CVE-2021-26691

CRITICALNVD 9.89.8—Elevated

9.8

In Apache HTTP Server versions 2.4.0 to 2.4.46 a specially crafted SessionHeader sent by an origin server could cause a heap overflow

CVSS v3: 9.8
EG Score: 9.8(medium)
EPSS: 97.8%
KEV: Not listed

Published

June 10, 2021

Last Modified

November 21, 2024

References (26)

security@apachehttp://httpd.apache.org/security/vulnerabilities_24.html
security@apachehttp://www.openwall.com/lists/oss-security/2021/06/10/7
security@apachehttps://lists.apache.org/thread.html/r50cae1b71f1e7421069036b213c26da7d8f47dd59874e3bd956959fe%40%3Cannounce.httpd.apache.org%3E
security@apachehttps://lists.apache.org/thread.html/r7f2b70b621651548f4b6f027552f1dd91705d7111bb5d15cda0a68dd%40%3Cdev.httpd.apache.org%3E
security@apachehttps://lists.apache.org/thread.html/re026d3da9d7824bd93b9f871c0fdda978d960c7e62d8c43cba8d0bf3%40%3Ccvs.httpd.apache.org%3E
security@apachehttps://lists.debian.org/debian-lts-announce/2021/07/msg00006.html
security@apachehttps://lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/SPBR6WUYBJNACHKE65SPL7TJOHX7RHWD/
security@apachehttps://lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/ZNCYSR3BXT36FFF4XTCPL3HDQK4VP45R/
security@apachehttps://security.gentoo.org/glsa/202107-38
security@apachehttps://security.netapp.com/advisory/ntap-20210702-0001/
security@apachehttps://www.debian.org/security/2021/dsa-4937
security@apachehttps://www.oracle.com/security-alerts/cpujan2022.html
security@apachehttps://www.oracle.com/security-alerts/cpuoct2021.html
af854a3a-2127-422b-91ae-364da2661108http://httpd.apache.org/security/vulnerabilities_24.html
af854a3a-2127-422b-91ae-364da2661108http://www.openwall.com/lists/oss-security/2021/06/10/7

Patch Availability(6)

Vendor / Ecosystem	Fixed in / Patch	Released	Source
ubuntu	apache2-bin (2.4.46-4ubuntu1.1) @ hirsute	2026-05-23	ubuntu
ubuntu	libapache2-mod-proxy-html (1:2.4.7-1ubuntu4.22+esm1) @ trusty	2026-05-23	ubuntu
redhat	httpd-0:2.4.6-97.el7_9.4	2022-01-17	redhat
redhat	httpd	2021-11-10	redhat
redhat	jbcs-httpd24-openssl-pkcs11-0:0.4.10-22.jbcs.el7	2021-11-10	redhat
redhat	httpd:2.4-8040020211008164252.522a0ee4	2021-10-12	redhat

Patches are aggregated from vendor advisories (Red Hat, Microsoft, Cisco, GitHub) and package ecosystems (OSV, GHSA). Multiple rows for the same upstream release have been deduplicated.

Weakness Classification(2)

MITRE Common Weakness Enumeration — the root-cause categories this CVE belongs to.

All Vendor Advisories

(7)

Every vendor that published an advisory referencing this CVE — pulled from our cve_vendor_advisories aggregation. Click any row for the vendor's original advisory page.

Data Freshness Timeline

(refreshed 9× in last 7d / 17× in last 30d)

Every time one of our enrichment pipelines (NVD, MITRE cvelistV5, EPSS, CISA KEV, GHSA, OSV, vendor advisories) ran against this CVE. Most recent first.

2026-06-02 20:11 UTCepss_batch
2026-06-02 20:11 UTCepss_batch
2026-06-02 20:11 UTCepss_batch
2026-05-31 22:29 UTCepss_batch
2026-05-31 22:29 UTCepss_batch
2026-05-31 00:15 UTCepss_batch
2026-05-29 13:42 UTCepss_batch
2026-05-28 13:43 UTCepss_batch
2026-05-28 13:43 UTCepss_batch
2026-05-27 13:39 UTCepss_batch
2026-05-27 13:39 UTCepss_batch
2026-05-26 13:42 UTCepss_batch
2026-05-26 07:17 UTCepss_batch
2026-05-26 07:17 UTCepss_batch
2026-05-23 08:13 UTCEG score recompute
2026-05-23 08:13 UTCVendor advisory
2026-05-23 08:13 UTCGHSA enrichment

Frequently asked(5)

What is CVE-2021-26691?

CVE-2021-26691 is a critical vulnerability published on June 10, 2021. In Apache HTTP Server versions 2.4.0 to 2.4.46 a specially crafted SessionHeader sent by an origin server could cause a heap overflow

When was CVE-2021-26691 disclosed?

CVE-2021-26691 was first published in the National Vulnerability Database on June 10, 2021, with the most recent update on November 21, 2024. EchelonGraph re-ingests CVE updates from NVD on a 2-hour cycle, so this page reflects the latest published state.

Is CVE-2021-26691 actively exploited?

CVE-2021-26691 is not currently on CISA's Known Exploited Vulnerabilities catalog. FIRST EPSS estimates a 97.8% percentile likelihood of exploitation in the next 30 days — higher percentiles indicate greater predicted risk.

What is the CVSS score of CVE-2021-26691?

CVE-2021-26691 has a CVSS v3 base score of 9.8 (NVD).

How do I remediate CVE-2021-26691?

Patch to the fixed version published by the affected vendor. Where vendor advisories exist for CVE-2021-26691, EchelonGraph cross-links them in the Vendor Advisories panel below — those typically contain the canonical remediation steps, fixed version numbers, and any vendor-specific mitigations.

Dependency Blast Radius

See which npm, PyPI, Go, and Maven packages are affected by CVE-2021-26691

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Is Your Infrastructure Affected by CVE-2021-26691?

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CVE-2021-26691

Published

Last Modified

References (26)

Patch Availability(6)

Weakness Classification(2)

All Vendor Advisories

Data Freshness Timeline

Frequently asked(5)

Dependency Blast Radius

Is Your Infrastructure Affected by CVE-2021-26691?

Same vendor

Same CWE

CVE-2021-26691

📅 Published

🔄 Last Modified

🔗 References (26)

Patch Availability(6)

Weakness Classification(2)

All Vendor Advisories

Data Freshness Timeline

❓ Frequently asked(5)

Dependency Blast Radius

Is Your Infrastructure Affected by CVE-2021-26691?

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References (26)

Frequently asked(5)

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