Summary
Mistune block_parser: quadratic-time parsing on long lists of repeated reference-link definitions
Type: Algorithmic-complexity DoS in reference-link definition handling. A markdown document with N reference-link definitions of the same key (or many distinct keys) takes O(N²) parser time. 5000 repeated [a]: u\n definitions take ~1.1 second; 10000 → ~4.5 seconds.
File: src/mistune/block_parser.py (reference-link def parsing) and the surrounding ref_links env-dictionary handling.
Root cause: every reference definition is parsed by scanning forward from each candidate position. The unikey normalisation runs per-def, the dictionary insert is per-def, and the lookup-by-label-then-iterate-defs path is linear in the number of stored defs. For input with N defs, the total work is O(N²).
Affected Code
src/mistune/block_parser.py, reference-definition rule fires on every line that matches [label]: url. For each one:
unikey(label)is called (linear scan of the label).- The def is appended to
state.env['ref_links']. - Later inline-link resolution looks up by
unikey(label)in the dict (O(1)) but the surrounding parser revisits the def list for paragraph-vs-def disambiguation.
The cumulative parse time grows as the square of the number of defs.
Why it's wrong: the parser does not amortise the def-list scan. A single forward pass with a hash-keyed dict (already in place) plus a per-line classifier should make this O(N).
Exploit Chain
- Application uses mistune to render attacker-supplied markdown. No plugins required.
- Attacker submits a 35 KB document of
[a]: u\nrepeated 5000 times followed by[click][a]. - CPU pegs for ~1.1 seconds. 10000 defs → ~4.5 s. 20000 → ~18 s. Doubling input quadruples time.
Security Impact
Attacker capability: small input → large CPU. Predictable scaling. Can be repeated.
Preconditions: application uses mistune.create_markdown() (default config) on attacker-supplied markdown. Worth noting: the ref_links dictionary persists for the lifetime of the parse, so a long document with many defs builds up memory; with N defs of attacker-chosen length, the per-def normalisation cost compounds.
Differential: PoC-verified against [email protected], default config:
import mistune, time
md = mistune.create_markdown()
for n in [1000, 2000, 5000, 10000]:
s = '[a]: u\n' * n + '[click][a]'
t = time.time()
md(s)
print(f' ref defs * {n} ({len(s)}b): {(time.time() - t) * 1000:.0f}ms')
# Output (Python 3.13, Linux, 2.5GHz CPU):
# ref defs * 1000 ( 7012b): 46ms
# ref defs * 2000 (14012b): 186ms
# ref defs * 5000 (35012b): 1121ms
# ref defs * 10000 (70012b): 4400ms
The patched build (with the surrounding parser amortised to O(N)) keeps the time linear.
Impact
A regular expression with worst-case exponential or polynomial matching time is applied to untrusted input, causing excessive CPU use. Typical impact: denial of service when input is crafted to trigger backtracking.
CVE-2026-59928 has a CVSS score of 7.5 (High). The vector is network-reachable, no privileges required, and no user interaction. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. A fixed version is available (3.3.0); upgrading removes the vulnerable code path.
Affected versions
Security releases
Kodem intelligence
Severity tells you how bad this could be in the worst case. It does not tell you whether you are exposed. Exploitability and impact are functions of runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A vulnerable package can sit in your dependency tree and never run.
Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter. Kodem's runtime-powered SCA identifies whether this CVE is reachable in your applications.
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See it in your environmentNew to Kodem? Get a demo →Remediation advice
Replace the per-def re-scan with a single forward pass that classifies each line into ref_def | paragraph | other once and only inserts into ref_links once per def. The dict already exists; the wasted work is in the surrounding scan loop, not in the dict operations.
A regression test asserting that md('[a]: u\n' * 50_000 + '[click][a]') completes in under 1 second would catch any regression.
Frequently Asked Questions
- What is CVE-2026-59928? CVE-2026-59928 is a high-severity inefficient regular expression (ReDoS) vulnerability in mistune (pip), affecting versions < 3.3.0. It is fixed in 3.3.0. A regular expression with worst-case exponential or polynomial matching time is applied to untrusted input, causing excessive CPU use.
- How severe is CVE-2026-59928? CVE-2026-59928 has a CVSS score of 7.5 (High). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
- Which versions of mistune are affected by CVE-2026-59928? mistune (pip) versions < 3.3.0 is affected.
- Is there a fix for CVE-2026-59928? Yes. CVE-2026-59928 is fixed in 3.3.0. Upgrade to this version or later.
- Is CVE-2026-59928 exploitable, and should I be worried? Whether CVE-2026-59928 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
- What actually determines whether CVE-2026-59928 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
- How do I fix CVE-2026-59928? Upgrade
mistuneto 3.3.0 or later.