CVE-2026-59928 | Mistune block_parser: quadratic-time parsing on long lists of repeated reference-link definitions

Exp

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.

Published: 2026-07-08 Last update: 2026-07-09 Assigner: [email protected] Source: [email protected]
NVD Status：Analyzed，CVE State: published
View at NVD, CVE.org, EUVD

CVE-2026-59928 is rated Exploit Available (55/100): CVSS High severity, with low exploitation likelihood (EPSS 0.37%). 1 public exploit reference(s) are indexed (Exploit-DB). Public exploits are available—assess exposure, apply mitigations, and prioritize patching.

Risk is dynamic; we continuously reassess and refresh what is shown on this page as upstream context changes.

Public exploit references (Exploit-DB) for CVE-2026-59928

EDB-ID Source Kind Published Link
— nvd_ref exploit_tag Exploit-DB ↗

Exploit prediction scoring system (EPSS) score for CVE-2026-59928

EPSS lead: Daily EPSS estimates relative likelihood of exploitation; percentile ranks this CVE among scored vulnerabilities (higher = more severe relative rank).

# Date Old EPSS score New EPSS score Delta (New - Old)
1 2026-07-10 0.35% 0.37% +0.02%
2 2026-07-09 — 0.35% —

Full EPSS history (2 records total)

Common vulnerability scoring system (CVSS) metrics for CVE-2026-59928

CVSS metrics for this CVE.

Base score Version Severity Vector Exploitability Impact Score source
7.5 3.1 HIGH
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H Click to expand
Attack vector (AV:N)
Could be attacked over the internet or any normal routed network—not just someone sitting at the machine.
Attack complexity (AC:L)
Once they can reach the bug, pulling it off is straightforward—no weird race conditions or rare setup.
Privileges required (PR:N)
No account or special rights needed—anonymous or random user is enough.
User interaction (UI:N)
Nobody has to click “OK” or open a trap file; it can work without a victim helping.
Scope (S:U)
Damage stays in the same “trust bubble” as the broken component—no big spill into unrelated systems.
Confidentiality (C:N)
Doesn’t really leak secrets in a meaningful way.
Integrity (I:N)
Data isn’t meaningfully altered or forged.
Availability (A:H)
Could take the service down hard or make it unusable for people who depend on it.
 3.9 3.6 [email protected]

Weakness enumeration for CVE-2026-59928

GitHub Security Advisory for CVE-2026-59928

GHSA-ffq3-xpv3-j92q · Severity: high · Ecosystem: pip — Mistune block_parser: quadratic-time parsing on long lists of repeated reference-link definitions

OS Trackers for CVE-2026-59928

vendor priority summary link
debian not yet assigned CVE-2026-59928 not yet assigned priority: Debian including 1 source packages (mistune), 5 status rows across 5 suites (bookworm, bullseye, forky, sid, trixie): open 5. https://security-tracker.debian.org/tracker/CVE-2026-59928
redhat medium — https://access.redhat.com/security/cve/CVE-2026-59928
suse medium CVE-2026-59928 severity moderate: SUSE including 3 source package names (python-mistune, python3-mistune, python311-mistune), 21 product×package rows across 11 product lines (SUSE Linux Enterprise High Performance Computing 15 SP4-LTSS, SUSE Linux Enterprise High Performance Computing 15 SP5-LTSS, … (11 product lines)): Known Not Affected 21. https://www.suse.com/security/cve/CVE-2026-59928/
ubuntu medium CVE-2026-59928 medium priority: Ubuntu including 1 source packages (mistune), 7 status rows across 7 suites (bionic, focal, jammy, noble, questing, resolute, upstream): needs-triage 6, ignored 1. https://ubuntu.com/security/CVE-2026-59928

Affected software / configurations for CVE-2026-59928

Vendor Product Version Raw CPE
mistune_project mistune < 3.3.0 cpe:2.3:a:mistune_project:mistune:*:*:*:*:*:*:*:*

References for CVE-2026-59928

cvelogic Threat Intelligence