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NVD / CVE Program· CNA apache

CVE-2021-33193: Request splitting via HTTP/2 method injection and mod_proxy

Noroxi assessment

Priority: Plan (action score 44/100)

  • The sources we track (CISA KEV, Exploit-DB, Metasploit, Nuclei, GitHub) show no public exploit code or exploitation in the wild.
  • Probability of exploitation in the next 30 days: 46.1% (FIRST EPSS); higher than 98.8% of all records.
  • Anyone who can reach it over the internet can trigger it. No account or password is required. No user action is required. No special conditions are required; it is repeatable.
  • Fixed version: httpd (AlmaLinux:8) 2.4.37-47.module_el8.6.0+2935+fb177b09.2, 2.4.37-47.module_el8.6.0+2872+fe0ff7aa.1.alma; httpd-devel (AlmaLinux:8) 2.4.37-47.module_el8.6.0+2872+fe0ff7aa.1.alma, 2.4.37-47.module_el8.6.0+2935+fb177b09.2; httpd-filesystem (AlmaLinux:8) 2.4.37-47.module_el8.6.0+2872+fe0ff7aa.1.alma, 2.4.37-47.module_el8.6.0+2935+fb177b09.2.
  • Mentioned 1 time in Turkish national cyber security advisories (siberguvenlik.gov.tr).

Add it to your regular patch cycle. Upgrade to httpd (AlmaLinux:8) 2.4.37-47.module_el8.6.0+2935+fb177b09.2, 2.4.37-47.module_el8.6.0+2872+fe0ff7aa.1.alma; httpd-devel (AlmaLinux:8) 2.4.37-47.module_el8.6.0+2872+fe0ff7aa.1.alma, 2.4.37-47.module_el8.6.0+2935+fb177b09.2; httpd-filesystem (AlmaLinux:8) 2.4.37-47.module_el8.6.0+2872+fe0ff7aa.1.alma, 2.4.37-47.module_el8.6.0+2935+fb177b09.2 or later.

Generated automatically from the record's data: CVSS (NVD and CNA), CISA KEV and SSVC, FIRST EPSS, exploit sources, fixed versions. How we calculate it

Primary sources: NVD · CVE.org · FIRST EPSS

A crafted method sent through HTTP/2 will bypass validation and be forwarded by mod_proxy, which can lead to request splitting or cache poisoning. This issue affects Apache HTTP Server 2.4.17 to 2.4.48.

HighCVSS 7.5 · v3.1—No exploit Fix available
Published
Aug 16, 2021
Updated
Jun 16, 2026
EPSS
46.1% · 98th percentile
CWE
—

Report tools

JSON

Action score

44

Plan

Add it to your regular patch cycle.

CVSS
30 / 40 · 7.5 / 10
CISA KEV
0 / 30 · Not listed
EPSS
14 / 30 · 46.1%

Get told when this record changes

  • Not in KEV: no record of exploitation in the wild
  • No public exploit code on record
  • A fix has been released

If any of these changes, we'll tell you in the daily email digest.

Follow CVE-2021-33193

When you'll hear from us

  • when it enters CISA KEV
  • when exploit code appears or is weaponized
  • when the record is updated (including a fixed version)
  • when EPSS rises 20 points in a week

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Noroxi analysis

What does it mean in your environment?

The assessment above is generated automatically from the record's data; it doesn't know your environment. Our team verifies on your systems whether this vulnerability is actually exploitable and which mitigation is enough.

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Affected systems

Is your version affected?

Pick the product and type the version you run. It's compared with this record's version ranges in your browser; the version isn't sent anywhere.

VendorProduct
debiandebian linux
apachehttp server
fedoraprojectfedora
tenabletenable.sc
oraclesecure backup
oraclezfs storage appliance kit

Affected versions

NVD version ranges (for catalog products). Add the product to your stack with a version and matching uses these.

  • apache http server2.4.17 and later · before 2.4.49

Versions reported by the vendor

Affected version ranges reported by the assigning authority (apache). Independent of NVD's CPE analysis and usually ahead of it.

  • Apache Software Foundation Apache HTTP Server

    • Apache HTTP Server 2.4 2.4.17 to 2.4.48affected

Package-level exposure

OSV and GitHub Advisory data: ecosystem, package and range. SBOM matching uses this table.

EcosystemPackageAffected rangeFix
AlmaLinux:8httpdbefore 2.4.37-47.module_el8.6.0+2935+fb177b09.22.4.37-47.module_el8.6.0+2935+fb177b09.2
AlmaLinux:8httpdbefore 2.4.37-47.module_el8.6.0+2872+fe0ff7aa.1.alma2.4.37-47.module_el8.6.0+2872+fe0ff7aa.1.alma
AlmaLinux:8httpd-develbefore 2.4.37-47.module_el8.6.0+2872+fe0ff7aa.1.alma2.4.37-47.module_el8.6.0+2872+fe0ff7aa.1.alma
AlmaLinux:8httpd-develbefore 2.4.37-47.module_el8.6.0+2935+fb177b09.22.4.37-47.module_el8.6.0+2935+fb177b09.2
AlmaLinux:8httpd-filesystembefore 2.4.37-47.module_el8.6.0+2872+fe0ff7aa.1.alma2.4.37-47.module_el8.6.0+2872+fe0ff7aa.1.alma
AlmaLinux:8httpd-filesystembefore 2.4.37-47.module_el8.6.0+2935+fb177b09.22.4.37-47.module_el8.6.0+2935+fb177b09.2
AlmaLinux:8httpd-manualbefore 2.4.37-47.module_el8.6.0+2872+fe0ff7aa.1.alma2.4.37-47.module_el8.6.0+2872+fe0ff7aa.1.alma
AlmaLinux:8httpd-manualbefore 2.4.37-47.module_el8.6.0+2935+fb177b09.22.4.37-47.module_el8.6.0+2935+fb177b09.2
AlmaLinux:8httpd-toolsbefore 2.4.37-47.module_el8.6.0+2872+fe0ff7aa.1.alma2.4.37-47.module_el8.6.0+2872+fe0ff7aa.1.alma
AlmaLinux:8httpd-toolsbefore 2.4.37-47.module_el8.6.0+2935+fb177b09.22.4.37-47.module_el8.6.0+2935+fb177b09.2
AlmaLinux:8mod_http2before 1.15.7-5.module_el8.6.0+2872+fe0ff7aa1.15.7-5.module_el8.6.0+2872+fe0ff7aa
AlmaLinux:8mod_ldapbefore 2.4.37-47.module_el8.6.0+2935+fb177b09.22.4.37-47.module_el8.6.0+2935+fb177b09.2
AlmaLinux:8mod_ldapbefore 2.4.37-47.module_el8.6.0+2872+fe0ff7aa.1.alma2.4.37-47.module_el8.6.0+2872+fe0ff7aa.1.alma
AlmaLinux:8mod_mdbefore 1:2.0.8-8.module_el8.6.0+2872+fe0ff7aa1:2.0.8-8.module_el8.6.0+2872+fe0ff7aa
AlmaLinux:8mod_proxy_htmlbefore 1:2.4.37-47.module_el8.6.0+2935+fb177b09.21:2.4.37-47.module_el8.6.0+2935+fb177b09.2
AlmaLinux:8mod_proxy_htmlbefore 1:2.4.37-47.module_el8.6.0+2872+fe0ff7aa.1.alma1:2.4.37-47.module_el8.6.0+2872+fe0ff7aa.1.alma
AlmaLinux:8mod_sessionbefore 2.4.37-47.module_el8.6.0+2935+fb177b09.22.4.37-47.module_el8.6.0+2935+fb177b09.2
AlmaLinux:8mod_sessionbefore 2.4.37-47.module_el8.6.0+2872+fe0ff7aa.1.alma2.4.37-47.module_el8.6.0+2872+fe0ff7aa.1.alma
AlmaLinux:8mod_sslbefore 1:2.4.37-47.module_el8.6.0+2935+fb177b09.21:2.4.37-47.module_el8.6.0+2935+fb177b09.2
AlmaLinux:8mod_sslbefore 1:2.4.37-47.module_el8.6.0+2872+fe0ff7aa.1.alma1:2.4.37-47.module_el8.6.0+2872+fe0ff7aa.1.alma
Alpine:v3.11apache2before 2.4.49-r02.4.49-r0
Debian:10apache2before 2.4.38-3+deb10u92.4.38-3+deb10u9
Debian:12apache2before 2.4.48-42.4.48-4
openSUSE:Leap 15.2apache2before 2.4.43-lp152.2.18.12.4.43-lp152.2.18.1

+64

Same product

debian: all records

Other highest-scoring records for the same primary product.

  • CVE-2016-9079A use-after-free vulnerability in SVG Animation has been discovered.KEV
    86Now
  • CVE-2024-44308The issue was addressed with improved checks.KEV
    68This week
  • CVE-2021-25216A second vulnerability in BIND's GSSAPI security policy negotiation can be targeted by a buffer overflow attack
    64This week

Published before and after this one in the same product

  • ← EarlierCVE-2021-36086Jul 1, 2021 · The CIL compiler in SELinux 3.2 has a use-after-free in cil_reset_classpermission (called from cil_reset_classperms_set and cil_reset_classp
    13Monitor
  • ← EarlierCVE-2021-31799Jul 30, 2021 · In RDoc 3.11 through 6.x before 6.3.1, as distributed with Ruby through 3.0.1, it is possible to execute arbitrary code via | and tags in a
    28Monitor
  • Later →CVE-2021-20001Feb 11, 2022 · It was discovered, that debian-edu-config, a set of configuration files used for the Debian Edu blend, before 2.12.16 configured insecure pe
    39Monitor

Remediation

Which version to upgrade to

Fix versions compiled from the vendor, package registries and Microsoft. Verify the vendor's note before upgrading.

Product / packageFixed versionSource
almalinux:httpd2.4.37-47.module_el8.6.0+2935+fb177b09.2 · AlmaLinux:8Package registry (OSV)
almalinux:httpd-devel2.4.37-47.module_el8.6.0+2872+fe0ff7aa.1.alma · AlmaLinux:8Package registry (OSV)
almalinux:httpd-filesystem2.4.37-47.module_el8.6.0+2872+fe0ff7aa.1.alma · AlmaLinux:8Package registry (OSV)
almalinux:httpd-manual2.4.37-47.module_el8.6.0+2935+fb177b09.2 · AlmaLinux:8Package registry (OSV)
almalinux:httpd-tools2.4.37-47.module_el8.6.0+2935+fb177b09.2 · AlmaLinux:8Package registry (OSV)
almalinux:mod_http21.15.7-5.module_el8.6.0+2872+fe0ff7aa · AlmaLinux:8Package registry (OSV)
almalinux:mod_ldap2.4.37-47.module_el8.6.0+2935+fb177b09.2 · AlmaLinux:8Package registry (OSV)
almalinux:mod_md1:2.0.8-8.module_el8.6.0+2872+fe0ff7aa · AlmaLinux:8Package registry (OSV)
almalinux:mod_proxy_html1:2.4.37-47.module_el8.6.0+2872+fe0ff7aa.1.alma · AlmaLinux:8Package registry (OSV)
almalinux:mod_session2.4.37-47.module_el8.6.0+2872+fe0ff7aa.1.alma · AlmaLinux:8Package registry (OSV)
almalinux:mod_ssl1:2.4.37-47.module_el8.6.0+2872+fe0ff7aa.1.alma · AlmaLinux:8Package registry (OSV)
alpine:apache22.4.49-r0 · Alpine:v3.11Package registry (OSV)
debian:apache22.4.48-4 · Debian:12Package registry (OSV)
opensuse:apache22.4.43-lp152.2.18.1 · openSUSE:Leap 15.2Package registry (OSV)
red hat:httpd0:2.4.37-47.module+el8.6.0+14529+083145da.1 · Red Hat:enterprise_linux:8::appstreamPackage registry (OSV)
red hat:httpd-debuginfo0:2.4.37-47.module+el8.6.0+14529+083145da.1 · Red Hat:enterprise_linux:8::appstreamPackage registry (OSV)
red hat:httpd-debugsource0:2.4.37-47.module+el8.6.0+14529+083145da.1 · Red Hat:enterprise_linux:8::appstreamPackage registry (OSV)
red hat:httpd-devel0:2.4.37-47.module+el8.6.0+14529+083145da.1 · Red Hat:enterprise_linux:8::appstreamPackage registry (OSV)
red hat:httpd-filesystem0:2.4.37-47.module+el8.6.0+14529+083145da.1 · Red Hat:enterprise_linux:8::appstreamPackage registry (OSV)
red hat:httpd-manual0:2.4.37-47.module+el8.6.0+14529+083145da.1 · Red Hat:enterprise_linux:8::appstreamPackage registry (OSV)
red hat:httpd-tools0:2.4.37-47.module+el8.6.0+14529+083145da.1 · Red Hat:enterprise_linux:8::appstreamPackage registry (OSV)
red hat:httpd-tools-debuginfo0:2.4.37-47.module+el8.6.0+14529+083145da.1 · Red Hat:enterprise_linux:8::appstreamPackage registry (OSV)
red hat:mod_http20:1.15.7-5.module+el8.6.0+13996+01710940 · Red Hat:enterprise_linux:8::appstreamPackage registry (OSV)
red hat:mod_http2-debuginfo0:1.15.7-5.module+el8.6.0+13996+01710940 · Red Hat:enterprise_linux:8::appstreamPackage registry (OSV)
red hat:mod_http2-debugsource0:1.15.7-5.module+el8.6.0+13996+01710940 · Red Hat:enterprise_linux:8::appstreamPackage registry (OSV)
red hat:mod_ldap0:2.4.37-47.module+el8.6.0+14529+083145da.1 · Red Hat:enterprise_linux:8::appstreamPackage registry (OSV)
rocky linux:httpd0:2.4.37-47.module+el8.6.0+823+f143cee1.1 · Rocky Linux:8Package registry (OSV)
rocky linux:mod_http20:1.15.7-5.module+el8.6.0+823+f143cee1 · Rocky Linux:8Package registry (OSV)
rocky linux:mod_md1:2.0.8-8.module+el8.5.0+695+1fa8055e · Rocky Linux:8Package registry (OSV)
suse:apache22.4.23-29.77.2 · SUSE:Linux Enterprise Software Development Kit 12 SP5Package registry (OSV)

Exploit status

No known public exploit

No public exploit has been observed yet. That doesn't mean you're safe, only that the bar is a little higher.

Research context

For pentesters and researchers: attack profile, score disagreement, timeline, patch commits, credits, variant and chain candidates, bug bounty scope. All derived from existing data; no exploit code.

Timeline

From publication to today: proof of concept, Metasploit module, CISA KEV and fix record. Dates are as reported by the sources.

No dated events beyond publication.

EPSS, last 120 days

FIRST EPSS daily score; only changes of 0.01 or more are recorded (step chart).

Patch and commit links

Commit, PR and diff links among the references. A starting point for patch-diffing and variant hunting; fixes, not exploits.

Credits

All researchers

Finders, reporters and analysts named in the CNA record. Click a name for that researcher’s other records.

Variant candidates

Same product, same weakness class, within 18 months. If the patch missed the root cause, the sibling bug is here.

No nightly-computed relations.

Chain candidates

An authentication bypass and a privilege-requiring bug in the same product, published close together: combined they may become an unauthenticated path.

—

Bug bounty scope

The product’s vendor appears in a public program. Matched by name; verify the scope text in the program.

National notice (Türkiye Cybersecurity Directorate / USOM)

Official security notices citing this record; remediation advice is on the agency's page.

All national notices →

Technical details

Attack conditions

  • Anyone who can reach it over the internet can trigger it.
  • No account or password is required.
  • No user action is required.
  • No special conditions are required; it is repeatable.

If successful

Confidentiality
none
Integrity
high · data or configuration can be modified
Availability
none
Attack vector
Network
Attack complexity
Low
Privileges required
None
User interaction
None
Scope
Unchanged
Confidentiality impact
None
Integrity impact
High
Availability impact
None

Weakness class (CWE)

—

CVSS vector

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:N

nvd-primary

Attack context

MITRE CAPEC attack patterns and ATT&CK techniques for this weakness class (CWE). A starting point for detection rules and threat hunting.

MITRE has no CAPEC/ATT&CK mapping for this CWE.

Change log

  1. Fix✗ → ✓

For records you follow, these changes also arrive as notifications. →

References

Show all references (11 more)

Vendor advisories and official records. Exploit/PoC links are deliberately left out.

All records