SB2026091530 - Multiple vulnerabilities in libexpat

Description

This security bulletin contains information about 4 vulnerabilities.

1) Inefficient Algorithmic Complexity (CVE-ID: CVE-2026-66046)

CWE-ID: CWE-407 - Inefficient Algorithmic Complexity

CVSSv4: 8.7 [CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N]

The vulnerability allows a remote attacker to cause a denial of service.

The vulnerability exists due to algorithmic complexity in attribute declaration handling when parsing XML documents with numerous attributes. A remote attacker can submit a crafted XML document to cause a denial of service.

2) Inefficient Algorithmic Complexity (CVE-ID: CVE-2026-76641)

CWE-ID: CWE-407 - Inefficient Algorithmic Complexity

CVSSv4: 8.7 [CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N]

The vulnerability allows a remote attacker to cause a denial of service.

The vulnerability exists due to an inefficient algorithm in attribute isCdata lookups when parsing crafted XML input. A remote attacker can submit moderately sized crafted XML input to cause a denial of service.

A compression layer around XML can significantly reduce the minimum attack payload size.

3) Unchecked Return Value (CVE-ID: CVE-2026-76956)

CWE-ID: CWE-252 - Unchecked Return Value

CVSSv4: 8.2 [CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N]

The vulnerability allows a remote attacker to cause a denial of service.

The vulnerability exists due to inverted getentropy() return handling in the entropy extraction logic when initializing hash randomization. A remote attacker can conduct a hash-flooding attack to cause a denial of service.

The issue applies where getentropy is configured or detected as the only high-quality entropy extractor.

4) Use-after-free (CVE-ID: CVE-2026-76957)

CWE-ID: CWE-416 - Use After Free

CVSSv4: 6.9 [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]

The vulnerability allows a remote attacker to compromise vulnerable system.

The vulnerability exists due to a lack of handler call depth tracking with custom encoding callbacks. A remote attacker can pass specially crafted XML input to the application, trigger a use-after-free error and perform a denial of service attack.

Remediation

Install update from vendor's website.

References

• https://blog.hartwork.org/posts/expat-2-8-4-released/
• https://github.com/libexpat/libexpat/pull/1321
• https://www.openwall.com/lists/oss-security/2026/08/31/13
• https://github.com/libexpat/libexpat/pull/1331
• https://github.com/libexpat/libexpat/pull/1326