VulnerabilityPublished 6 Mar 2026

CVE-2026-27807: MarkUsProject Markus · XML Entity Expansion

Affects MarkUsProject MarkusSource CVE Database V5

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Not known to be exploited · CVSS 4.9 Medium : upgrade to 2.9.4 when convenient.

Data as of 29 Oct 2026, 19:04 UTC

CVSS 3.1 base score

4.9Medium

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

EPSS: 0.2% chance of exploitation in 30 days – higher than 22% of CVEsExploit Prediction Scoring System: the likelihood attackers will use this flaw soon.
Patch: Available – fixed in 2.9.4

CVSS:3.1/AV:N/AC:L/PR:H/UI:N/S:U/C:N/I:N/A:H

Attack Vector: Network
Attack Complexity: Low
Privileges Required: High
User Interaction: None
Scope: Unchanged
Confidentiality: None
Integrity: None
Availability: High

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

Attack Vector: Network: The vulnerable component is bound to the network stack and the set of possible attackers extends beyond the other options listed, 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: Specialized access conditions or extenuating circumstances do not exist. An attacker can expect repeatable success against the vulnerable component.
Privileges Required: High: The attacker is authorized with (i.e., requires) privileges that provide significant (e.g., administrative) control over the vulnerable component that could affect component-wide settings and files.
User Interaction: None: The vulnerable system can be exploited without any interaction from any user.
Scope: Unchanged: An exploited vulnerability can only affect resources managed by the same security authority. In this case, the vulnerable component and the impacted component are either the same, or both are managed by the same security authority.
Confidentiality: None: There is no loss of confidentiality within the impacted component.
Integrity: None: There is no loss of integrity within the impacted component.
Availability: High: There is total loss of availability, resulting in the attacker being able to fully deny access to resources in the impacted component; this loss is either sustained (while the attacker continues to deliver the attack) or persistent (the condition persists even after the attack has completed). Alternatively, the attacker has the ability to deny some availability, but the loss of availability presents a direct, serious consequence to the impacted component (e.g., the attacker cannot disrupt existing connections, but can prevent new connections; the attacker can repeatedly exploit a vulnerability that, in each instance of a successful attack, leaks a only small amount of memory, but after repeated exploitation causes a service to become completely unavailable).

Definitions: FIRST CVSS v3.1 specification.

Exploitation evidence

Disclosed: evidence found.
CVE published 6 Mar 2026
Detectable: no evidence on record.
No public nuclei template indexed
Weaponized: 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 29 Oct 2026.

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.
Weaponized: A Metasploit exploit module or Exploit-DB entry exists (Metasploit scanner modules do not count), or CISA SSVC or the CVSS exploit maturity 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 Markus and markusproject/Markus to 2.9.4.

Affected software

Product	Affected	Fixed in
MarkUsProjectMarkus	Affected <2.9.4	Fixed in 2.9.4
GitHubmarkusproject/Markus`pkg:github/markusproject/Markus`Other threats	Affected <2.9.4	Fixed in 2.9.4

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

AI-suggested

To mitigate CVE-2026-27807, organizations should promptly upgrade Markus to version 2.9.4 or later, where the vulnerability is patched. Until upgrading, restrict YAML file upload permissions strictly to trusted instructors and monitor uploads for suspicious content. Implement application-layer controls to validate and sanitize YAML files before parsing, specifically disabling or limiting alias usage and recursive references. Employ runtime resource limits on the application server to prevent excessive CPU and memory consumption during parsing. Conduct regular audits of user privileges to minimize the number of accounts with upload capabilities. Additionally, monitor application logs for unusual parsing errors or performance degradation that may indicate attempted exploitation. Educate instructors on secure file handling practices and enforce strong authentication mechanisms to reduce the risk of account compromise. Finally, maintain up-to-date backups and incident response plans to quickly recover from potential denial of service incidents.

Description

MarkUs is a web application for the submission and grading of student assignments. Prior to version 2.9.4, MarkUs allows course instructors to upload YAML files to create/update various entities (e.g., assignment settings). These YAML files are parsed with aliases enabled. This issue has been patched in version 2.9.4.

Impact

The primary impact of CVE-2026-27807 is denial of service through resource exhaustion caused by recursive entity expansion during YAML file parsing. This can disrupt the availability of the Markus application, preventing instructors and students from accessing assignment submission and grading functionalities. While confidentiality and integrity are not directly affected, prolonged downtime can hinder educational workflows and delay grading processes. Organizations relying on Markus for course management may experience operational disruptions, especially during peak academic periods. Since exploitation requires authenticated instructor privileges, the risk is somewhat contained within trusted users, but insider threats or compromised instructor accounts could leverage this vulnerability. The absence of known exploits in the wild reduces immediate risk, but unpatched systems remain vulnerable to targeted attacks. The impact is more pronounced in institutions with high reliance on Markus and limited incident response capabilities.

CWE-776Improper Restriction of Recursive Entity References in DTDs (Billion Laughs Attack) (opens in a new tab)
Base weaknessLikelihood of exploit: MediumOWASP Top 10 A02:2025: Security Misconfiguration (opens in a new tab)
The product uses XML documents and allows their structure to be defined with a Document Type Definition (DTD), but it does not properly control the number of recursive definitions of entities.
Operation
If possible, prohibit the use of DTDs or use an XML parser that limits the expansion of recursive DTD entities.
Implementation
Before parsing XML files with associated DTDs, scan for recursive entity declarations and do not continue parsing potentially explosive content.
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
Exponential Data Expansion (opens in a new tab)
From MITRE CWE, MITRE CAPEC and OWASP (Top 10, OpenCRE).

References

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Exploitation evidence

What to do

Affected software

Description

AI analysisMachine-generated, updated 6 Mar 2026, 03:46 UTC

WeaknessesCWE-776

Technical details

References

Community reviews

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