How severe is CVE-2021-35209?

CVE-2021-35209 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-2021-35209 being actively exploited?

It is not currently listed in CISA's KEV catalog. Its EPSS exploit-prediction score is 3% (86th percentile), an estimate of the probability of exploitation in the next 30 days.

What products are affected by CVE-2021-35209?

CVE-2021-35209 primarily affects Zimbra Collaboration. In total, 34 product configurations (CPEs) are listed as vulnerable; see the affected-products list for the exact versions.

How do I fix CVE-2021-35209?

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-2021-35209 published?

CVE-2021-35209 was published on 2021-07-02 and last updated on 2026-06-17.

CVE-2021-35209

CVE-2021-35209 is a critical-severity vulnerability in Zimbra Collaboration 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-918.

Key facts

Severity: Critical (CVSS 3.x base score 9.8)
CVSS v2: 7.5
EPSS exploit prediction: 3% (86th percentile)
Actively exploited: Not listed in CISA KEV
Weakness: CWE-918
Affected product: Zimbra Collaboration
Published: 2021-07-02
Last modified: 2026-06-17

Description

An issue was discovered in ProxyServlet.java in the /proxy servlet in Zimbra Collaboration Suite 8.8 before 8.8.15 Patch 23 and 9.x before 9.0.0 Patch 16. The value of the X-Host header overwrites the value of the Host header in proxied requests. The value of X-Host header is not checked against the whitelist of hosts Zimbra is allowed to proxy to (the zimbraProxyAllowedDomains setting).

Frequently asked questions

What is CVE-2021-35209?: An issue was discovered in ProxyServlet.java in the /proxy servlet in Zimbra Collaboration Suite 8.8 before 8.8.15 Patch 23 and 9.x before 9.0.0 Patch 16. The value of the X-Host header overwrites the value of the Host header in proxied requests. The value of X-Host header is not checked against the whitelist of hosts Zimbra is allowed to proxy to (the zimbraProxyAllowedDomains setting).
How severe is CVE-2021-35209?: CVE-2021-35209 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-2021-35209 being actively exploited?: It is not currently listed in CISA's KEV catalog. Its EPSS exploit-prediction score is 3% (86th percentile), an estimate of the probability of exploitation in the next 30 days.
What products are affected by CVE-2021-35209?: CVE-2021-35209 primarily affects Zimbra Collaboration. In total, 34 product configurations (CPEs) are listed as vulnerable; see the affected-products list for the exact versions.
How do I fix CVE-2021-35209?: 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-2021-35209 published?: CVE-2021-35209 was published on 2021-07-02 and last updated on 2026-06-17.

References

Affected products (34)

cpe:2.3:a:zimbra:collaboration:*:*:*:*:*:*:*:*
cpe:2.3:a:zimbra:collaboration:8.8.15:-:*:*:*:*:*:*
cpe:2.3:a:zimbra:collaboration:8.8.15:p1:*:*:*:*:*:*
cpe:2.3:a:zimbra:collaboration:8.8.15:p10:*:*:*:*:*:*
cpe:2.3:a:zimbra:collaboration:8.8.15:p11:*:*:*:*:*:*
cpe:2.3:a:zimbra:collaboration:8.8.15:p12:*:*:*:*:*:*
cpe:2.3:a:zimbra:collaboration:8.8.15:p13:*:*:*:*:*:*
cpe:2.3:a:zimbra:collaboration:8.8.15:p14:*:*:*:*:*:*
cpe:2.3:a:zimbra:collaboration:8.8.15:p15:*:*:*:*:*:*
cpe:2.3:a:zimbra:collaboration:8.8.15:p16:*:*:*:*:*:*
cpe:2.3:a:zimbra:collaboration:8.8.15:p17:*:*:*:*:*:*
cpe:2.3:a:zimbra:collaboration:8.8.15:p18:*:*:*:*:*:*
cpe:2.3:a:zimbra:collaboration:8.8.15:p19:*:*:*:*:*:*
cpe:2.3:a:zimbra:collaboration:8.8.15:p2:*:*:*:*:*:*
cpe:2.3:a:zimbra:collaboration:8.8.15:p3:*:*:*:*:*:*
cpe:2.3:a:zimbra:collaboration:8.8.15:p4:*:*:*:*:*:*
cpe:2.3:a:zimbra:collaboration:8.8.15:p5:*:*:*:*:*:*
cpe:2.3:a:zimbra:collaboration:8.8.15:p6:*:*:*:*:*:*
cpe:2.3:a:zimbra:collaboration:8.8.15:p7:*:*:*:*:*:*
cpe:2.3:a:zimbra:collaboration:8.8.15:p8:*:*:*:*:*:*
cpe:2.3:a:zimbra:collaboration:8.8.15:p9:*:*:*:*:*:*
cpe:2.3:a:zimbra:collaboration:9.0.0:-:*:*:*:*:*:*
cpe:2.3:a:zimbra:collaboration:9.0.0:p1:*:*:*:*:*:*
cpe:2.3:a:zimbra:collaboration:9.0.0:p10:*:*:*:*:*:*
cpe:2.3:a:zimbra:collaboration:9.0.0:p11:*:*:*:*:*:*
cpe:2.3:a:zimbra:collaboration:9.0.0:p12:*:*:*:*:*:*
cpe:2.3:a:zimbra:collaboration:9.0.0:p2:*:*:*:*:*:*
cpe:2.3:a:zimbra:collaboration:9.0.0:p3:*:*:*:*:*:*
cpe:2.3:a:zimbra:collaboration:9.0.0:p4:*:*:*:*:*:*
cpe:2.3:a:zimbra:collaboration:9.0.0:p5:*:*:*:*:*:*
cpe:2.3:a:zimbra:collaboration:9.0.0:p6:*:*:*:*:*:*
cpe:2.3:a:zimbra:collaboration:9.0.0:p7:*:*:*:*:*:*
cpe:2.3:a:zimbra:collaboration:9.0.0:p8:*:*:*:*:*:*
cpe:2.3:a:zimbra:collaboration:9.0.0:p9:*:*:*:*:*:*

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