VulnerabilityPublished 24 Jun 2019Updated 30 May 2025

CVE-2018-20843: Expat · XML External Entity (XXE)

Source CVE Database V5

Not known to be exploited · CVSS 7.5 High : upgrade to 2.2.7 soon.

Data as of 26 Oct 2026, 11:07 UTC

CVSS 3.1 base score

7.5High

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

EPSS: 7.1% chance of exploitation in 30 days – higher than 94% of CVEsExploit Prediction Scoring System: the likelihood attackers will use this flaw soon.
Patch: Available – fixed in 2.2.7

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

Attack Vector: Network
Attack Complexity: Low
Privileges Required: None
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: None: The attacker is unauthorized prior to attack, and therefore does not require any access to settings or files to carry out an attack.
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 24 Jun 2019
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 26 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.
Weaponised: Public exploit code exists in Metasploit or Exploit-DB, 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 libexpat/libexpat to 2.2.7.

Affected software

Product	Affected	Fixed in
GitHublibexpat/libexpat`pkg:github/libexpat/libexpat`Other threats	Affected <2.2.7	Fixed in 2.2.7

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 first identify all software and systems that use libexpat for XML parsing. This includes checking embedded libraries in applications, middleware, and network devices. The primary mitigation is to upgrade libexpat to version 2.2.7 or later, where this vulnerability is fixed. If upgrading is not immediately feasible, organizations should implement input validation and filtering to detect and block XML inputs containing excessive colon characters or other suspicious patterns that could trigger the vulnerability. Network-level protections such as Web Application Firewalls (WAFs) or Intrusion Prevention Systems (IPS) can be configured to detect and block malformed XML payloads. Additionally, rate limiting and resource usage monitoring can help detect and mitigate potential DoS attempts. Regular vulnerability scanning and patch management processes should be enforced to ensure timely updates. Finally, organizations should conduct security awareness and incident response planning to quickly identify and respond to potential DoS incidents stemming from XML parsing issues.

Description

In libexpat in Expat before 2.2.7, XML input including XML names that contain a large number of colons could make the XML parser consume a high amount of RAM and CPU resources while processing (enough to be usable for denial-of-service attacks).

Impact

For European organizations, the impact of this vulnerability can be significant, especially for those relying on software or services that incorporate libexpat for XML processing. Industries such as finance, telecommunications, healthcare, and government services often process XML data extensively and could be targeted for DoS attacks that disrupt critical operations. The denial-of-service effect could lead to downtime, loss of availability of essential services, and potential cascading effects on dependent systems. Since the vulnerability does not compromise confidentiality or integrity, data breaches are unlikely; however, service unavailability can still cause operational and reputational damage. Organizations running legacy or unpatched systems with embedded libexpat versions prior to 2.2.7 are at risk. Attackers could exploit this vulnerability remotely without authentication or user interaction, increasing the threat surface. The absence of known exploits in the wild suggests limited active exploitation, but the ease of triggering the vulnerability means it remains a credible threat if left unmitigated.

CWE-611Improper Restriction of XML External Entity Reference (opens in a new tab)
Base weaknessOWASP Top 10 A02:2025: Security Misconfiguration (opens in a new tab)
The product processes an XML document that can contain XML entities with URIs that resolve to documents outside of the intended sphere of control, causing the product to embed incorrect documents into its output.
Implementation · System Configuration
Many XML parsers and validators can be configured to disable external entity expansion.
OWASP Cheat Sheets
Deserialization (opens in a new tab)
XML Security (opens in a new tab)
XML External Entity Prevention (opens in a new tab)
OWASP ASVS 4.0.3
V5.5.2 (opens in a new tab)
Attack patterns
Data Serialization External Entities Blowup (opens in a new tab)
From MITRE CWE, MITRE CAPEC and OWASP (Top 10, OpenCRE).

References

Community reviews

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Exploitation evidence

What to do

Affected software

Description

AI analysisMachine-generated, updated 8 Jul 2025, 14:10 UTC

WeaknessesCWE-611

Technical details

References

Community reviews

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