{"id":"CVE-2026-49851","aliases":["GHSA-qcq2-496w-v96p","PYSEC-2026-2652"],"title":" Mistune: Potential DoS via quadratic-time parsing in parse_link_text","summary":" Mistune: Potential DoS via quadratic-time parsing in parse_link_text","severity":"high","cvss":7.5,"cvssVector":"CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H","vendor":"mistune","product":"mistune","ecosystem":"pip","affected":["mistune < 3.3.0"],"patched":["mistune 3.3.0"],"published":"2026-07-09","updated":"2026-09-10","sourceUpdated":"2026-09-10T03:51:11.807497442Z","source":"OSV","sourceUrl":"https://osv.dev/vulnerability/GHSA-qcq2-496w-v96p","references":[{"url":"https://github.com/lepture/mistune/security/advisories/GHSA-qcq2-496w-v96p"},{"url":"https://nvd.nist.gov/vuln/detail/CVE-2026-49851"},{"url":"https://access.redhat.com/security/cve/CVE-2026-49851"},{"url":"https://bugzilla.redhat.com/show_bug.cgi?id=2492304"},{"url":"https://github.com/lepture/mistune"},{"url":"https://security.access.redhat.com/data/csaf/v2/vex/2026/cve-2026-49851.json"},{"url":"https://github.com/advisories/GHSA-qcq2-496w-v96p"}],"tags":["osv","pip","ghsa"],"epss":0.0063,"epssPercentile":0.48833,"cwe":["CWE-400","CWE-1333"],"ingestedAt":"2026-07-09T23:53:27.194Z","slug":"CVE-2026-49851","body":"## Overview\n\n### Summary\nMistune is vulnerable to a CPU exhaustion DoS due to superlinear (approximately O(n²)) behavior in parse_link_text. A relatively small input consisting of repeated [ characters causes significant parsing slowdown.\n\n### Affected component\nmistune/inline_parser.py → **parse_link_text**\n\n### Description\nWhen parsing Markdown containing many consecutive [ characters, parse_link_text repeatedly scans the input using a regex search inside a loop. Each iteration re-scans a large portion of the remaining string, resulting in quadratic-time behavior.\nAn attacker-controlled Markdown input can therefore trigger excessive CPU usage with a very small payload.\n\n### Root cause\nThe vulnerability stems from a two-loop interaction:\n- The outer loop in `InlineParser.parse()` (inline_parser.py) advances \n only 1 character at a time when parse_link() returns None\n- Each failed attempt calls `parse_link_text()` which performs an O(n) \n scan to the end of the string looking for a closing `]`\n- With n consecutive `[` characters, this results in O(n) × O(n) = O(n²) \n total work\n\n### PoC\nRun below python script\n```\nimport mistune\nimport time\n\nmd = mistune.create_markdown()\n\ns = \"[\" * 6400\n\nt = time.perf_counter()\nmd(s)\nprint(time.perf_counter() - t)\n```\n\"image\"\n\n**Benmark poc**\nRun below code for benchmark\n```\nimport mistune\nimport time\n\nmd = mistune.create_markdown()\n\nsizes = [100,200,400,800,1600,3200,6400]\n\nfor n in sizes:\n s = \"[\" * n\n\n t0 = time.perf_counter()\n md(s)\n dt = time.perf_counter() - t0\n\n print(f\"{n:6d} {dt:.6f}\")\n```\n\"image\"\n\n\n### Observed behaviour\n```\npython3 benchmark.py \n 100 0.001609\n 200 0.003207\n 400 0.012906\n 800 0.050220\n 1600 0.197307\n 3200 0.801172\n 6400 3.190393\n```\nExecution time grows superlinearly, consistent with O(n²) complex\n\n### Impact\nThis can be used as a denial-of-service attack in any application that parses user-supplied Markdown using Mistune, including:\n\n- Web applications (comments, posts, content rendering)\n- API services processing Markdown\n- Documentation rendering systems\n- A small (~6 KB) payload can block CPU for multiple seconds.\n\n### Suggested fix\nReturn the furthest scanned position from parse_link_text even on failure, so the outer loop can skip ahead instead of advancing 1 character at a time\n\n### Security Classification\nCWE-400: Uncontrolled Resource Consumption\nDenial of Service (CPU exhaustion)\n\n## Affected packages\n\n- `mistune < 3.3.0`\n\n## Remediation\n\nUpgrade to a patched release:\n\n- `mistune 3.3.0`","depth":"twilight","depthScore":41,"depthScoreParts":{"impact":41.3,"likelihood":0.1,"exploitation":0,"ransomware":0},"changes":[]}