fast-xml-parser: Incorrect Regular Expression
CVE-2026-25896 is a critical-severity incorrect regular expression vulnerability in fast-xml-parser from NaturalIntelligence, with a CVSS 3.1 base score of 9.3 out of 10. It can be exploited remotely over the network by an unauthenticated attacker, without any action from a user. Affected fast-xml-parser versions: >= 5.0.0, < 5.3.5; >= 4.1.3, < 4.5.4. The record does not name a fixed version, so check the vendor advisory. There is no confirmed exploitation so far. EPSS estimates a 0.53% chance of exploitation within 30 days, which is low compared with other CVEs.
Critical severity: a successful attack would have severe impact.
What to do
- Check which version of fast-xml-parser you run against the affected versions below, and install the vendor's latest release.
- Read the advisory on github.com for workarounds and exact fixed builds.
- Until you can update, restrict network access to the affected service so it is not reachable from untrusted networks.
Affected products and versions
| Vendor | Product | Affected versions | Fixed in |
|---|---|---|---|
| NaturalIntelligence | fast-xml-parser | >= 5.0.0, < 5.3.5; >= 4.1.3, < 4.5.4 | Not stated |
| Red Hat | Red Hat Advanced Cluster Security 4.8 | Not specified in the record | Not stated |
| Red Hat | Red Hat Advanced Cluster Security 4.9 | Not specified in the record | Not stated |
| Red Hat | Red Hat Developer Hub 1.8 | Not specified in the record | Not stated |
| Red Hat | Red Hat Developer Hub 1.9 | Not specified in the record | Not stated |
| Red Hat | Red Hat Openshift Data Foundation 4.18 | Not specified in the record | Not stated |
| Red Hat | Red Hat Openshift Data Foundation 4.19 | Not specified in the record | Not stated |
| Red Hat | Red Hat Openshift Data Foundation 4.20 | Not specified in the record | Not stated |
| Red Hat | Red Hat Satellite 6.18 | Not specified in the record | Not stated |
| Red Hat | Migration Toolkit for Applications 8 | Not specified in the record | Not stated |
| Red Hat | Red Hat OpenShift GitOps | Not specified in the record | Not stated |
| Red Hat | Red Hat OpenShift Virtualization 4 | Not specified in the record | Not stated |
| Red Hat | Red Hat Satellite 6 | Not specified in the record | Not stated |
| Red Hat | Self-service automation portal 2 | Not specified in the record | Not stated |
Version data comes from the organisation that assigned the CVE. Confirm against the vendor advisory before deciding you are unaffected.
How the severity score breaks down
- Attack vector
- NetworkCan be exploited remotely over a network, including the internet.
- Attack complexity
- LowNo special conditions are needed; the attack works reliably.
- Privileges required
- NoneNo account or login is needed.
- User interaction
- NoneNo action by a victim is required.
- Scope
- ChangedThe attack can reach beyond the vulnerable component into other systems.
- Confidentiality
- LowSome restricted data can be read.
- Integrity
- HighData or code can be modified at will.
- Availability
- NoneAvailability is not affected.
Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:L/I:H/A:N. Scored by CNA (github.com).
| Scoring system | Score | Severity | Scored by |
|---|---|---|---|
| CVSS 3.1 | 9.3 | Critical | CNA (github.com) |
| CVSS 3.1 | 7.1 | High | CNA |
Official description
fast-xml-parser allows users to validate XML, parse XML to JS object, or build XML from JS object without C/C++ based libraries and no callback. From 4.1.3to before 5.3.5, a dot (.) in a DOCTYPE entity name is treated as a regex wildcard during entity replacement, allowing an attacker to shadow built-in XML entities (<, >, &, ", ') with arbitrary values. This bypasses entity encoding and leads to XSS when parsed output is rendered. This vulnerability is fixed in 5.3.5.
Source: the CVE record as published by its numbering authority, via NVD and CVE.org.
The underlying weakness: Incorrect Regular Expression
The product specifies a regular expression in a way that causes data to be improperly matched or compared.
How developers prevent this class of flaw
- Regular expressions can become error prone when defining a complex language even for those experienced in writing grammars. Determine if several smaller regular expressions simplify one large regular expression. Also, subject the regular expression to thorough testing techniques such as equivalence partitioning, boundary value analysis, and robustness. After testing and a reasonable confidence level is achieved, a regular expression may not be foolproof. If an exploit is allowed to slip through, then record the exploit and refactor the regular expression.
Timeline
- CVE record published.
- CISA published its SSVC assessment (exploitation: poc, automatable: yes, technical impact: partial).
- Record last changed materially (score, affected products, status or references).
References
Advisories
Patches and release notes
Other references
- access.redhat.com/errata/RHSA-2026:40984
- access.redhat.com/errata/RHSA-2026:41941
- access.redhat.com/errata/RHSA-2026:41944
- access.redhat.com/errata/RHSA-2026:51349
- access.redhat.com/errata/RHSA-2026:6174
- access.redhat.com/errata/RHSA-2026:6802
- access.redhat.com/errata/RHSA-2026:7110
- access.redhat.com/errata/RHSA-2026:7128
- access.redhat.com/security/cve/CVE-2026-25896
- bugzilla.redhat.com/show_bug.cgi?id=2441501
- security.access.redhat.com/data/csaf/v2/vex/2026/cve-2026-25896.json
Other vulnerabilities in fast-xml-parser
- 8.7 High severity
- 7.5 High severity
- 7.5 High severity
- 7.5 High severity
- 7.5 High severity
- 7.5 High severity
Recent Incorrect Regular Expression vulnerabilities
- 7.5 High severity
- 9.8 Critical severity
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