CVE-2021-26691

CRITICALNVD 9.89.8—
EchelonGraph scoreMEDIUM confidence

Score 9.8 from GitHub Security Advisory (severity: CRITICAL) published 2022-05-24. NVD baseline CVSS 9.8; sources differ by 0.0.

Triggered by: GitHub Security Advisory CVSS
Sources: epss, ghsa, nvd
Elevated
9.8EG
EchelonGraph verdictPatch this weekExploitation is likely or a public exploit exists.
  • 891 internet-exposed hosts are running an affected version right now
  • High exploitation likelihood — EPSS 68%
CISA-KEV: Not listedEPSS PROB: 68%CVSS: 9.8Exploit: Elevated riskExposed: 891

A fix is available — apply it.

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

Live · internet exposure

891 internet-exposed hosts are running an affected version of CVE-2021-26691 right now.

across 64 countries (United States, Germany, Japan, France, United Kingdom)top: http_server
See the live KEV-Exposure radar →Found passively from public internet-scan data — aggregate, host-redacted.

EchelonGraph is the only CVE feed that fuses live vulnerability intelligence with its own live internet-exposure radar — so you see not just that a CVE is exploited, but how much of the internet is exposed to it right now.

CVSS v3
9.8
EG Score
9.8(medium)
EG Risk
76(Track*)
EG Risk 76/100SSVC: Track*

EG Risk is EchelonGraph's 0–100 priority score: it fuses intrinsic severity with real-world exploitation and automatability so you can rank equal-severity CVEs and fix the most dangerous first. Higher = act sooner. Distinct from the 0–10 EG Score (severity).

How it’s computed
Severity98% × 45%
Exploitation68% × 40%
Automatability30% × 15%
Action: Watch closely — could escalate to Attend.
EPSS PROB
68%
EPSS %ILE
99%
KEV
Not listed

Published

June 10, 2021

Last Modified

November 21, 2024

Advisory Details (10)

Auto-updated Sep 5, 2026
⚠️ Active exploitation confirmed. Patch available.
generic

NetApp Product Security

https://security.netapp.com/advisory/ntap-20210702-0001/
generic

Apache: Multiple vulnerabilities (GLSA 202107-38) — Gentoo security

https://security.gentoo.org/glsa/202107-38
generic

[SECURITY] Fedora 35 Update: httpd-2.4.49-1.fc35 - package-announce - Fedora mailing-lists

https://lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/ZNCYSR3BXT36FFF4XTCPL3HDQK4VP45R/
generic

[SECURITY] Fedora 34 Update: httpd-2.4.49-1.fc34 - package-announce - Fedora mailing-lists

https://lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/SPBR6WUYBJNACHKE65SPL7TJOHX7RHWD/
generic Patch Available

[SECURITY] [DLA 2706-1] apache2 security update

https://lists.debian.org/debian-lts-announce/2021/07/msg00006.html
generic
https://lists.apache.org/thread.html/re026d3da9d7824bd93b9f871c0fdda978d960c7e62d8c43cba8d0bf3%40%3Ccvs.httpd.apache.org%3E
generic
https://lists.apache.org/thread.html/r7f2b70b621651548f4b6f027552f1dd91705d7111bb5d15cda0a68dd%40%3Cdev.httpd.apache.org%3E
generic
https://lists.apache.org/thread.html/r50cae1b71f1e7421069036b213c26da7d8f47dd59874e3bd956959fe%40%3Cannounce.httpd.apache.org%3E
generic

oss-security - CVE-2021-26691: Apache httpd: mod_session response handling heap overflow

http://www.openwall.com/lists/oss-security/2021/06/10/7
generic Patch Available🔴 Active Exploitation

Apache HTTP Server 2.4 vulnerabilities - The Apache HTTP Server Project

http://httpd.apache.org/security/vulnerabilities_24.html

Vendor Advisories for CVE-2021-26691(4)

These vendors published their own advisory mentioning this CVE — often with vendor-specific remediation steps + affected product lists not in NVD.

Patch Availability(6)

Vendor / EcosystemFixed in / PatchReleasedSource
ubuntuapache2-bin (2.4.46-4ubuntu1.1) @ hirsute2026-05-23ubuntu
ubuntulibapache2-mod-proxy-html (1:2.4.7-1ubuntu4.22+esm1) @ trusty2026-05-23ubuntu
redhathttpd-0:2.4.6-97.el7_9.42022-01-17redhat
redhathttpd2021-11-10redhat
redhatjbcs-httpd24-openssl-pkcs11-0:0.4.10-22.jbcs.el72021-11-10redhat
redhathttpd:2.4-8040020211008164252.522a0ee42021-10-12redhat

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.

Affected Packages

(21 across 21 ecosystems)
Alpine:v3.10(1)
PackageVulnerable rangeFixed inDependents
apache2—2.4.48-r0—
Alpine:v3.11(1)
PackageVulnerable rangeFixed inDependents
apache2—2.4.48-r0—
Alpine:v3.12(1)
PackageVulnerable rangeFixed inDependents
apache2—2.4.48-r0—
Alpine:v3.13(1)
PackageVulnerable rangeFixed inDependents
apache2—2.4.48-r0—
Alpine:v3.14(1)
PackageVulnerable rangeFixed inDependents
apache2—2.4.48-r0—
Alpine:v3.15(1)
PackageVulnerable rangeFixed inDependents
apache2—2.4.48-r0—
Alpine:v3.16(1)
PackageVulnerable rangeFixed inDependents
apache2—2.4.48-r0—
Alpine:v3.17(1)
PackageVulnerable rangeFixed inDependents
apache2—2.4.48-r0—
Alpine:v3.18(1)
PackageVulnerable rangeFixed inDependents
apache2—2.4.48-r0—
Alpine:v3.19(1)
PackageVulnerable rangeFixed inDependents
apache2—2.4.48-r0—
Alpine:v3.20(1)
PackageVulnerable rangeFixed inDependents
apache2—2.4.48-r0—
Alpine:v3.21(1)
PackageVulnerable rangeFixed inDependents
apache2—2.4.48-r0—
Alpine:v3.22(1)
PackageVulnerable rangeFixed inDependents
apache2—2.4.48-r0—
Alpine:v3.23(1)
PackageVulnerable rangeFixed inDependents
apache2—2.4.48-r0—
Alpine:v3.24(1)
PackageVulnerable rangeFixed inDependents
apache2—2.4.48-r0—
Debian:10(1)
PackageVulnerable rangeFixed inDependents
apache22.4.38-3, 2.4.38-3+deb10u1, 2.4.38-3+deb10u2, 2.4.38-3+deb10u3, 2.4.38-3+deb10u42.4.38-3+deb10u5—
Debian:11(1)
PackageVulnerable rangeFixed inDependents
apache2—2.4.46-6—
Debian:12(1)
PackageVulnerable rangeFixed inDependents
apache2—2.4.46-6—
Debian:13(1)
PackageVulnerable rangeFixed inDependents
apache2—2.4.46-6—
Debian:14(1)
PackageVulnerable rangeFixed inDependents
apache2—2.4.46-6—
Debian:9(1)
PackageVulnerable rangeFixed inDependents
apache22.4.25-3 ... 2.4.25-3+deb9u9 (10 versions)2.4.25-3+deb9u10—

Weakness Classification(2)

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

Additional Vendor Advisories

(3)

Vendors that published advisories for this CVE beyond the curated set above. Broader coverage but minimal per-row detail — click through for the original advisory.

Data Freshness Timeline

(refreshed 6× in last 7d / 23× in last 30d)

Each row is a source pipeline that fetched or updated this CVE on that date, with what changed. For example, "NVD update" means NVD published or revised its analysis for this CVE; "MITRE cvelistV5" means we ingested or refreshed it from the CNA feed. Most recent first.

Showing the most recent 100 of 115 total refreshes for this CVE.

  1. 2026-09-06 13:44 UTCEPSS rescore
  2. 2026-09-05 15:25 UTCEPSS rescore
  3. 2026-09-04 13:57 UTCEPSS rescore
  4. 2026-09-04 05:03 UTCEPSS rescore
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  6. 2026-09-02 14:08 UTCEPSS rescore
  7. 2026-09-01 13:50 UTCEPSS rescore
  8. 2026-09-01 07:23 UTCOSV refresh
  9. 2026-08-30 19:14 UTCEPSS rescore
  10. 2026-08-28 21:37 UTCEPSS rescore
  11. 2026-08-26 14:42 UTCEPSS rescore
  12. 2026-08-25 13:45 UTCEPSS rescore
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  20. 2026-08-15 06:37 UTCOSV refresh
  21. 2026-08-15 01:27 UTCEPSS rescore
  22. 2026-08-12 13:47 UTCEPSS rescore
  23. 2026-08-11 13:38 UTCEPSS rescore
  24. 2026-08-07 16:23 UTCEPSS rescore
  25. 2026-08-06 13:43 UTCEPSS rescore
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  1. 2026-08-04 10:34 UTCEPSS rescore
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  3. 2026-07-30 18:50 UTCOSV refresh
  4. 2026-07-30 16:25 UTCEPSS rescore
  5. 2026-07-30 16:25 UTCEPSS rescore
  6. 2026-07-30 01:28 UTCEPSS rescore
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  8. 2026-07-28 15:33 UTCEPSS rescore
  9. 2026-07-26 14:52 UTCEPSS rescore
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  11. 2026-07-24 14:15 UTCEPSS rescore
  12. 2026-07-23 14:16 UTCEPSS rescore
  13. 2026-07-23 01:58 UTCEG score recompute
  14. 2026-07-22 22:45 UTCEG score recompute
  15. 2026-07-22 14:06 UTCEPSS rescore
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  17. 2026-07-21 15:22 UTCEPSS rescore
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  19. 2026-07-20 17:05 UTCEPSS rescore
  20. 2026-07-19 14:28 UTCEPSS rescore
  21. 2026-07-18 10:02 UTCEPSS rescore
  22. 2026-07-18 10:02 UTCEPSS rescore
  23. 2026-07-15 16:55 UTCEPSS rescore
  24. 2026-07-15 16:55 UTCEPSS rescore
  25. 2026-07-15 01:58 UTCEPSS rescore
  26. 2026-07-13 06:10 UTCEPSS rescore
  27. 2026-07-13 03:37 UTCOSV refresh
  28. 2026-07-12 05:44 UTCEPSS rescore
  29. 2026-07-12 05:44 UTCEPSS rescore
  30. 2026-07-11 08:25 UTCEPSS rescore
  31. 2026-07-11 08:25 UTCEPSS rescore
  32. 2026-07-09 19:07 UTCEPSS rescore
  33. 2026-07-06 16:25 UTCEPSS rescore
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  35. 2026-07-02 16:57 UTCEPSS rescore
  36. 2026-07-01 15:03 UTCEPSS rescore
  37. 2026-06-30 23:20 UTCEPSS rescore
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  39. 2026-06-29 14:04 UTCEPSS rescore
  40. 2026-06-28 04:54 UTCEPSS rescore
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  42. 2026-06-25 13:48 UTCEPSS rescore
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  44. 2026-06-24 14:03 UTCEPSS rescore
  45. 2026-06-24 14:03 UTCEPSS rescore
  46. 2026-06-24 13:02 UTCOSV refresh
  47. 2026-06-23 21:31 UTCEPSS rescore
  48. 2026-06-23 21:31 UTCEPSS rescore
  49. 2026-06-18 17:51 UTCEPSS rescore
  50. 2026-06-18 17:51 UTCEPSS rescore
  51. 2026-06-17 17:51 UTCEPSS rescore
  52. 2026-06-16 17:51 UTCEPSS rescore
  53. 2026-06-15 17:46 UTCEPSS rescore
  54. 2026-06-14 23:15 UTCEPSS rescore
  55. 2026-06-13 22:58 UTCEPSS rescore
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  57. 2026-06-12 23:10 UTCEPSS rescore
  58. 2026-06-11 13:58 UTCEPSS rescore
  59. 2026-06-10 22:16 UTCEPSS rescore
  60. 2026-06-10 13:20 UTCEPSS rescore
  61. 2026-06-08 14:15 UTCEPSS rescore
  62. 2026-06-07 17:46 UTCOSV refresh
  63. 2026-06-07 15:23 UTCEPSS rescore
  64. 2026-06-07 15:23 UTCEPSS rescore
  65. 2026-06-06 13:46 UTCEPSS rescore
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  67. 2026-06-06 13:46 UTCEPSS rescore
  68. 2026-06-05 22:45 UTCEPSS rescore
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  70. 2026-06-05 06:09 UTCEPSS rescore
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  72. 2026-06-04 13:10 UTCEPSS rescore
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  74. 2026-06-02 20:11 UTCEPSS rescore
  75. 2026-06-02 20:11 UTCEPSS rescore

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 68% probability of exploitation in the next 30 days, which ranks it in the top 0.7% of all scored CVEs.
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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