-- Google Analytics 4 BigQuery query
SELECT
  (SELECT value.string_value FROM UNNEST(event_params) WHERE key = 'campaign_name') as campaign,
  COUNT(*) as sessions
FROM `project.dataset.events_*`
WHERE _TABLE_SUFFIX BETWEEN FORMAT_DATE('%Y%m%d', DATE_SUB(CURRENT_DATE(), INTERVAL 30 DAY))
  AND FORMAT_DATE('%Y%m%d', CURRENT_DATE())
  AND (SELECT value.string_value FROM UNNEST(event_params) WHERE key = 'campaign_name') LIKE '%\%%'
GROUP BY campaign
ORDER BY sessions DESC

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FAQ

Q: What if my URL has %20 for spaces? Is that invalid?

A: %20 is valid (2 and 0 are hex digits). But it's better to avoid needing encoding by using hyphens instead of spaces.

Q: Can I fix invalid encoding by double-encoding?

A: No! That makes it worse. Fix by removing the encoding and using clean characters only.

Q: Why does %2G appear in my URLs?

A: Likely a broken encoding tool that's using the wrong character set or has a bug. Stop using that tool and switch to proper URL builders.

Q: Will browsers automatically fix invalid encoding?

A: No. Different browsers handle it differently, creating inconsistent data. Some show literal characters, others throw errors.

Q: How do I know if encoding is uppercase or lowercase hex?

A: Both work technically, but RFC 3986 recommends uppercase (%2F not %2f). Most importantly: ensure both characters after % are valid hex (0-9, A-F).

Q: Can analytics tools decode invalid percent encoding?

A: No. If the encoding is malformed, analytics receives and stores the malformed value. Garbage in, garbage out.

Q: What's the most common cause of invalid encoding?

A: Broken URL builder tools, manual encoding attempts, and copy-paste corruption through multiple systems.

Q: Should I decode, clean, then re-encode URLs?

A: No. Decode, clean to safe characters (a-z, 0-9, hyphens), and don't re-encode. This prevents all encoding issues.

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