CVE-2025-68142VulnerabilityPublished 16 Dec 2025

CVE-2025-68142: facelessuser pymdown-extensions · ReDoS

Affects facelessuser pymdown-extensionsSource CVE Database V5

Not known to be exploited · CVSS 2.7 Low : upgrade to 10.16.1 when convenient.

Data as of 26 Oct 2026, 06:26 UTC

CVSS 4.0 base score

2.7Low

How severe the flaw is on the 0–10 CVSS scale.

EPSS: 0.4% chance of exploitation in 30 days – higher than 41% of CVEsExploit Prediction Scoring System: the likelihood attackers will use this flaw soon.
Patch: Available – fixed in 10.16.1

CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:L/SC:N/SI:N/SA:N/E:U

Base metrics

Attack Vector: Network
Attack Complexity: Low
Attack Requirements: None
Privileges Required: None
User Interaction: None
Vulnerable system confidentiality: None
Vulnerable system integrity: None
Vulnerable system availability: Low
Subsequent system confidentiality: None
Subsequent system integrity: None
Subsequent system availability: None

Threat metrics

Exploit Maturity: Unreported

Bold values are the worst case: easiest to exploit or largest impact.

Attack Vector: Network: The vulnerable system is bound to the network stack and the set of possible attackers extends beyond the other options listed below, up to and including the entire Internet. Such a vulnerability is often termed “remotely exploitable” and can be thought of as an attack being exploitable at the protocol level one or more network hops away (e.g., across one or more routers).
Attack Complexity: Low: The attacker must take no measurable action to exploit the vulnerability. The attack requires no target-specific circumvention to exploit the vulnerability. An attacker can expect repeatable success against the vulnerable system.
Attack Requirements: None: The successful attack does not depend on the deployment and execution conditions of the vulnerable system. The attacker can expect to be able to reach the vulnerability and execute the exploit under all or most instances of the vulnerability.
Privileges Required: None: The attacker is unauthorized prior to attack, and therefore does not require any access to settings or files of the vulnerable system to carry out an attack.
User Interaction: None: The vulnerable system can be exploited without interaction from any human user, other than the attacker.
Vulnerable system confidentiality: None: There is no loss of confidentiality within the Vulnerable System.
Vulnerable system integrity: None: There is no loss of integrity within the Vulnerable System.
Vulnerable system availability: Low: Performance is reduced or there are interruptions in resource availability. Even if repeated exploitation of the vulnerability is possible, the attacker does not have the ability to completely deny service to legitimate users. The resources in the Vulnerable System are either partially available all of the time, or fully available only some of the time, but overall there is no direct, serious consequence to the Vulnerable System.
Subsequent system confidentiality: None: There is no loss of confidentiality within the Subsequent System or all confidentiality impact is constrained to the Vulnerable System.
Subsequent system integrity: None: There is no loss of integrity within the Subsequent System or all integrity impact is constrained to the Vulnerable System.
Subsequent system availability: None: There is no impact to availability within the Subsequent System or all availability impact is constrained to the Vulnerable System.
Exploit Maturity: Unreported: Based on threat intelligence sources each of the following must apply: · No knowledge of publicly available proof-of-concept · No knowledge of reported attempts to exploit this vulnerability · No knowledge of publicly available solutions used to simplify attempts to exploit the vulnerability

Definitions: FIRST CVSS v4.0 specification.

Exploitation evidence

Disclosed: evidence found.
CVE published 16 Dec 2025
Detectable: no evidence on record.
No public nuclei template indexed
Weaponised: no evidence on record.
No public exploit code indexed in Metasploit or Exploit-DB
Exploited in the wildExploited: no evidence on record.
Not in CISA KEV and no source reports exploitation
Observed by OffSeq MirageSeen by Mirage: no evidence on record.
No attack traffic recorded by OffSeq Mirage sensors

A missing step is not evidence of safety. Sources: cve.org (opens in a new tab), the Metasploit, Exploit-DB and nuclei-templates index as of 2026-10-26.

Disclosed: The CVE record is public on cve.org. Its publication is t = 0.
Detectable: A public nuclei template can find exposed instances, for defenders and attackers alike.
Weaponised: Public exploit code exists in Metasploit or Exploit-DB, or CISA SSVC reports a public proof of concept.
Exploited in the wild: CISA KEV lists it, CISA SSVC rates exploitation active, or a source reports attacks.
Observed by OffSeq Mirage: OffSeq Mirage honeypot sensors recorded attack traffic for this CVE, or found exposed hosts without attack traffic.

Moments are spaced evenly, not to scale; a dashed line marks a gap of six months or more. Deltas count from t = 0. Sources that give only a date (CISA KEV, Exploit-DB, or a time of exactly 00:00Z) read “same day” rather than hours. Metasploit modules carry the flaw’s disclosure date, not their release date, and nuclei templates carry no date, so both appear above but not on the timeline.

What to do

Upgrade facelessuser pymdown-extensions to 10.16.1.

Affected software

Product	Affected	Fixed in
PyPIpymdown-extensions`pkg:pypi/pymdown-extensions`Other threats	Affected <10.16.1	Fixed in 10.16.1

Check your own hosts for these packages with threat-finder (opens in a new tab), our free open-source scanner.

AI-suggested

European organizations should immediately upgrade facelessuser pymdown-extensions to version 10.16.1 or later to remediate the ReDOS vulnerability. Until upgrades are possible, disable or avoid using the pymdownx.blocks.caption extension when processing untrusted markdown content. Implement input validation and size limits on markdown content submitted by users to reduce the risk of resource exhaustion. Employ processing timeouts or sandboxing techniques for markdown rendering services to prevent long hangs. Monitor application logs and performance metrics for unusual spikes in CPU or processing time that may indicate exploitation attempts. Educate developers and DevOps teams about the vulnerability and ensure dependencies are regularly audited for security updates. Consider using alternative markdown processing libraries without known ReDOS issues if upgrading is delayed.

Description

### Impact This issue describes a ReDOS bug found within the figure caption extension (`pymdownx.blocks.caption` ). In systems that take unchecked user content, this could cause long hangs when processing the data if a malicious payload was crafted. ### Patches This issue is patched in Release [10.16.1](https://pypi.org/project/pymdown-extensions/10.16.1/). ### Workarounds Some possible workarounds If users are concerned about this vulnerability and process unknown user content without timeouts or other safeguards in place to prevent really large, malicious content being aimed at systems, the use of `pymdownx.blocks.caption` could be avoided until the library is updated to 10.16.1+. ### References The original issue https://github.com/facelessuser/pymdown-extensions/issues/2716. ### Description The original issue came through PyMdown Extensions' normal issue tracker instead of the typical security flow: https://github.com/facelessuser/pymdown-extensions/issues/2716. Because this came through the normal issue flow, it was handled as a normal issue. In the future, PyMdown Extensions will ensure such issues, even if prematurely made public through the normal issue flow, are redirected through the typical security process. The regular expression pattern in question is as follows: ```py RE_FIG_NUM = re.compile(r'^(\^)?([1-9][0-9]*(?:.[1-9][0-9]*)*)(?= |$)') ``` The POC was provided by @ShangzhiXu ```py import re import time regex_pattern = re.compile(r'^(\^)?([1-9][0-9]*(?:.[1-9][0-9]*)*)(?= |$)') for i in range(50, 500, 50): long_string = '1' * i + 'a' start_time = time.time() match = re.match(regex_pattern, long_string) end_time = time.time() print(f"long_string execution time: {end_time - start_time:.6f} s") ``` The issue with the above pattern is that `.` was used, which accepts any character when we meant to use `\.`. The fix was to update the pattern to: ```py RE_FIG_NUM = re.compile(r'^(\^)?([1-9][0-9]*(?:\.[1-9][0-9]*)*)(?= |$)') ``` Relevant PR with fix: https://github.com/facelessuser/pymdown-extensions/pull/2717 ### Version(s) & System Info - Operating System: Any - Python Version: Any

Impact

For European organizations, the primary impact is availability degradation due to potential denial-of-service conditions when processing malicious markdown content with vulnerable pymdown-extensions versions. This can affect web applications, content management systems, documentation platforms, or any service that renders or processes user-generated markdown using the vulnerable extension. Organizations relying on automated markdown rendering pipelines or collaborative documentation tools could experience service interruptions or degraded performance. Although the vulnerability does not expose sensitive data or allow code execution, the disruption of services can impact business continuity and user experience. The low CVSS score reflects the limited scope and ease of mitigation, but organizations with high markdown content exposure should prioritize patching to prevent potential abuse. The absence of known exploits reduces immediate risk but does not eliminate the threat of future attacks.

Summary

CVE-2025-68142 is a vulnerability classified under CWE-1333, related to inefficient regular expression complexity, found in the facelessuser pymdown-extensions Python library. The issue exists in the figure caption extension (pymdownx.blocks.caption) prior to version 10.16.1. When processing markdown content, this extension uses a regular expression that can exhibit exponential backtracking behavior if fed with specially crafted input, leading to a Regular Expression Denial of Service (ReDOS). This causes the markdown processor to hang or consume excessive CPU resources, resulting in denial of service. The vulnerability can be exploited remotely without authentication or user interaction by submitting malicious markdown content to systems that utilize the vulnerable extension for rendering or processing user-supplied markdown. The vulnerability is mitigated by upgrading to pymdown-extensions version 10.16.1 or later, where the problematic regex has been fixed. As a temporary workaround, disabling or avoiding the use of the figure caption extension when processing untrusted content can reduce risk. The vulnerability does not affect confidentiality or integrity but impacts availability by enabling resource exhaustion. No known exploits are currently reported in the wild.

CWE-1333Inefficient Regular Expression Complexity (opens in a new tab)
Base weaknessLikelihood of exploit: High
The product uses a regular expression with a worst-case computational complexity that is inefficient and possibly exponential.
Architecture and Design
Use regular expressions that do not support backtracking, e.g. by removing nested quantifiers.
System Configuration
Set backtracking limits in the configuration of the regular expression implementation, such as PHP's pcre.backtrack_limit. Also consider limits on execution time for the process.
Implementation
Do not use regular expressions with untrusted input. If regular expressions must be used, avoid using backtracking in the expression.
OWASP Cheat Sheets
File Upload (opens in a new tab)
OWASP ASVS 4.0.3
V12.1.1 (opens in a new tab)
Attack patterns
Regular Expression Exponential Blowup (opens in a new tab)
From MITRE CWE, MITRE CAPEC and OWASP (Top 10, OpenCRE).

References

Community reviews

Loading reviews…

Exploitation evidence

What to do

Affected software

Description

AI analysisMachine-generated, updated 23 Dec 2025, 19:26 UTC

WeaknessesCWE-1333

Technical details

References

Community reviews

Related threats