CVE-2026-59928: Mistune block_parser: quadratic-time parsing on long lists of repeated reference-link definitions
Vulnerability summary
CVE-2026-59928 is a high (CVSS 7.5) Inefficient Algorithmic Complexity vulnerability in lepture mistune. EPSS estimates a 0.65% chance of exploitation in the next 30 days, higher than 51.2% 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.
- 08-07-2026Last modifiedVulnerability information updated.
Description
Mistune is a Python Markdown parser with renderers and plugins. Prior to 3.3.0, a Markdown document containing many repeated or distinct reference-link definitions causes quadratic work in src/mistune/block_parser.py and the ref_links environment dictionary handling, 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
0.65%
Chance of exploitation in the next 30 days
Percentile
51%
Versus all other CVEs
30-day change
No change 0.00 pp
From 0.65% 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-ffq3-xpv3-j92qX Refsource CONFIRMWEBEVIDENCEREPORTExploitThird Party Advisory
- github.com/lepture/mistune/commit/2b04d7ba341c16ac78fe82d3076bdd5c3de87c69X Refsource MISCWEBFIXPatch
- github.com/lepture/mistunePACKAGE
