Skip to main content
No exploitation reportedFix available

CVE-2021-26691

No exploitation reported by the sources we read.

  • Published2021-06-10
  • WeaknessCWE-122 · CWE-787

Am I affected?

With a domain, we read the version its public headers and pages expose. A distribution build can carry the fix under an older number.

Description

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

Timeline

dates in UTC
  1. 2021-06-1007:15CVE publishedNVD
  2. 2026-10-2319:23EPSS crossed 10 %FIRST EPSS

Observed activity

isMalicious corpus · updated 2026-10-23 21:37 UTC
Indicators tagged with this CVE, per day0 indicators over 7 days
Block these indicators
  • Firewall list (EDL, text): https://api.ismalicious.com/edl/cve/CVE-2021-26691?format=txt
  • CSV export: https://api.ismalicious.com/edl/cve/CVE-2021-26691?format=csv
  • TAXII collection of exploited CVEs
  • The EDL and CSV take an API key on the Pro plan.

Versions per branch

NVD
Oracle instantis enterprisetrack
BranchAffectedFixed in
17.1= 17.1no fix published
17.2= 17.2no fix published
17.3= 17.3no fix published
Debian debian linux
BranchAffectedFixed in
9.0= 9.0no fix published
10.0= 10.0no fix published
Fedoraproject fedora
BranchAffectedFixed in
34.x= 34no fix published
35.x= 35no fix published

6 more products in the sources.

as publishedRanges as NVD and OSV publish them; a distribution’s build can be fixed under an older number.

Distribution builds and vendor advisories

Packages from OSV.dev and products from the vendors' CSAF advisories.

version not verifiedVersions not verified: read as OSV.dev and the vendors publish them, not checked against each vendor's release list.

Packages

  • httpdAlmaLinux:8all versions · fixed in 2.4.37-43.module_el8.5.0+2597+c4b14997.almaand 6 other packages
  • mod_http2AlmaLinux:8all versions · fixed in 1.15.7-3.module_el8.5.0+2609+b30d9eec
  • mod_mdAlmaLinux:8all versions · fixed in 1:2.0.8-8.module_el8.5.0+2609+b30d9eec
  • mod_mdAlmaLinux:8all versions · fixed in 1:2.0.8-8.module_el8.6.0+2872+fe0ff7aa
  • mod_sslAlmaLinux:8all versions · fixed in 1:2.4.37-43.module_el8.5.0+2597+c4b14997.almaand 1 other package

Full record on NVD

Fix

Advisories and patches

Detection

Nuclei template
none so far
ProjectDiscovery
Exploit-DB
no entry
Exploit-DB
Proof-of-concept repositories
none listed
PoC-in-GitHub

Sources

  • NVDCVSS 3.19.8read 2026-10-23
  • NVDCVSS 2.07.5read 2026-10-23
  • CNAweakness (CWE)CWE-122read 2026-10-23
  • NVDweakness (CWE)CWE-787read 2026-10-23
  • FIRST EPSSEPSS score68.3 %read 2026-10-23
  • OSVaffected packages—

Technical details

CVSS v3 Vector
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H
Published
2021-06-10
Last Modified
2026-06-17
NVD status
Modified

Frequently asked questions

What is CVE-2021-26691?

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

Is CVE-2021-26691 actively exploited?

Active exploitation of CVE-2021-26691 has not been confirmed. Its EPSS score was 68.3% on 2026-10-22, the estimated probability of exploitation in the next 30 days.

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

CVE-2021-26691 has a CVSS v3 base score of 9.8 (CRITICAL severity), with vector string CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H.

Running Apache HTTP Server? Hear about its next vulnerability

Surveillance reads the versions in your stack — a domain, a lockfile or a list — checks each CVE against the ranges its sources publish, and alerts you when one you run is exploited or enters CISA KEV. A version it cannot read stays « version not verified ».

Get alerts for this productCISA KEV additions, October 2026Search CVEs

No credit card required · No monthly cap on website lookups · 250 API requests/month

Go further

Related vulnerabilities

Same product first, then the same vendor advisory and the same publication week.

Back to CVE Database