{"description":"Trending threats, MITRE ATT\u0026CK coverage, and detection metadata. Fed continuously.","feed_url":"https://feed.craftedsignal.io/vendors/mistune/feed.json","home_page_url":"https://feed.craftedsignal.io/","items":[{"_cs_actors":[],"_cs_cpes":[],"_cs_cves":[{"cvss":7.5,"id":"CVE-2026-49851"}],"_cs_exploited":false,"_cs_has_poc":false,"_cs_poc_references":[],"_cs_products":["mistune (\u003c 3.3.0)"],"_cs_severities":["medium"],"_cs_tags":["denial-of-service","python","library","vulnerability","markdown"],"_cs_type":"advisory","_cs_vendors":["Mistune"],"content_html":"\u003cp\u003eA significant CPU exhaustion Denial of Service (DoS) vulnerability, tracked as CVE-2026-49851, has been identified in the Mistune Python Markdown parser. This vulnerability stems from inefficient parsing logic within the \u003ccode\u003eparse_link_text\u003c/code\u003e function, which exhibits superlinear (approximately O(n²)) time complexity when processing specific malicious inputs. Attackers can exploit this by submitting Markdown content containing a large number of consecutive \u003ccode\u003e[\u003c/code\u003e characters. This forces Mistune to repeatedly scan and re-scan the input, leading to excessive CPU utilization. The issue affects Mistune versions prior to 3.3.0 and poses a critical risk to any application that uses Mistune to process user-supplied Markdown, including web applications, API services, and documentation rendering systems.\u003c/p\u003e\n\u003ch2 id=\"attack-chain\"\u003eAttack Chain\u003c/h2\u003e\n\u003col\u003e\n\u003cli\u003eAn attacker crafts a malicious Markdown payload consisting of a large number of consecutive opening square bracket characters (e.g., \u003ccode\u003es = \u0026quot;[\u0026quot; * 6400\u003c/code\u003e).\u003c/li\u003e\n\u003cli\u003eThe malicious Markdown payload is delivered to a target application that uses Mistune for parsing user-supplied content, via HTTP request body, API input, or other means.\u003c/li\u003e\n\u003cli\u003eThe application processes the input, invoking the \u003ccode\u003emistune.create_markdown()\u003c/code\u003e function to parse the content.\u003c/li\u003e\n\u003cli\u003eDuring parsing, the \u003ccode\u003eInlineParser.parse()\u003c/code\u003e function in \u003ccode\u003emistune/inline_parser.py\u003c/code\u003e attempts to interpret the malformed link text.\u003c/li\u003e\n\u003cli\u003eWhen \u003ccode\u003eparse_link()\u003c/code\u003e fails to find a valid link, the outer loop in \u003ccode\u003eInlineParser.parse()\u003c/code\u003e advances only one character at a time.\u003c/li\u003e\n\u003cli\u003eEach failed attempt triggers \u003ccode\u003eparse_link_text()\u003c/code\u003e which performs an O(n) scan to the end of the string searching for a closing \u003ccode\u003e]\u003c/code\u003e.\u003c/li\u003e\n\u003cli\u003eThe combination of the O(n) scan in \u003ccode\u003eparse_link_text()\u003c/code\u003e within an outer loop that iterates O(n) times results in a total O(n²) CPU exhaustion, causing the application to become unresponsive or crash.\u003c/li\u003e\n\u003c/ol\u003e\n\u003ch2 id=\"impact\"\u003eImpact\u003c/h2\u003e\n\u003cp\u003eSuccessful exploitation of CVE-2026-49851 can lead to a denial of service for any application that parses user-supplied Markdown using vulnerable versions of Mistune. This includes web applications handling comments or posts, API services processing Markdown content, and documentation rendering systems. With a relatively small malicious payload (e.g., 6,400 characters, approximately 6 KB), an attacker can cause CPU exhaustion for multiple seconds on the targeted server, significantly degrading performance or rendering the service completely unavailable. There is no specific victim count or sector mentioned, but any organization using vulnerable Mistune installations in internet-facing applications is at risk.\u003c/p\u003e\n\u003ch2 id=\"recommendation\"\u003eRecommendation\u003c/h2\u003e\n\u003cul\u003e\n\u003cli\u003eImmediately patch the Mistune library to version 3.3.0 or later to address \u003cstrong\u003eCVE-2026-49851\u003c/strong\u003e.\u003c/li\u003e\n\u003cli\u003eImplement robust input validation and sanitization for all user-supplied Markdown content processed by your applications to prevent malformed inputs that exploit \u003cstrong\u003eCVE-2026-49851\u003c/strong\u003e.\u003c/li\u003e\n\u003cli\u003eMonitor application CPU usage for unexpected spikes as a potential indicator of a DoS attack attempt.\u003c/li\u003e\n\u003c/ul\u003e\n","date_modified":"2026-07-09T23:53:38Z","date_published":"2026-07-09T23:53:38Z","id":"https://feed.craftedsignal.io/briefs/2026-07-mistune-dos/","summary":"The Mistune Python Markdown parser is vulnerable to a CPU exhaustion Denial of Service (DoS) attack, identified as CVE-2026-49851, due to a superlinear (O(n²)) parsing behavior in the `parse_link_text` function when processing specially crafted input containing repeated square brackets, allowing an attacker to significantly degrade application performance with a small payload.","title":"Mistune Markdown Parser Vulnerable to CPU Exhaustion DoS (CVE-2026-49851)","url":"https://feed.craftedsignal.io/briefs/2026-07-mistune-dos/"}],"language":"en","title":"CraftedSignal Threat Feed - Mistune","version":"https://jsonfeed.org/version/1.1"}