BlackTreeCVE Intelligence
← Back to the CVE catalogue
Full vulnerability report · 2021
CVE-2021-33193High confidence

Request splitting via HTTP/2 method injection and mod_proxy

Apache Software Foundation · Apache HTTP Server

7.5HighCVSS 3.1
Recommended action
Within 7 days

High technical severity with public exploit material referenced by a structured source; prioritise exposed affected systems while verifying vendor guidance.

Patch available
R
Operational reassessment

Published severity in operational context

Open reassessment dashboard →
Published severityHighOperational priority:Critical, raised one band.upgradedsince 3 Aug 2024

Evidence used

  • No CISA KEV confirmation is currently recorded.
  • A structured source references public exploit or proof-of-concept material.
  • The selected CVSS metric records a network-reachable, unauthenticated path with no user interaction.
  • EPSS is 46.18% for the current model date.

Compensating controls

  • Validate the affected product branch and deploy the verified fixed release.
  • Restrict the affected network interface to trusted sources where business-safe.
  • Increase monitoring for the attack path and post-exploitation behaviour described in the report.

Verification

  1. Confirm that the asset runs Apache Software Foundation Apache HTTP Server and falls inside the recorded affected range.
  2. Verify the installed build against the product-specific fixed version after deployment.
  3. Validate exposure, authentication requirements and compensating controls in the actual environment.
  4. Reopen this reassessment when CVSS, KEV, EPSS, exploit evidence or remediation changes.
Mitigation target: Within 3 daysRemediation target: Within 90 days

This automated reassessment organises public evidence. It does not know asset exposure, business impact or control effectiveness and does not replace CVSS or a human risk decision.

Distribution package intelligence

Release-specific package status

Alpine, Debian findings are scoped to the named distribution, release and source package. An absent finding does not mean a package is unaffected.

7 package states
Repository candidate not checked

A published vendor fix does not prove that a matching update is enabled and installable on a particular asset. Confirm the local package candidate before scheduling remediation.

Distribution releaseSource packageVendor stateFixed versionEvidence
Alpine v3.23v3.23 · mainapache2Vendor fix publishedAlpine records a security fix at this version. An absent entry does not mean the package is unaffected.2.4.49-r0Alpine Security Database ↗Source updated 3 Oct 2026
Alpine v3.22v3.22 · mainapache2Vendor fix publishedAlpine records a security fix at this version. An absent entry does not mean the package is unaffected.2.4.49-r0Alpine Security Database ↗Source updated 3 Oct 2026
Alpine v3.21v3.21 · mainapache2Vendor fix publishedAlpine records a security fix at this version. An absent entry does not mean the package is unaffected.2.4.49-r0Alpine Security Database ↗Source updated 3 Oct 2026
Debian trixietrixie · sourceapache2Vendor fix publishedDebian records a fixed source-package version for this release.2.4.48-4Debian Security Tracker ↗Source updated 5 Oct 2026
Debian bookwormbookworm · sourceapache2Vendor fix publishedDebian records a fixed source-package version for this release.2.4.48-4Debian Security Tracker ↗Source updated 5 Oct 2026
Debian forkyforky · sourceapache2Vendor fix publishedDebian records a fixed source-package version for this release.2.4.48-4Debian Security Tracker ↗Source updated 5 Oct 2026
Debian sidsid · sourceapache2Vendor fix publishedDebian records a fixed source-package version for this release.2.4.48-4Debian Security Tracker ↗Source updated 5 Oct 2026
Direct vendor intelligence

Authoritative vendor CSAF and VEX advisories

Structured product status and remediation from the issuing vendor. Product-state explanations are always visible; large lists can be searched or downloaded.

1 current
CVE-2021-33193 · CSAF 2.0 · revision 3 · finalRed Hat Product Securityhttpd: Request splitting via HTTP/2 method injection and mod_proxy
15 known affected

The vendor explicitly identifies these products as affected by this CVE.

  • httpd as a component of Red Hat Enterprise Linux 6
  • httpd-devel as a component of Red Hat Enterprise Linux 6
  • httpd-manual as a component of Red Hat Enterprise Linux 6
  • httpd-tools as a component of Red Hat Enterprise Linux 6
  • httpd.src as a component of Red Hat Enterprise Linux 6
  • mod_ssl as a component of Red Hat Enterprise Linux 6
  • httpd as a component of Red Hat Enterprise Linux 7
  • httpd-devel as a component of Red Hat Enterprise Linux 7
  • httpd-manual as a component of Red Hat Enterprise Linux 7
  • httpd-tools as a component of Red Hat Enterprise Linux 7
  • httpd.src as a component of Red Hat Enterprise Linux 7
  • mod_ldap as a component of Red Hat Enterprise Linux 7
Summary
A NULL pointer dereference was found in Apache httpd mod_h2. The highest threat from this flaw is to system integrity.
Remediation
For details on how to apply this update, which includes the changes described in this advisory, refer to: https://access.redhat.com/articles/11258 Applications using the APR libraries, such as httpd, must be restarted for this update to take effect. After installing the updated packages, the httpd daemon will be restarted automatically.
Optional official sources

National CERT insights
?CERT means Computer Emergency Response Team; CSIRT is the closely related term Computer Security Incident Response Team.

Choose official national sources for this report. Each advisory shows its original language. Your selection is remembered on this device and included in shared links.

Official European source

ENISA European Vulnerability Database

Official EUVD identifiers, advisory evidence and known-exploited context. Missing fields are not treated as evidence of low risk.

1 current
ENISA EUVD identifier

EUVD-2021-19907

No EUVD known-exploited evidence

ENISA has published the identifier mapping but no EUVD description has been stored yet.

EUVD state
Present in the current official mapping
Known exploitation
Not present in the current ENISA EUVD known-exploited dataset. This is not proof of no exploitation.
ENISA score
Not supplied in the stored EUVD record
Advisory evidence
No linked advisory details stored yet
Recommended actionWithin 7 days

High technical severity with public exploit material referenced by a structured source; prioritise exposed affected systems while verifying vendor guidance.

Patch available
01

What, why and how

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.

What

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.

Why

The current structured CVE record identifies a security weakness, but the root cause requires confirmation in the linked vendor material.

How

An attacker operating through a network path may attempt exploitation without authentication or user interaction. If successful, the issue may cause the confidentiality, integrity or availability impact described by the vendor.

What

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.

Why

The current structured CVE record identifies a security weakness, but the root cause requires confirmation in the linked vendor material.

How

An attacker operating through a network path may attempt exploitation without authentication or user interaction. If successful, the issue may cause the confidentiality, integrity or availability impact described by the vendor.

02

Exploit reality and attack path

CVSS severity, EPSS forecast probability, public exploit material and CISA-confirmed exploitation are separate signals.

Observed exploitation
?Confirmed exploitation and public exploit material are separate signals. Attacks can occur without public proof-of-concept or exploit code.
No confirmed evidence

No CISA KEV match was present at the last successful refresh. This means no confirmation from that source, not proof of no exploitation.

Public PoC / exploit material
?Confirmed exploitation and public exploit material are separate signals. Attacks can occur without public proof-of-concept or exploit code.
Reference recorded

A structured CVE source labels at least one public reference as exploit material. BlackTree has not independently validated that it is safe, reliable or weaponised.

Likely attack path
a network path → vulnerable operation → cause the confidentiality, integrity or availability impact described by the vendor
Attack surface
Network
Privileges required
None: unauthenticated exploitation is possible
User interaction
None
Attack complexity
Low: no specialised conditions are recorded
Security boundary
Unchanged: impact remains within the vulnerable component's security authority
Weakness
?CWE means Common Weakness Enumeration.
CWE not yet assigned
CVSS vector
?CVSS means Common Vulnerability Scoring System. The vector records the metric values used to calculate technical severity.
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:N

Common Vulnerability Scoring System 3.1: the compact vector below is decoded into plain language.

AVNetworkAttack vector: The vulnerable component can be reached over a network.ACLowAttack complexity: No specialised conditions are required beyond attacker-controlled input.PRNonePrivileges required: The attacker does not need an account or existing privileges.UINoneUser interaction: No action by another user is required.SUnchangedScope: The security impact remains within the vulnerable component's authority.CNoneConfidentiality impact: No direct loss is represented by this metric.IHighIntegrity impact: A successful attack can cause a major loss.ANoneAvailability impact: No direct loss is represented by this metric.
Post-exploitation / living off the land
No specific living-off-the-land technique is confirmed in the structured sources. Monitor normal administration tools for activity inconsistent with the affected service's baseline.
NetworkUnauthenticatedPublic exploit reference
A

Official authority intelligence

Only matched European and national findings are included. Language selectors and unavailable sources are omitted.

BSI · German · WID-SEC-2024-1591Juniper JUNOS: Mehrere Schwachstellen

Ein Angreifer kann mehrere Schwachstellen in Juniper JUNOS ausnutzen, um einen Denial of Service zu verursachen, Informationen offenzulegen, Privilegien zu erweitern und Sicherheitsmechanismen inklusive zu umgehen.

Official advisory ↗
BSI · German · WID-SEC-2022-0722Apache HTTP Server: Schwachstelle ermöglicht Denial of Service

Ein entfernter, anonymer Angreifer kann eine Schwachstelle in Apache HTTP Server ausnutzen, um einen Denial of Service Angriff durchzuführen.

Official advisory ↗
Cyber Security Agency of Singapore · English · CSA-SB-20210818Security Bulletin 18 Aug 2021

The Cyber Security Agency of Singapore included this CVE in its official Security Bulletin 18 Aug 2021, published on 18 August 2021. Open the linked bulletin for the product, severity and reference information published in that issue.

Official advisory ↗
CERT-FR · French · CERTFR-2024-AVI-0575Multiples vulnérabilités dans les produits Juniper Networks

ord?id=CVE-2021-29469 Référence CVE CVE-2021-30641 https://www.cve.org/CVERecord?id=CVE-2021-30641 Référence CVE CVE-2021-31535 https://www.cve.org/CVERecord?id=CVE-2021-31535 Référence CVE CVE-2021-31618 https://www.cve.org/CVERecord?id=CVE-2021-31618 Référence CVE CVE-2021-3177 https://www.cve.org/CVERecord?id=CVE-2021-3177 Référence CVE CVE-2021-32803 https://www.cve.org/CVERecord?id=CVE-2021-32803 Référence CVE CVE-2021-32804 https://www.cve.org/CVERecord?id=CVE-2021-32804 Référence CVE CVE-2021-33033 https://www.cve.org/CVERecord?id=CVE-2021-33033 Référence CVE CVE-2021-33034 https://www.cve.org/CVERecord?id=CVE-2021-33034 Référence CVE CVE-2021-33193 https://www.cve.org/CVERecord?id=CVE-2021-33193 Référence CVE CVE-2021-3347 https://www.cve.org/CVERecord?id=CVE-2021-3347 Référence CVE CVE-2021-33909 https://www.cve.org/CVERecord?id=CVE-2021-33909 Référence CVE CVE-2021-34798 https://www.cve.org/CVERecord?id=CVE-2021-34798 Référence CVE CVE-2021-35604 https://www.cve.org/CVERecord?id=CVE-2021-35604 Référence CVE CVE-2021-35624 https://www.cve.org/CVERecord?id=CVE-2021-35624 Référence CVE CVE-2021-36160 https://www.cve.org/CVERecord?id=CVE-2021-36160 Référence CVE CVE-2021-37701 https://www.cve.org/CVERecord?id=CVE-2021-37701 Référence CVE CVE-2021-37712 https://www.cve.org/CVERecord?id=CV

Official advisory ↗
CERT-FR · French · CERTFR-2021-AVI-931Multiples vulnérabilités dans les produits Cisco

De multiples vulnérabilités ont été découvertes dans les produits Cisco. Elles permettent à un attaquant de provoquer un déni de service à distance, une atteinte à l'intégrité des données et une atteinte à la confidentialité des données.

Official advisory ↗
CERT-FR · French · CERTFR-2021-AVI-875Multiples vulnérabilités dans les produits Check Point

De multiples vulnérabilités ont été découvertes dans les produits Check Point. Certaines d'entre elles permettent à un attaquant de provoquer un déni de service à distance, un contournement de la politique de sécurité et une atteinte à l'intégrité des données.

Official advisory ↗
CERT-FR · French · CERTFR-2021-AVI-800Multiples vulnérabilités dans Tenable.sc

De multiples vulnérabilités ont été découvertes dans Tenable.sc. Elles permettent à un attaquant de provoquer un déni de service à distance, une atteinte à l'intégrité des données et une atteinte à la confidentialité des données.

Official advisory ↗
CERT-FR · French · CERTFR-2021-AVI-713Multiples vulnérabilités dans Apache httpd

De multiples vulnérabilités ont été découvertes dans Apache httpd. Elles permettent à un attaquant de provoquer un problème de sécurité non spécifié par l'éditeur.

Official advisory ↗
JVN iPedia · Japanese · JVNDB-2021-010682Apache HTTP Server における脆弱性

Apache HTTP Server には、不特定の脆弱性が存在します。

Official advisory ↗
JVN iPedia · Japanese · JVNDB-2021-002672Apache HTTP Server における mod_proxy_uwsgi が割り当てられたメモリを超えて読み取られる脆弱性

Apache HTTP Server には、mod_proxy_uwsgi が割り当てられたメモリを超えて読み取られる、およびサービス運用妨害 (クラッシュ) 状態にされる脆弱性が存在します。

Official advisory ↗
03

Patch and workaround

Operational remediation based on structured source evidence.

Status
?Patch availability is based on structured fixed-version fields and authoritative update references. If no fix is verified, check the vendor advisory before making a change.
Patch available
Affected
Fixed
Action
For details on how to apply this update, which includes the changes described in this advisory, refer to: https://access.redhat.com/articles/11258 Applications using the APR libraries, such as httpd, must be restarted for this update to take effect. After installing the updated packages, the httpd daemon will be restarted automatically.
Workaround
No verified workaround is recorded. If business-safe, reduce exposure to the affected interface and allow only trusted sources until authoritative guidance is available.
04

Evidence and provenance

Published 16 Aug 2021 · Last source change 3 Aug 2024, 23:42 UTC · CWE not yet assigned

CVE recordCVE.org · 5.1
CVSS sourceNIST NVD
EPSS source
?The date BlackTree first stored a score for this CVE from the daily FIRST EPSS feed.
FIRST · tracked since 2026-08-14
European sourceENISA EUVD · EUVD-2021-19907
Product sourceVendor CSAF · Red Hat Product Security
Remediation sourceVendor CSAF · Red Hat Product Security
CWE sourceUnavailable
NVD statusNVD enriched

Missing structured fields: CWE classification. Missing data is not evidence of low risk; review the primary advisory.

Material change intelligence

What changed after publication

View recent updates ↗
  1. Affected versionsThe structured affected or fixed version information changed.
    Before
    Apache HTTP Server 2.4 2.4.17 to 2.4.48 · Fixed: An authoritative update reference is available, but the fixed version is not recorded in the structured CVE fields. Check the linked vendor advisory for the applicable release.
    After
    Apache HTTP Server 2.4 2.4.17 to 2.4.48 · Fixed: For details on how to apply this update, which includes the changes described in this advisory, refer to: https://access.redhat.com/articles/11258 Applications using the APR libraries, such as httpd, must be restarted for this update to take effect. After installing the updated packages, the httpd daemon will be restarted automatically.
    Red Hat Product Security ↗
Material fields only · duplicate refreshes suppressed · history retained for the configured operational retention period
CVE published
Fixed release or patch reference recorded
Fixed release recorded from official guidance
ENISA EUVD · EUVD-2021-19907↗Red Hat Product Security security advisory↗Patch or update reference↗Patch or update reference↗Patch or update reference↗Patch or update reference↗Public exploit reference↗Official CVE record (CNA and ADP data)↗NVD record↗
Technical terms and abbreviations used in this report
CVE
Common Vulnerabilities and Exposures: the public identifier for one disclosed vulnerability.
CVSS
Common Vulnerability Scoring System: a technical severity framework; it is not patching priority by itself.
EPSS
Exploit Prediction Scoring System: FIRST's estimate of the probability that exploitation activity will be observed in the next 30 days; it is a forecast, not confirmation.
CWE
Common Weakness Enumeration: the standard category describing the underlying software or hardware weakness.
CNA
CVE Numbering Authority: an organisation authorised to assign and publish CVE records.
CISA ADP
Cybersecurity and Infrastructure Security Agency Authorized Data Publisher: structured enrichment added to a CVE record.
NVD
National Vulnerability Database: NIST's enrichment service for CVE records.
CERT / CSIRT
A computer security incident response team that publishes warnings or coordinates incident response.
PoC
Proof of concept: public material that demonstrates or helps reproduce exploitation.
CSAF
Common Security Advisory Framework: a machine-readable format for security advisories.
LoTL
Living off the land: abuse of legitimate tools or system functions during an attack.
Free version - for non-commercial use only.CVE-2021-33193 · cve.blacktree.nl