CVE-2026-59922: Mistune plugins/formatting: quadratic-time parsing on long runs of `~~x~~`, `==x==`, and `^^x^^` markers (strikethrough / mark / insert)
Vulnerability summary
CVE-2026-59922 is a high (CVSS 7.5) Inefficient Algorithmic Complexity vulnerability in lepture mistune. EPSS estimates a 0.64% chance of exploitation in the next 30 days, higher than 50.9% of scored vulnerabilities. 1 public exploit is recorded.
Public exploit (GitHub, Date unknown)
Source freshness unavailable
How is this decided?Advisory lineage
Upstream 0 · Downstream 2 · Related 2Timeline
- 08-07-2026PublishedVulnerability first disclosed.
- 09-07-2026Last modifiedVulnerability information updated.
Description
Mistune is a Python Markdown parser with renderers and plugins. Prior to 3.3.0, a run of closed tilde, equals-sign, or caret marker pairs around a character causes quadratic work in src/mistune/plugins/formatting.py when the strikethrough, mark, or insert plugin scans for matching markers from each possible start position, allowing denial of service through CPU exhaustion. This issue is fixed in version 3.3.0.
CVSS metrics
5 scores · 5 sources · 1 version- OSV · PypiEcosystem advisory · calculated7.5HighCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
- OSV · UbuntuEcosystem advisory · calculated7.5HighCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
- OSV · DebianEcosystem advisory · calculated7.5HighCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
- NVDNIST7.5HighCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
EPSS trend
Probability
0.64%
Chance of exploitation in the next 30 days
Percentile
51%
Versus all other CVEs
30-day change
No change 0.00 pp
From 0.64% on 04-10-2026
Known exploited
Compare KEV catalogs
CISA KEVNot in the CISA KEV catalog.
ENISA CNWNot in the ENISA CNW catalog.
CIRCLNot in the CIRCL catalog.
Public exploits
1 foundMistune
Maturity: poc
Weaknesses
CWE · root cause- CWE-407Inefficient Algorithmic ComplexityAn algorithm in a product has an inefficient worst-case computational complexity that may be detrimental to system performance and can be triggered by an attacker, typically using crafted manipulations that ensure that the worst case is being reached.
- CWE-1333Inefficient Regular Expression ComplexityThe product uses a regular expression with a worst-case computational complexity that is inefficient and possibly exponential.
Affected systems
Known conditions: network-reachable, no authentication, no user interaction (CVSS vector). Other conditions: not stated in public data.
Debian1 product
- mistuneAll versions (No fix recorded)
Lepture1 product
- mistune< 3.3.0
Mistune_project1 product
- mistune< 3.3.0
PyPI1 product
- mistune< 3.3.0
Ubuntu1 product
- mistuneAll versions (No fix recorded)
5 vendors · 5 products
References
9 links- github.com/lepture/mistune/security/advisories/GHSA-c8j7-8cv4-2xmqX Refsource CONFIRMWEBEVIDENCEREPORTExploitVendor Advisory
- github.com/lepture/mistune/commit/96d0f57f8fe9eeb06bb4cff521962a27d7c402e7X Refsource MISCWEBFIXPatch
- github.com/lepture/mistunePACKAGE
