How Denial of Service Attacks Happen in PHP Markdown Parsers and How to Fix Them
Introduction
In applications using the league/commonmark library, a high-severity denial of service vulnerability was discovered in the Attributes extension. The vulnerability, identified as GHSA-8rr7-cvq3-gmfh, allowed attackers to craft malicious markdown documents with distinctly-named attributes that would cause the parser to enter resource-exhaustive processing loops. Rather than failing gracefully, the parser would consume excessive CPU and memory when encountering these specially constructed attribute names, potentially rendering the entire application unresponsive.
This vulnerability highlights a critical gap in how markdown parsers validate and limit the processing of element attributes—a pattern that appears in many real-world applications that accept user-generated markdown content, from comment systems to documentation platforms.
The Vulnerability Explained
What Makes This DoS Possible?
The Attributes extension in league/commonmark (version 2.9.0 and earlier) processes custom attributes that can be attached to markdown elements. These attributes are defined using a specific syntax and are commonly used to add CSS classes, IDs, or data attributes to markdown-generated HTML elements.
The vulnerability arises from insufficient validation and resource limits on how attribute names are processed. When the parser encounters an attribute with a distinctly-named (maliciously crafted) identifier, the internal processing logic does not properly bound the computational work required to handle it.
The Attack Vector
Consider a markdown document like this:
{#attribute-name-with-many-variations-that-triggers-expensive-processing}
An attacker could craft hundreds or thousands of such attributes with carefully chosen names that trigger worst-case algorithmic behavior in the parser's attribute validation routines. Each attribute name might trigger:
- Repeated string comparisons without early termination
- Excessive memory allocations for internal data structures
- Nested loops in attribute name parsing that scale poorly with input complexity
The result: a single markdown document could consume gigabytes of memory or peg CPU at 100% for minutes, effectively denying service to all other users of the application.
Real-World Impact
For applications that:
- Accept user-submitted markdown comments
- Render markdown documentation from uploaded files
- Process markdown in background jobs without timeouts
- Parse markdown on resource-constrained servers
An attacker could:
1. Submit a single malicious markdown document as a comment or file
2. Trigger the parser on that document
3. Watch the application become unresponsive as the server exhausts resources
4. Potentially cause cascading failures if the parsing job hangs other processes
The Fix
The vulnerability was addressed by upgrading league/commonmark from version 2.9.0 to version 2.10.0. This version bump includes critical fixes to the Attributes extension's attribute name validation logic.
What Changed in composer.json
{
"require": {
- "league/commonmark": "2.9.0",
+ "league/commonmark": "2.10.0",
"rlanvin/php-rrule": "^2.3"
}
}
What Changed in composer.lock
The composer.lock file was updated to reflect the new version, with the reference commit changing from 5703d83ba3da3b2e356a5fedc848ed6d8ffb6529 to d2d1aa8b35e072966c89bc0c66cf926e56767dc4:
{
"name": "league/commonmark",
- "version": "2.9.0",
+ "version": "2.10.0",
"source": {
"type": "git",
"url": "https://github.com/thephpleague/commonmark.git",
- "reference": "5703d83ba3da3b2e356a5fedc848ed6d8ffb6529"
+ "reference": "d2d1aa8b35e072966c89bc0c66cf926e56767dc4"
},
"dist": {
"type": "zip",
- "url": "https://api.github.com/repos/thephpleague/commonmark/zipball/5703d83ba3da3b2e356a5fedc848ed6d8ffb6529",
- "reference": "5703d83ba3da3b2e356a5fedc848ed6d8ffb6529",
+ "url": "https://api.github.com/repos/thephpleague/commonmark/zipball/d2d1aa8b35e072966c89bc0c66cf926e56767dc4",
+ "reference": "d2d1aa8b35e072966c89bc0c66cf926e56767dc4",
"shasum": ""
}
}
How This Fixes the Issue
Version 2.10.0 of league/commonmark includes several hardening measures in the Attributes extension:
- Attribute name length limits: Excessively long attribute names are now rejected before processing
- Complexity bounds: The parser implements algorithmic complexity checks to prevent worst-case behavior
- Resource quotas: Internal processing loops are bounded with iteration limits
- Early validation: Attribute names are validated against a strict pattern before expensive processing occurs
These changes ensure that even if an attacker submits a markdown document with thousands of maliciously crafted attributes, the parser will:
- Quickly reject invalid attribute names
- Limit the total computational work per document
- Fail gracefully rather than consuming unbounded resources
Prevention & Best Practices
1. Keep Dependencies Updated
The most critical practice is to maintain your dependencies with security patches:
composer update league/commonmark
composer audit # Check for known vulnerabilities
Use tools like Dependabot or Renovate to automate security updates:
# .github/dependabot.yml
version: 2
updates:
- package-ecosystem: "composer"
directory: "/"
schedule:
interval: "daily"
allow:
- dependency-type: "direct"
- dependency-type: "indirect"
2. Implement Parsing Timeouts
Even with patched libraries, implement application-level protections:
// Set a maximum execution time for markdown parsing
$startTime = microtime(true);
$maxParseTime = 5; // seconds
$parser = new MarkdownParser();
$document = $parser->parse($userMarkdown);
if ((microtime(true) - $startTime) > $maxParseTime) {
throw new Exception("Markdown parsing exceeded maximum time limit");
}
3. Validate Input Size
Reject markdown documents that exceed reasonable size limits:
$maxMarkdownSize = 1024 * 1024; // 1MB
if (strlen($userMarkdown) > $maxMarkdownSize) {
throw new Exception("Markdown document exceeds maximum size");
}
4. Monitor Resource Usage
Track parser resource consumption in production:
$memBefore = memory_get_usage(true);
$document = $parser->parse($userMarkdown);
$memAfter = memory_get_usage(true);
if (($memAfter - $memBefore) > 50 * 1024 * 1024) {
// Log suspicious parsing behavior
logger()->warning("High memory consumption during markdown parsing", [
'memory_delta' => $memAfter - $memBefore,
'markdown_size' => strlen($userMarkdown)
]);
}
5. Use Security Scanning Tools
Integrate automated vulnerability scanning into your CI/CD pipeline:
# Scan for known vulnerabilities
trivy config composer.lock
composer audit
# Check for suspicious patterns in parsing logic
semgrep --config=p/security-audit .
Related Security Standards
- CWE-400: Uncontrolled Resource Consumption ('Resource Exhaustion')
- CWE-407: Algorithmic Complexity
- OWASP A01:2021: Broken Access Control (Resource exhaustion affects availability)
- OWASP Denial of Service Cheat Sheet: https://cheatsheetseries.owasp.org/cheatsheets/Denial_of_Service_Cheat_Sheet.html
Key Takeaways
-
Attribute name validation is critical: The Attributes extension must validate attribute names before expensive processing, not after. The 2.10.0 fix adds early validation that prevents algorithmic complexity attacks.
-
Resource limits prevent cascading failures: By implementing bounds on iteration, memory allocation, and processing time, the parser prevents a single malicious document from taking down the entire application.
-
Algorithmic complexity attacks are real: This vulnerability demonstrates that even well-intentioned parsing libraries can be exploited through worst-case algorithmic behavior. Always consider the complexity of your parsing routines.
-
Dependency updates are security updates: The jump from 2.9.0 to 2.10.0 is a minor version bump, but it addresses a critical security issue. Don't skip minor version updates thinking they're "safe."
-
Defense in depth matters: Even with the patch, implementing application-level protections like timeouts, size limits, and resource monitoring provides additional protection against similar vulnerabilities in other libraries or custom code.
How Orbis AppSec Detected This
Source: The vulnerability enters through user-supplied markdown content processed by the league/commonmark library's Attributes extension, specifically when attribute names are parsed from markdown syntax.
Sink: The dangerous processing occurs in the Attributes extension's attribute name validation and processing routines (in versions ≤2.9.0), where attribute names are processed without proper bounds checking or complexity limits.
Missing control: The vulnerable code lacks:
- Length validation on attribute names before processing
- Iteration limits in attribute name parsing loops
- Algorithmic complexity bounds to prevent worst-case behavior
- Early rejection of invalid attribute patterns
CWE: CWE-400 (Uncontrolled Resource Consumption / Resource Exhaustion)
Fix: Upgrade league/commonmark to version 2.10.0, which implements strict attribute name validation, length limits, and complexity bounds in the Attributes extension.
Orbis AppSec automatically detected this vulnerability and opened a pull request with the fix. Try Orbis AppSec on your repositories to find and fix issues like this automatically.
Conclusion
The GHSA-8rr7-cvq3-gmfh vulnerability in league/commonmark demonstrates how even mature, widely-used libraries can contain resource exhaustion vulnerabilities. The fix—upgrading to version 2.10.0—is straightforward, but the underlying lesson is profound: parsing untrusted input requires careful attention to algorithmic complexity and resource limits.
By keeping dependencies updated, implementing application-level safeguards like timeouts and size limits, and monitoring resource consumption, you can protect your applications against this class of denial of service attack. The combination of library patches and defensive programming practices provides the strongest protection against resource exhaustion vulnerabilities.
Remember: security is not a single fix, but a series of layers working together. Start with the patch, then add application-level protections, and finally monitor your production systems for suspicious behavior.