Exception Handling Vulnerabilities (OWASP A10)

Purpose

Provide detection patterns for vulnerabilities related to improper exception and error handling, including XXE (XML External Entity) injection, stack trace disclosure, and authentication bypass via exceptions.

OWASP 2025 Mapping

Category: A10 - Mishandling of Exceptional Conditions

CWEs:

• CWE-390: Detection of Error Condition Without Action
• CWE-392: Missing Report of Error Condition
• CWE-460: Improper Cleanup on Thrown Exception
• CWE-611: Improper Restriction of XML External Entity Reference (XXE)
• CWE-755: Improper Handling of Exceptional Conditions

When to Use

Activate this skill when:

• Searching for XML parsing vulnerabilities
• Reviewing error handling code
• Looking for information disclosure via exceptions
• Finding authentication/authorization bypass via exception paths

---

XXE (XML External Entity) Injection

Overview

XXE occurs when XML parsers process external entity references in untrusted XML input, allowing attackers to read files, perform SSRF, or cause DoS.

Detection Patterns

Java

# Vulnerable XML parsers
grep -rniE "DocumentBuilderFactory|SAXParserFactory|XMLInputFactory|TransformerFactory|SchemaFactory|XMLReader" --include="*.java"

# Check for disabled external entities (SAFE patterns)
grep -rniE "setFeature.*FEATURE_SECURE_PROCESSING|setFeature.*disallow-doctype-decl|setExpandEntityReferences.*false" --include="*.java"

Vulnerable Pattern:

// VULNERABLE: Default parser allows XXE
DocumentBuilderFactory dbf = DocumentBuilderFactory.newInstance();
DocumentBuilder db = dbf.newDocumentBuilder();
Document doc = db.parse(userInputStream);  // XXE possible

Secure Pattern:

// SAFE: External entities disabled
DocumentBuilderFactory dbf = DocumentBuilderFactory.newInstance();
dbf.setFeature("http://apache.org/xml/features/disallow-doctype-decl", true);
dbf.setFeature("http://xml.org/sax/features/external-general-entities", false);
dbf.setFeature("http://xml.org/sax/features/external-parameter-entities", false);

Python

# Vulnerable XML parsers
grep -rniE "xml\.etree\.ElementTree|xml\.dom\.minidom|xml\.sax|lxml\.etree" --include="*.py"

# Check for defusedxml (SAFE)
grep -rniE "defusedxml|defused" --include="*.py"

Vulnerable Pattern:

# VULNERABLE: Standard library allows XXE
import xml.etree.ElementTree as ET
tree = ET.parse(user_input)  # XXE possible

Secure Pattern:

# SAFE: Use defusedxml
import defusedxml.ElementTree as ET
tree = ET.parse(user_input)  # XXE blocked

PHP

# Vulnerable XML functions
grep -rniE "simplexml_load|DOMDocument|XMLReader|SimpleXMLElement" --include="*.php"

# Check for entity loader disabled (SAFE)
grep -rniE "libxml_disable_entity_loader|LIBXML_NOENT" --include="*.php"

Vulnerable Pattern:

// VULNERABLE: External entities enabled
$xml = simplexml_load_string($userInput);  // XXE possible

Secure Pattern:

// SAFE: Disable external entities (PHP < 8.0)
libxml_disable_entity_loader(true);
$xml = simplexml_load_string($userInput, 'SimpleXMLElement', LIBXML_NOENT);

Go

# XML parsing
grep -rniE "xml\.Unmarshal|xml\.Decoder|xml\.NewDecoder" --include="*.go"

Note: Go's encoding/xml does not process external entities by default, making it safe from XXE. However, third-party libraries may be vulnerable.

TypeScript/JavaScript

# XML parsing libraries
grep -rniE "xml2js|fast-xml-parser|libxmljs|DOMParser|parseFromString" --include="*.ts" --include="*.js"

Vulnerable Pattern:

// VULNERABLE: Some libraries allow XXE
const parser = new DOMParser();
const doc = parser.parseFromString(userInput, "text/xml");

---

Stack Trace / Error Disclosure

Overview

Exposing stack traces or detailed error messages to users reveals internal paths, library versions, and application structure.

Detection Patterns

Java

# Stack trace printing
grep -rniE "printStackTrace|getStackTrace|\.getMessage\(\)" --include="*.java"

# Exposed to response
grep -rniE "response\.getWriter\(\).*exception|sendError.*getMessage" --include="*.java"

Vulnerable Pattern:

// VULNERABLE: Stack trace sent to user
catch (Exception e) {
    response.getWriter().println(e.getMessage());
    e.printStackTrace(response.getWriter());
}

Python

# Traceback exposure
grep -rniE "traceback\.print_exc|traceback\.format_exc|sys\.exc_info" --include="*.py"

# Debug mode
grep -rniE "DEBUG.*=.*True|app\.debug.*=.*True" --include="*.py"

Vulnerable Pattern:

# VULNERABLE: Traceback in response
except Exception as e:
    return jsonify({"error": traceback.format_exc()})  # Exposes internals

PHP

# Error display
grep -rniE "display_errors|error_reporting|var_dump|print_r" --include="*.php"

Vulnerable Pattern:

// VULNERABLE: Errors displayed to users
ini_set('display_errors', 1);
error_reporting(E_ALL);

Go

# Stack trace functions
grep -rniE "debug\.PrintStack|runtime\.Stack|debug\.Stack" --include="*.go"

# Error exposure
grep -rniE "http\.Error.*err\.Error\(\)|json\..*err\.Error\(\)" --include="*.go"

Vulnerable Pattern:

// VULNERABLE: Internal error exposed
if err != nil {
    http.Error(w, err.Error(), 500)  // May leak internal paths
}

TypeScript

# Stack exposure
grep -rniE "\.stack|Error\(\)\.stack|console\.error" --include="*.ts"

---

Empty Catch Blocks

Overview

Empty catch blocks silently swallow exceptions, potentially hiding security failures or allowing bypasses.

Detection Patterns

# Java - Empty catch
grep -rniE "catch\s*\([^)]+\)\s*\{\s*\}" --include="*.java"

# Python - pass in except
grep -rniE "except.*:\s*pass" --include="*.py"

# PHP - empty catch
grep -rniE "catch\s*\([^)]+\)\s*\{\s*\}" --include="*.php"

# TypeScript - empty catch
grep -rniE "catch\s*\([^)]*\)\s*\{\s*\}" --include="*.ts"

Vulnerable Pattern:

// VULNERABLE: Authentication failure silently ignored
try {
    authenticateUser(token);
} catch (AuthenticationException e) {
    // Empty - user proceeds as authenticated!
}

---

Exception-Based Authentication Bypass

Overview

When authentication/authorization logic is inside try blocks, exceptions may allow bypass.

Detection Patterns

# Auth in try blocks
grep -rniE "try.*\{[^}]*(authenticate|authorize|checkPermission|isAdmin)" --include="*.java" --include="*.py" --include="*.go" --include="*.ts" --include="*.php"

# Catch blocks that continue execution
grep -rniE "catch.*\{[^}]*(continue|return true|return null)" --include="*.java"

Vulnerable Pattern:

// VULNERABLE: Exception allows bypass
boolean isAuthorized = false;
try {
    isAuthorized = authService.checkPermission(user, resource);
} catch (Exception e) {
    // Exception occurs, isAuthorized stays false... or does it?
    log.error("Auth check failed", e);
}
// If exception + isAuthorized not checked properly = bypass

---

Resource Exhaustion via Exceptions

Overview

Repeated exception generation can exhaust resources (CPU, memory, logs).

Detection Patterns

# Exceptions in loops
grep -rniE "(for|while).*\{[^}]*throw new|try.*\{[^}]*(for|while)" --include="*.java" --include="*.py" --include="*.go"

# Unbounded recursion
grep -rniE "catch.*\{[^}]*throw|except.*raise" --include="*.java" --include="*.py"

---

Language-Specific Secure Patterns

Java - Safe Exception Handling

// Log internally, return generic message
catch (Exception e) {
    logger.error("Operation failed", e);  // Internal log with stack
    throw new ApiException("An error occurred", 500);  // Generic to user
}

Python - Safe Exception Handling

# Log internally, return generic message
except Exception as e:
    logger.exception("Operation failed")  # Logs full traceback
    return jsonify({"error": "An error occurred"}), 500  # Generic to user

Go - Safe Exception Handling

// Wrap errors, don't expose internals
if err != nil {
    log.Printf("operation failed: %v", err)  // Internal log
    http.Error(w, "Internal server error", 500)  // Generic to user
}

PHP - Safe Exception Handling

// Production error handling
ini_set('display_errors', 0);
ini_set('log_errors', 1);
error_log($e->getMessage());  // Log internally
echo json_encode(["error" => "An error occurred"]);  // Generic to user

TypeScript - Safe Exception Handling

// Sanitize errors before response
catch (error) {
    logger.error('Operation failed', { error });  // Internal log
    res.status(500).json({ error: 'An error occurred' });  // Generic
}

---

Verification Steps

1. Find XML parsers → Check if external entities are disabled
2. Find catch/except blocks → Check if they're empty or expose details
3. Find error responses → Check if stack traces or internal errors are included
4. Find auth in try blocks → Check exception handling doesn't bypass auth
5. Test XXE → Send payload with external entity reference

XXE Test Payloads

<!-- File read -->
<?xml version="1.0"?>
<!DOCTYPE foo [<!ENTITY xxe SYSTEM "file:///etc/passwd">]>
<foo>&xxe;</foo>

<!-- SSRF -->
<?xml version="1.0"?>
<!DOCTYPE foo [<!ENTITY xxe SYSTEM "http://internal-server/admin">]>
<foo>&xxe;</foo>

<!-- DoS (Billion Laughs) -->
<?xml version="1.0"?>
<!DOCTYPE lolz [
  <!ENTITY lol "lol">
  <!ENTITY lol2 "&lol;&lol;&lol;&lol;&lol;&lol;&lol;&lol;&lol;&lol;">
  <!ENTITY lol3 "&lol2;&lol2;&lol2;&lol2;&lol2;&lol2;&lol2;&lol2;&lol2;&lol2;">
]>
<lolz>&lol3;</lolz>

---

CWE References

CWE	Name	Description
CWE-390	Detection Without Action	Exception caught but not handled
CWE-392	Missing Report	Exception not logged/reported
CWE-460	Improper Cleanup	Resources not cleaned on exception
CWE-611	XXE	XML parser processes external entities
CWE-755	Improper Handling	Generic exception handling issues