How severe is CVE-2021-21828?

CVE-2021-21828 has a CVSS 3.x base score of 9.8, rated critical severity. It is exploitable over network with low attack complexity, requires no privileges and no user interaction. Impact on confidentiality is high, integrity high, and availability high.

Is CVE-2021-21828 being actively exploited?

It is not currently listed in CISA's KEV catalog. Its EPSS exploit-prediction score is 1% (63rd percentile), an estimate of the probability of exploitation in the next 30 days.

What products are affected by CVE-2021-21828?

CVE-2021-21828 affects Att Xmill. See the affected-products list for the exact vulnerable versions.

How do I fix CVE-2021-21828?

Review the linked vendor and NVD advisories for patched versions and mitigations, then upgrade or apply the recommended workaround. Given its critical severity, prioritise patching exposed systems.

When was CVE-2021-21828 published?

CVE-2021-21828 was published on 2021-08-20 and last updated on 2026-06-17.

CVE-2021-21828

CVE-2021-21828 is a critical-severity vulnerability in Att Xmill with a CVSS 3.x base score of 9.8. It is not currently listed as actively exploited by CISA, and its EPSS exploit-prediction score is low. The underlying weakness is classified as CWE-787.

Key facts

Severity: Critical (CVSS 3.x base score 9.8)
CVSS v2: 7.5
EPSS exploit prediction: 1% (63rd percentile)
Actively exploited: Not listed in CISA KEV
Weakness: CWE-787
Affected product: Att Xmill
Published: 2021-08-20
Last modified: 2026-06-17

Description

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.

Frequently asked questions

What is CVE-2021-21828?: 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.
How severe is CVE-2021-21828?: CVE-2021-21828 has a CVSS 3.x base score of 9.8, rated critical severity. It is exploitable over network with low attack complexity, requires no privileges and no user interaction. Impact on confidentiality is high, integrity high, and availability high.
Is CVE-2021-21828 being actively exploited?: It is not currently listed in CISA's KEV catalog. Its EPSS exploit-prediction score is 1% (63rd percentile), an estimate of the probability of exploitation in the next 30 days.
What products are affected by CVE-2021-21828?: CVE-2021-21828 affects Att Xmill. See the affected-products list for the exact vulnerable versions.
How do I fix CVE-2021-21828?: Review the linked vendor and NVD advisories for patched versions and mitigations, then upgrade or apply the recommended workaround. Given its critical severity, prioritise patching exposed systems.
When was CVE-2021-21828 published?: CVE-2021-21828 was published on 2021-08-20 and last updated on 2026-06-17.

References

https://talosintelligence.com/vulnerability_reports/TALOS-2021-1291

Affected products (1)

cpe:2.3:a:att:xmill:0.7:*:*:*:*:*:*:*

More vulnerabilities in Att Xmill

CVE-2022-26507 — Critical (CVSS 9.8): A heap-based buffer overflow exists in XML Decompression DecodeTreeBlock in AT&T Labs Xmill 0.7. A crafted input file…
CVE-2021-21811 — Critical (CVSS 9.8): A memory corruption vulnerability exists in the XML-parsing CreateLabelOrAttrib functionality of AT&T Labs’ Xmill…
CVE-2021-21827 — Critical (CVSS 9.8): A heap-based buffer overflow vulnerability exists in the XML Decompression DecodeTreeBlock functionality of AT&T Labs…
CVE-2021-21826 — Critical (CVSS 9.8): A heap-based buffer overflow vulnerability exists in the XML Decompression DecodeTreeBlock functionality of AT&T Labs…
CVE-2021-21825 — Critical (CVSS 9.8): A heap-based buffer overflow vulnerability exists in the XML Decompression PlainTextUncompressor::UncompressItem…
CVE-2021-21810 — Critical (CVSS 9.8): A memory corruption vulnerability exists in the XML-parsing ParseAttribs functionality of AT&T Labs’ Xmill 0.7. A…

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