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CVE-2026-115349

Mistune 3.0.0 before 3.3.0 Denial of Service via Nested Bracket Parsing

8.7 High

Mistune from 3.0.0 before 3.3.0 contains an algorithmic complexity denial of service vulnerability in the parse_link_text function in src/mistune/helpers.py. Attackers can submit markdown with deeply nested square brackets triggering quadratic parsing time, exhausting server CPU with small crafted requests.

Attack profile

How easy it is to exploit

Attack vector
Network
Complexity
Low
Privileges needed
None
User interaction
None

What an attacker can affect

Confidentiality
None
Integrity
None
Availability
High

Exploit likelihood

0.34%Unchanged since Oct 23, 2026
90%50%10%1%

What each source reports

CVE List Score used8.7

Mistune 3.0.0 before 3.3.0 Denial of Service via Nested Bracket Parsing

Mistune from 3.0.0 before 3.3.0 contains an algorithmic complexity denial of service vulnerability in the parse_link_text function in src/mistune/helpers.py. Attackers can submit markdown with deeply nested square brackets triggering quadratic parsing time, exhausting server CPU with small crafted requests.

Record
CVE-2026-115349
CVSS 4.0
8.7 HighCVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N
CVSS 3.1
7.5 HighCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
Weakness
CWE-1333
Updated
2026-11-02
EUVDEUVD-2026-1084188.7

Mistune from 3.0.0 before 3.3.0 contains an algorithmic complexity denial of service vulnerability in the parse_link_text function in src/mistune/helpers.py. Attackers can submit markdown with deeply nested square brackets triggering quadratic parsing time, exhausting server CPU with small crafted requests.

Record
EUVD-2026-108418
CVSS 4.0
8.7 HighCVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N
Updated
2026-11-02
GitHub AdvisoriesGHSA-4239-8pr8-4pwv8.7

Mistune from 3.0.0 before 3.3.0 contains an algorithmic complexity denial of service...

Mistune from 3.0.0 before 3.3.0 contains an algorithmic complexity denial of service vulnerability in the parse_link_text function in src/mistune/helpers.py. Attackers can submit markdown with deeply nested square brackets triggering quadratic parsing time, exhausting server CPU with small crafted requests.

Record
GHSA-4239-8pr8-4pwv
CVSS 4.0
8.7 HighCVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N/E:X/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/MVI:X/MVA:X/MSC:X/MSI:X/MSA:X/S:X/AU:X/R:X/V:X/RE:X/U:X
CVSS 3.1
7.5 HighCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
Weakness
CWE-1333
Updated
2026-11-02
NVD8.7

Mistune from 3.0.0 before 3.3.0 contains an algorithmic complexity denial of service vulnerability in the parse_link_text function in src/mistune/helpers.py. Attackers can submit markdown with deeply nested square brackets triggering quadratic parsing time, exhausting server CPU with small crafted requests.

Record
CVE-2026-115349
CVSS 4.0
8.7 HighCVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N/E:X/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/MVI:X/MVA:X/MSC:X/MSI:X/MSA:X/S:X/AU:X/R:X/V:X/RE:X/U:X
CVSS 3.1
7.5 HighCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
Weakness
CWE-1333
Updated
2026-11-02
OSV8.7

Mistune 3.0.0 before 3.3.0 Denial of Service via Nested Bracket Parsing

Mistune from 3.0.0 before 3.3.0 contains an algorithmic complexity denial of service vulnerability in the parse_link_text function in src/mistune/helpers.py. Attackers can submit markdown with deeply nested square brackets triggering quadratic parsing time, exhausting server CPU with small crafted requests.

Record
CVE-2026-115349
CVSS 4.0
8.7 HighCVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N
Weakness
CWE-1333
Updated
2026-11-04
OSVDEBIAN-CVE-2026-1153498.7

Mistune from 3.0.0 before 3.3.0 contains an algorithmic complexity denial of service vulnerability in the parse_link_text function in src/mistune/helpers.py. Attackers can submit markdown with deeply nested square brackets triggering quadratic parsing time, exhausting server CPU with small crafted requests.

Record
DEBIAN-CVE-2026-115349
CVSS 4.0
8.7 HighCVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N/E:X/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/MVI:X/MVA:X/MSC:X/MSI:X/MSA:X/S:X/AU:X/R:X/V:X/RE:X/U:X
Updated
2026-11-02
OSVOESA-2026-47927.5

python-mistune security update

The python-mistune package is a fast yet powerful Python Markdown parser with renderers and plugins, compatible with sane CommonMark rules.

Security Fix(es):

Mistune before 3.3.0 contains an inefficient algorithmic complexity vulnerability in parse_link_text that allows remote attackers to cause denial of service through unbalanced square brackets. Attackers can submit markdown containing long runs of unmatched brackets to mistune.html(), triggering repeated forward scans that stall worker processes for minutes.(CVE-2026-115347)

Mistune from 3.0.0 before 3.3.0 contains an algorithmic complexity vulnerability in the inline link parser's parse_link_text function that allows unauthenticated attackers to exhaust CPU with unbalanced brackets. Attackers can submit Markdown containing many unbalanced '[' or '![' characters, forcing repeated unbounded forward scans and tying up worker processes for minutes.(CVE-2026-115348)

Mistune from 3.0.0 before 3.3.0 contains an algorithmic complexity denial of service vulnerability in the parse_link_text function in src/mistune/helpers.py. Attackers can submit markdown with deeply nested square brackets triggering quadratic parsing time, exhausting server CPU with small crafted requests.(CVE-2026-115349)

Mistune before 3.3.0 contains an algorithmic complexity vulnerability in the inline link parser that allows unauthenticated attackers to exhaust CPU by submitting long runs of unmatched '[' characters. Attackers can send small markdown payloads that trigger O(N^2) parse_link_text and parse_link_label probing, occupying rendering workers for 30+ seconds per 16 KB input.(CVE-2026-115352)

Record
OESA-2026-4792
CVSS 3.1
7.5 HighCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
Updated
2026-11-11
OSVUBUNTU-CVE-2026-1153498.7

Mistune from 3.0.0 before 3.3.0 contains an algorithmic complexity denial of service vulnerability in the parse_link_text function in src/mistune/helpers.py. Attackers can submit markdown with deeply nested square brackets triggering quadratic parsing time, exhausting server CPU with small crafted requests.

Record
UBUNTU-CVE-2026-115349
CVSS 4.0
8.7 HighCVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N
CVSS 3.1
7.5 HighCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
Updated
2026-11-02
OSVopenSUSE-SU-2026:12407-1No score

python313-mistune-3.4.0-2.1 on GA media

These are all security issues fixed in the python313-mistune-3.4.0-2.1 package on the GA media of openSUSE Tumbleweed.

Record
openSUSE-SU-2026:12407-1
Updated
2026-11-04

Affected products

Affected packages

Affected packages
PackageAffectedFixed
mistune
Debian:12
All versions, plus listed versions
2.0.4-12.0.4-23.0.2-13.0.2-23.1.3-13.1.4-13.3.4-13.4.0-1
None known
mistune
Debian:13
All versions, plus listed versions
3.1.3-13.1.4-13.3.4-13.4.0-1
None known
mistune
Debian:14
All versions, plus listed versions
3.1.3-13.1.4-13.3.4-13.4.0-1
None known
python-mistune
openEuler:22.03-LTS-SP4
From an advisory covering 4 CVEs
before 0.8.3-9.oe2203sp4
0.8.3-9.oe2203sp4
mistune
Ubuntu:16.04:LTS
All versions, plus listed versions
0.6-20.7.1-1
None known
mistune
Ubuntu:18.04:LTS
All versions, plus listed versions
0.7.4-10.8-10.8.1-10.8.3-2
None known
mistune
Ubuntu:20.04:LTS
All versions, plus listed versions
0.8.4-10.8.4-2
None known
mistune
Ubuntu:22.04:LTS
All versions, plus listed versions
0.8.4-40.8.4-52.0.0-1+really0.8.4-1
None known
mistune
Ubuntu:24.04:LTS
All versions, plus listed versions
2.0.4-23.0.2-1
None known
mistune
Ubuntu:26.10
All versions, plus listed versions
3.1.4-1
None known
mistune
Ubuntu:26.04:LTS
All versions, plus listed versions
3.1.3-13.1.4-1
None known
python-mistune
openSUSE:Tumbleweed
From an advisory covering 4 CVEs
before 3.4.0-2.1
3.4.0-2.1

History

History
RecordedChangedSources
affected, aliases, referencesOSV
affected, facts
affected, aliases, facts, referencesOSV
first indexedCVE List, EUVD, GitHub Advisories, NVD, OSV

Dates are when Every CVE recorded each change. Upstream dates may differ.

In short

CVE-2026-115349 is a high vulnerability (CVSS 8.7) in Lepture Mistune, published October 22, 2026. No source lists public exploit code. EPSS puts the chance of exploitation in the next 30 days at 0.34%, the 28th percentile. No source names a fixed version.

References

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