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How CVE-2026-71488 Works
This vulnerability affects the PHP library league/commonmark, which is used to parse and render Markdown. It allows an unauthenticated attacker to cause a denial of service (DoS) by submitting specially crafted Markdown that triggers super-linear or quadratic time complexity during parsing. The core of the issue lies in how the parser handles three specific parsing paths, all of which can be triggered by a single line of input.
1. Fenced Code Block Detection (Quadratic): The parser uses a regular expression to detect the start of a fenced code block. The pattern includes a lookahead to enforce a rule about backticks. On a line with a long run of backticks, filler text, and a single trailing backtick, the regex engine’s quantifier gives back characters one by one, re-running the lookahead each time. This results in quadratic work. A 320 KB line can take ~27 seconds to convert, while a similar payload that fails an early guard takes only 0.011 seconds.
2. Reference Link Label Lookup (Quadratic): When the parser encounters a shortcut or collapsed reference link (e.g.,
</code>), it copies the text between the brackets and normalizes it. This normalization involves up to four passes (trim, regex replace, encoding check, and case conversion). With nested brackets, this lookup is performed for each closing bracket on a progressively longer span. With a single reference definition present to unlock this path, 64,000 nested brackets can take 22 seconds to process, compared to 0.59 seconds without it. 3. Emphasis, Strikethrough, and Highlight Delimiter Processing (Super-Linear): This path processes delimiters like ``, <code>_</code>, <code>~</code>, and <code>=</code>. An optimization called `openersBottom` memoization aims to keep this process linear, but it relies on a cache key with an `O(1)` space. The `getCacheKey()` methods for these processors embed the delimiter's raw run length in the key, creating an unbounded key space. An attacker can use a growing number of distinct run lengths, each forcing a fresh, full backward re-scan of all openers. This leads to work growing at roughly <code>n^1.5</code>. An 800 KB input of asterisks, letters, and spaces can take ~27 seconds to convert. The overall impact is a denial of service. An attacker can use relatively small requests to consume excessive CPU time, potentially occupying all available PHP workers and preventing legitimate requests from completing. The issue is patched in version 2.9.1 and later. <h2 style="color: blue;">DailyCVE Form:</h2> Platform: PHP League CommonMark Version: 0.6.0 - 2.9.0 Vulnerability: Denial of Service (ReDoS) Severity: High (CVSS 7.5) date: 2026-09-01 <h2 style="color: blue;">Prediction: Patch available (2.9.1)</h2> <h2 style="color: blue;">What Undercode Say:</h2> The vulnerability stems from inefficient regular expression handling and algorithmic complexity in three distinct parsing paths within the `league/commonmark` library. <h2 style="color: blue;">To check your version, you can use Composer:</h2> [bash] composer show league/commonmark
To immediately mitigate the risk, you can enforce a strict per-line input length limit before passing data to the converter:
Example using PHP to limit line length
$maxLineLength = 4096;
$lines = explode("\n", $userInput);
foreach ($lines as $line) {
if (strlen($line) > $maxLineLength) {
// Reject or truncate the input
throw new Exception("Input line exceeds maximum length.");
}
}
Exploit: (Educational Purposes!)
An attacker can craft a single, long line of Markdown to trigger this vulnerability. For example, a line with a long sequence of backticks followed by filler text and a trailing backtick can cause the quadratic parsing in the fenced code block detection path.
`````````````````````````````````````````````````````````````````````````````````` (filler text) `
Or a deeply nested structure of brackets to exploit the reference link lookup path:
[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]
Protection:
The primary and most effective protection is to upgrade to version 2.9.1 or later.
composer require league/commonmark:^2.9.1
If an immediate upgrade is not possible, implement defensive input validation:
- Enforce a maximum length for individual lines before parsing. This is crucial as every known trigger fits on a single line.
- Apply total request-size limits to prevent large payloads.
- Consider rate-limiting and execution time limits for parsing operations.
- Restrict conversion to trusted users only, as applications processing only trusted Markdown are not remotely exploitable.
Impact:
- Availability: Unauthenticated attackers can cause a denial of service by exhausting CPU resources, preventing the application from responding to legitimate requests.
- Scope: This affects any application using a default `CommonMarkConverter()` or `GithubFlavoredMarkdownConverter()` that processes untrusted Markdown.
- Non-Impact: The vulnerability does not lead to data disclosure, does not alter the rendered output, and does not bypass any rendering restrictions.
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Sources:
Reported By: github.com
Extra Source Hub:
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