libexpat: Unexpected Status Code or Return Value
CVE-2026-76956 is a high-severity unexpected status code or return value vulnerability in libexpat from Libexpat Project, with a CVSS 3.1 base score of 7.5 out of 10. It can be exploited remotely over the network by an unauthenticated attacker, without any action from a user. The record lists version 2.8.4 as the first without the flaw, so updating libexpat to that release or later removes the exposure. There is no confirmed exploitation so far. EPSS estimates a 0.29% chance of exploitation within 30 days, which is low compared with other CVEs.
High severity with no evidence of exploitation so far.
What to do
- Update libexpat to 2.8.4 or a later version.
- 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 |
|---|---|---|---|
| Libexpat Project | libexpat | 2.8.2 up to, but not including, 2.8.4 | 2.8.4 |
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
- UnchangedImpact stays within the vulnerable component.
- Confidentiality
- NoneNo data is disclosed.
- Integrity
- NoneNothing can be modified.
- Availability
- HighThe component can be taken down completely.
Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H. Scored by NVD.
| Scoring system | Score | Severity | Scored by |
|---|---|---|---|
| CVSS 3.1 | 5.9 | Medium | CNA (mitre.org) |
| CVSS 3.1 | 7.5 | High | NVD |
Official description
In libexpat 2.8.2 and 2.8.3 before 2.8.4, misinterpretation of getentropy's return code leads to insufficient entropy, which results in being vulnerable to hash flooding attacks, causing a denial of service via crafted XML content.
Source: the CVE record as published by its numbering authority, via NVD and CVE.org.
The underlying weakness: Unexpected Status Code or Return Value
The product does not properly check when a function or operation returns a value that is legitimate for the function, but is not expected by the product.
More Unexpected Status Code or Return Value vulnerabilities (CWE-394)
Timeline
- CVE record published.
- CISA published its SSVC assessment (exploitation: none, automatable: yes, technical impact: partial).
- Record last changed materially (score, affected products, status or references).
References
Patches and release notes
- github.com/libexpat/libexpat/pull/1326Issue Tracking
- github.com/libexpat/libexpat/pull/1329Issue Tracking
Other vulnerabilities in libexpat
- 7.8 High severity
- 7.5 High severity
- 7.5 High severity
- 7.8 High severity
- 9.8 Critical severity
- 9.8 Critical severity
Recent Unexpected Status Code or Return Value vulnerabilities
- 9.8 Critical severity
- 7.3 High severity
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