CVE-2026-26825: A use-of-uninitialized memory vulnerability exists in libxls 1.6.3 when parsing malformed XLS files

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MEDIUM 5.3REMOTEDOWNGRADE

CVE-2026-26825 is a medium-severity flaw (CVSS 5.3) affecting libxls. Below: why it ranks where it does, which versions are affected, what to do, and the exact commands.

A use-of-uninitialized memory vulnerability exists in libxls 1.6.3 when parsing malformed XLS files. The issue is reachable via xls_parseWorkBook() and is triggered by uninitialized heap memory originating from the OLE layer (ole2_read). The flaw is detectable with MemorySanitizer (MSAN) and can lead to undefined behavior, incorrect parsing logic, or potential information disclosure.

Why this priority

Public exploit reference
Remotely exploitable, no login, no user action

No fix is published yet: follow the downgrade or mitigation steps.

Risk facts

CVSS score	5.3 (Medium)
Attack vector	Network (remote)
Attack complexity	Low
Privileges needed	None
User interaction	None
EPSS exploit chance	0.2% (top 85%)
CISA exploited list	No
Weakness	CWE-908
Published	2026-06-03
Last modified	2026-09-23

Publisher (CNA): MITRE · Vendor: libxls project

Last updated 2026-09-23 · Sources: CVE Program, NVD, CISA KEV, EPSS and vendor advisories · How we score

Am I affected?

Timeline

2026-06-03 20:16 UTCPublished CVE record published
2026-09-23 16:16 UTCUpdated Record last modified

libxls project libxls DOWNGRADE

Affected: 1.6.3

Fixed in: no fixed release known

Safe to downgrade to: a release before 1.6.3

1. Check installed version

apt-cache policy libxls || rpm -q libxls || libxls --version

2. No fixed release: DOWNGRADE to a release before 1.6.3 (Debian / Ubuntu)

apt-cache madison libxls sudo apt install libxls=<VERSION_BEFORE_1.6.3> sudo apt-mark hold libxls

2. No fixed release: DOWNGRADE (RHEL / Amazon Linux / Fedora)

dnf --showduplicates list libxls sudo dnf downgrade libxls-<VERSION_BEFORE_1.6.3> sudo dnf versionlock add libxls

Downgrade risk

Downgrading can break dependent software and may reopen older bugs. Read the distro advisory first and test on a non-production host.

After downgrade

Restart the service, and keep the package held until a fixed release ships.

Verify at the official source NVD · CVE Program · Report a problem with this page
Scores and fixes here are computed automatically from those and other public sources. Confirm with the vendor advisory before changing production systems.

Quick answers

How do I fix CVE-2026-26825?

libxls: downgrade or apply the listed mitigation. Exact commands for each OS are on this page.

Is CVE-2026-26825 being exploited?

CVE-2026-26825 is not on the CISA Known Exploited Vulnerabilities list at the time of the last update. Check again, as this can change.

What does CVE-2026-26825 affect?

CVE-2026-26825 affects libxls. Severity is medium (CVSS 5.3). A use-of-uninitialized memory vulnerability exists in libxls 1.6.3 when parsing malformed XLS files. The issue is reachable via xls_parseWorkBook() and is triggered by uninitialized heap memory originating from the OLE layer (ole2_read). Th

How urgent is CVE-2026-26825?

Priority 32/100. Public exploit reference; Remotely exploitable, no login, no user action

Related CVEs

CVE-2026-26824 MEDIUM 6.5 UPGRADE libxls through version 1.6.3 contains a use of uninitialized memory vulnerability in the OLE container parser

References

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