CVE-2021-21828: A heap-based buffer overflow vulnerability exists in the XML Decompression DecodeTreeBlock functionality of AT

This week

CRITICAL 9.8REMOTEDOWNGRADE

CVE-2021-21828 is a critical-severity flaw (CVSS 9.8) affecting xmill. Below: why it ranks where it does, which versions are affected, what to do, and the exact commands.

A heap-based buffer overflow vulnerability exists in the XML Decompression DecodeTreeBlock functionality of AT&T Labs Xmill 0.7. In the default case of DecodeTreeBlock a label is created via CurPath::AddLabel in order to track the label for later reference. An attacker can provide a malicious file to trigger this vulnerability.

Why this priority

Critical severity
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	9.8 (Critical)
Attack vector	Network (remote)
Attack complexity	Low
Privileges needed	None
User interaction	None
EPSS exploit chance	1.1% (top 34%)
CISA exploited list	No
Weakness	CWE-120, CWE-787
Published	2021-08-20
Last modified	2026-06-17

Publisher (CNA): Talos · Vendor: att

Last updated 2026-06-17 · Sources: CVE Program, NVD, CISA KEV, EPSS and vendor advisories · How we score

Am I affected?

Timeline

2021-08-20 22:15 UTCPublished CVE record published
2026-06-17 03:36 UTCUpdated Record last modified

att xmill DOWNGRADE

Affected: 0.7

Fixed in: no fixed release known

Safe to downgrade to: a release before 0.7

1. Check installed version

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

2. No fixed release: DOWNGRADE to a release before 0.7 (Debian / Ubuntu)

apt-cache madison xmill sudo apt install xmill=<VERSION_BEFORE_0.7> sudo apt-mark hold xmill

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

dnf --showduplicates list xmill sudo dnf downgrade xmill-<VERSION_BEFORE_0.7> sudo dnf versionlock add xmill

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-2021-21828?

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

Is CVE-2021-21828 being exploited?

CVE-2021-21828 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-2021-21828 affect?

CVE-2021-21828 affects xmill. Severity is critical (CVSS 9.8). A heap-based buffer overflow vulnerability exists in the XML Decompression DecodeTreeBlock functionality of AT&T Labs Xmill 0.7. In the default case of DecodeTreeBlock a label is created via CurPath::AddLabel in order to track the label for

How urgent is CVE-2021-21828?

Priority 61/100. Critical severity; Public exploit reference; Remotely exploitable, no login, no user action

Related CVEs

CVE-2022-26507 CRITICAL 9.8 DOWNGRADE UPGRADE A heap-based buffer overflow exists in XML Decompression DecodeTreeBlock in AT&T Labs Xmill 0.7
CVE-2021-21830 CRITICAL 9.8 DOWNGRADE A heap-based buffer overflow vulnerability exists in the XML Decompression LabelDict::Load functionality of AT
CVE-2021-21829 CRITICAL 9.8 DOWNGRADE A heap-based buffer overflow vulnerability exists in the XML Decompression EnumerationUncompressor::Uncompress
CVE-2021-21827 CRITICAL 9.8 DOWNGRADE A heap-based buffer overflow vulnerability exists in the XML Decompression DecodeTreeBlock functionality of AT
CVE-2021-21826 CRITICAL 9.8 DOWNGRADE A heap-based buffer overflow vulnerability exists in the XML Decompression DecodeTreeBlock functionality of AT

References

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