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CVE-2026-71488 is a high-severity vulnerability in the PHP library league/commonmark, which is widely used for parsing and rendering Markdown. The issue stems from a fundamental performance flaw where specially crafted Markdown input can cause the parser’s time complexity to become quadratic. This means that as the length of a malicious input line increases, the time required to parse it grows exponentially, leading to a denial-of-service condition.
The core of the vulnerability lies in the parser’s handling of character positions and byte positions. The parser identifies locations using character positions, but regular-expression matches report byte positions. These positions differ when a UTF-8 character uses more than one byte. Several parsing paths repeatedly rescan growing portions of the line to translate between the two positions, causing the inefficiency.
In current 2.x releases, a single non-ASCII character anywhere on a line can place that whole line on the slower multibyte path. An attacker can combine this with a long run of leading whitespace or repeated Markdown punctuation, causing increasingly large rescans. When the Autolink extension is enabled, repeated URL-like prefixes provide another trigger, even on ASCII-only lines, as the extension copies and validates the remaining line at every URL-like prefix.
An attacker can use a comparatively small request to consume disproportionate CPU time and allocation activity, and repeated or concurrent requests can occupy all available PHP workers, preventing legitimate requests from completing. This vulnerability affects CommonMarkConverter, GithubFlavoredMarkdownConverter, and custom environments, with the autolink-specific path affecting applications using `AutolinkExtension` or GithubFlavoredMarkdownExtension. The impact is limited to availability, not data disclosure or output alteration. The issue is patched in version 2.9.0 and later.
DailyCVE Form:
Platform: PHP Library
Version: 0.6.0 – 2.8.3
Vulnerability: Quadratic DoS
Severity: High
date: 2026-08-07
Prediction: 2026-08-07
What Undercode Say:
The vulnerability can be analyzed by examining the parser’s behavior with malicious input. The following Bash commands and code snippets demonstrate how to test for the vulnerability and understand its impact.
1. Testing for Vulnerability
To check if your application is using a vulnerable version of league/commonmark, you can use Composer:
composer show league/commonmark
This will display the installed version. If it is between 0.6.0 and 2.8.3, it is vulnerable.
2. Demonstrating the Quadratic Behavior
The following PHP code snippet illustrates how a single non-ASCII character can trigger the slow path:
<?php
require 'vendor/autoload.php';
use League\CommonMark\CommonMarkConverter;
$converter = new CommonMarkConverter();
// A line with a non-ASCII character and leading whitespace
$maliciousInput = str_repeat(' ', 10000) . 'ç' . str_repeat(' ', 10000);
$start = microtime(true);
$converter->convert($maliciousInput);
$end = microtime(true);
echo "Time: " . ($end - $start) . " seconds\n";
?>
This code creates a long line with a single non-ASCII character (ç) surrounded by spaces. The parser will repeatedly rescan the line to translate between character and byte positions, resulting in quadratic time complexity.
3. Autolink Extension Trigger
When the Autolink extension is enabled, even ASCII-only lines can trigger the vulnerability. The following example demonstrates a line with repeated URL-like prefixes:
<?php
require 'vendor/autolink.php';
use League\CommonMark\CommonMarkConverter;
use League\CommonMark\Extension\Autolink\AutolinkExtension;
$converter = new CommonMarkConverter();
$converter->getEnvironment()->addExtension(new AutolinkExtension());
// A line with repeated URL-like prefixes
$maliciousInput = str_repeat('http://', 5000) . 'example.com';
$start = microtime(true);
$converter->convert($maliciousInput);
$end = microtime(true);
echo "Time: " . ($end - $start) . " seconds\n";
?>
Here, the Autolink extension will copy and validate the remaining line at every http://` prefix, leading to quadratic time complexity.league/commonmark`. The attack does not require complex Markdown structure; a single long line is sufficient.
<h2 style="color: blue;">How Exploit:</h2>
An attacker can exploit this vulnerability by submitting specially crafted Markdown content to an application that uses a vulnerable version of
The attacker can craft a payload with a non-ASCII character and a long run of leading whitespace or repeated punctuation to trigger the core parsing issue. If the Autolink extension is enabled, the attacker can use repeated URL-like prefixes on an ASCII-only line.
By sending these payloads repeatedly or concurrently, the attacker can consume all available PHP workers, causing a denial of service for legitimate users. The attack is effective with relatively small request sizes, making it a cheap and efficient way to disrupt service.
Protection:
The primary and most effective protection is to upgrade to `league/commonmark` version 2.9.0 or later. This version includes patches that:
– Record UTF-8 character-to-byte positions incrementally.
– Convert regular-expression match positions without restarting from the beginning of the line.
– Match autolinks against the original line instead of copying every remaining suffix.
If an immediate upgrade is not possible, the following workarounds can be implemented:
1. Input Limits: Reject or truncate inputs with excessively long individual lines before passing them to the converter. A per-line limit is crucial because every demonstrated trigger fits on one line.
2. Request-Size Limits: Implement a total request-size limit as an additional layer of defense.
3. Trusted Users: Restrict conversion to trusted users only.
4. Execution-Time Limits: Apply strict execution-time limits for Markdown parsing.
5. Rate Limiting: Implement rate limiting to reduce the number of requests an attacker can send.
6. Disable Autolink: Disable `AutolinkExtension` and avoid `GithubFlavoredMarkdownExtension` to remove the autolink-specific trigger. However, note that the core multibyte parsing paths remain reachable in the standard parser.
Impact:
- Availability: The vulnerability can lead to a denial-of-service (DoS) condition. An attacker can consume disproportionate CPU time and memory, exhausting server resources and preventing legitimate requests from being processed.
- Performance: The quadratic time complexity means that processing time grows exponentially with input size, making even modest inputs cause significant performance degradation.
- Scope: The vulnerability affects any application that processes user-submitted Markdown using the vulnerable library versions, particularly web applications and content management systems.
- No Data Breach: The impact is limited to availability; it does not disclose data, change rendered output, or bypass rendering restrictions.
- Configuration Ineffective: Settings such as `html_input` and `allow_unsafe_links` do not mitigate the issue because the expensive work occurs before rendering.
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Sources:
Reported By: github.com
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