Evidence used
- No CISA KEV confirmation is currently recorded.
- The selected CVSS metric records a network-reachable, unauthenticated path with no user interaction.
- EPSS is 0.42% for the current model date.
BlackTreeCVE IntelligenceFull vulnerability report · 2026
CVE-2026-71554High confidence
h2: Duplicate Host header could facilitate request smuggling
python-hyper · h2
Official source article: GitHub GHSA-6HR6-W5QG-QMWG ↗. Check the applicable product and release in the original source.
5.3MediumCVSS 3.1
Patch only the product branches with a verified fix
Fix availability varies by productMedium technical severity with no CISA KEV confirmation; remediate through the normal risk-based patch cycle unless local exposure raises the priority. Verified remediation exists for at least one product or source, but 20 structured product or package states remain unresolved. Apply remediation only to the exact product branch confirmed by its source.
R
Open reassessment dashboard →Operational reassessment
Published severity in operational context
Published severityMedium→Operational priority:Medium, unchanged from published severity.unchanged
Compensating controls
- Validate the affected product branch and deploy the verified fixed release.
- Restrict the affected network interface to trusted sources where business-safe.
- Monitor vendor guidance and exploitation sources for a material change.
Verification
- Confirm that the asset runs python-hyper h2 and falls inside the recorded affected range.
- Verify the installed build against the product-specific fixed version after deployment.
- Validate exposure, authentication requirements and compensating controls in the actual environment.
- Reopen this reassessment when CVSS, KEV, EPSS, exploit evidence or remediation changes.
Remediation availability differs by product scope
Verified remediation exists for at least one product or source, but 20 structured product or package states remain unresolved. Apply remediation only to the exact product branch confirmed by its source.
- Red Hat Product Security: 16 affected or under-investigation product states without a fixed product state in the same current advisory
- Debian Security Tracker: 4 affected package states with no published fix
Distribution package intelligence
5 package statesRelease-specific package status
Debian, ubuntu findings are scoped to the named distribution, release and source package. An absent finding does not mean a package is unaffected.
Package result overrides the generic status
BlackTree has verified remediation for at least one product or source, but the relevant distribution still reports no fixed package for 4 affected package states shown here. Treat those rows as affected with no fix until that distribution publishes a fixed version.
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 release | Source package | Vendor state | Fixed version | Evidence |
|---|---|---|---|---|
| Debian trixietrixie · source | python-h2 | Affected, no fix publishedDebian currently tracks this release as open. | Not published in this feed | Debian Security Tracker ↗Source updated 6 Oct 2026 |
| Debian bookwormbookworm · source | python-h2 | Affected, no fix publishedDebian currently tracks this release as open. | Not published in this feed | Debian Security Tracker ↗Source updated 6 Oct 2026 |
| Debian forkyforky · source | python-h2 | Affected, no fix publishedDebian currently tracks this release as open. | Not published in this feed | Debian Security Tracker ↗Source updated 6 Oct 2026 |
| Debian sidsid · source | python-h2 | Affected, no fix publishedDebian currently tracks this release as open. | Not published in this feed | Debian Security Tracker ↗Source updated 6 Oct 2026 |
| Ubuntu 24.04 LTSnoble · standard archive | python-h2 | Under evaluationCanonical reports that the package might be affected and still needs evaluation or fixing. | Not published in this feed | Canonical Ubuntu Security ↗Source updated 5 Oct 2026 |
Open-source package ranges2 source-attributed ranges
These OSV and GitHub advisory ranges apply only to the named package and ecosystem. A listed fixed version is not a universal product patch or proof that an update is installed.
| Ecosystem and package | Affected range | First fixed version | Evidence |
|---|---|---|---|
| PyPIh2 | ECOSYSTEM: introduced 0; fixed 4.4.1 | 4.4.1 | OSV record ↗aggregator derived · 10 Sep 2026 |
| piph2 | <= 4.4.0 | 4.4.1 | GitHub advisory ↗upstream repository advisory · 6 Aug 2026 |
Direct vendor intelligence
1 currentAuthoritative 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.
CVE-2026-71554 · CSAF 2.0 · revision 3 · finalRed Hat Product Securityh2: h2: Request smuggling due to duplicate Host headers
The vendor explicitly identifies these products as affected by this CVE.
- rhmtc/openshift-migration-hook-runner-rhel8 as a component of Migration Toolkit for Containers
- ansible-automation-platform-24/ee-supported-rhel8 as a component of Red Hat Ansible Automation Platform 2
- ansible-automation-platform-24/ee-supported-rhel9 as a component of Red Hat Ansible Automation Platform 2
- ansible-automation-platform-24/platform-resource-runner-rhel8 as a component of Red Hat Ansible Automation Platform 2
- ansible-automation-platform-25/ee-supported-rhel8 as a component of Red Hat Ansible Automation Platform 2
- ansible-automation-platform-25/ee-supported-rhel9 as a component of Red Hat Ansible Automation Platform 2
- ansible-automation-platform-25/platform-resource-runner-rhel8 as a component of Red Hat Ansible Automation Platform 2
- ansible-automation-platform-26/ee-supported-rhel9 as a component of Red Hat Ansible Automation Platform 2
- ansible-automation-platform-27/ee-supported-rhel9 as a component of Red Hat Ansible Automation Platform 2
- rhai/guidellm-rhel9 as a component of Red Hat OpenShift AI (RHOAI)
- rhoai/odh-feature-server-rhel9 as a component of Red Hat OpenShift AI (RHOAI)
- rhoai/odh-llama-stack-core-rhel9 as a component of Red Hat OpenShift AI (RHOAI)
- Summary
- A flaw was found in h2, a Python library for HTTP/2. This vulnerability allows a remote attacker to perform request smuggling. The library incorrectly processes HTTP/2 requests that contain more than one Host header. When these requests are downgraded to HTTP/1.1 by a consuming application, the duplicate Host headers can be exploited to bypass security mechanisms and potentially gain unauthorized access or manipulate data.
- Remediation
- Update the bundled h2 Python library to 4.4.1 or later. Where an immediate upgrade is not possible, avoid HTTP/2 to HTTP/1.1 downgrade in front-end proxies for affected services, or reject requests carrying more than one Host header at the edge.
Medium technical severity with no CISA KEV confirmation; remediate through the normal risk-based patch cycle unless local exposure raises the priority. Verified remediation exists for at least one product or source, but 20 structured product or package states remain unresolved. Apply remediation only to the exact product branch confirmed by its source.
Fix availability varies by product01
What, why and how
h2 is a pure-Python implementation of a HTTP/2 protocol stack. Versions up to and including 4.4.0 accept request header blocks containing more than one Host header, and forward every Host header to the consuming application. Where the consumer downgrades HTTP/2 to HTTP/1.1, the resulting request carries two Host header lines, providing a request smuggling primitive. This issue is fixed in version 4.4.1.
What
h2 is a pure-Python implementation of a HTTP/2 protocol stack. Versions up to and including 4.4.0 accept request header blocks containing more than one Host header, and forward every Host header to the consuming application. Where the consumer downgrades HTTP/2 to HTTP/1.1, the resulting request carries two Host header lines, providing a request smuggling primitive. This issue is fixed in version 4.4.1.
Why
The product acts as an intermediary HTTP agent (such as a proxy or firewall) in the data flow between two entities such as a client and server, but it does not interpret malformed HTTP requests or responses in ways that are consistent with how the messages will be processed by those entities that are at the ultimate destination.
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
h2 is a pure-Python implementation of a HTTP/2 protocol stack. Versions up to and including 4.4.0 accept request header blocks containing more than one Host header, and forward every Host header to the consuming application. Where the consumer downgrades HTTP/2 to HTTP/1.1, the resulting request carries two Host header lines, providing a request smuggling primitive. This issue is fixed in version 4.4.1.
Why
The product acts as an intermediary HTTP agent (such as a proxy or firewall) in the data flow between two entities such as a client and server, but it does not interpret malformed HTTP requests or responses in ways that are consistent with how the messages will be processed by those entities that are at the ultimate destination.
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 No confirmed evidence
?
Confirmed exploitation and public exploit material are separate signals. Attacks can occur without public proof-of-concept or exploit code.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 None recorded
?
Confirmed exploitation and public exploit material are separate signals. Attacks can occur without public proof-of-concept or exploit code.No exploit-tagged reference or CISA SSVC proof-of-concept state is currently recorded. Research may still exist outside the structured feeds.
- Likely attack path
- a network path → Inconsistent Interpretation of HTTP Requests ('HTTP Request/Response Smuggling') → 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: a standard category for the underlying weakness. - CWE-444 ↗
CWE-444: Inconsistent Interpretation of HTTP Requests ('HTTP Request/Response Smuggling'). The product acts as an intermediary HTTP agent (such as a proxy or firewall) in the data flow between two entities such as a client and server, but it does not interpret malformed HTTP requests or responses in ways that are consistent with how the messages will be processed by those entities that are at the ultimate destination.
- 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:N/A:LCommon 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.INoneIntegrity impact: No direct loss is represented by this metric.ALowAvailability impact: A successful attack can cause a limited loss.- 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.
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. - Fix availability varies by product
- Affected
- rhmtc/openshift-migration-hook-runner-rhel8 as a component of Migration Toolkit for Containers
- ansible-automation-platform-24/ee-supported-rhel8 as a component of Red Hat Ansible Automation Platform 2
- ansible-automation-platform-24/ee-supported-rhel9 as a component of Red Hat Ansible Automation Platform 2
- ansible-automation-platform-24/platform-resource-runner-rhel8 as a component of Red Hat Ansible Automation Platform 2
- ansible-automation-platform-25/ee-supported-rhel8 as a component of Red Hat Ansible Automation Platform 2
- ansible-automation-platform-25/ee-supported-rhel9 as a component of Red Hat Ansible Automation Platform 2
- ansible-automation-platform-25/platform-resource-runner-rhel8 as a component of Red Hat Ansible Automation Platform 2
- ansible-automation-platform-26/ee-supported-rhel9 as a component of Red Hat Ansible Automation Platform 2
- ansible-automation-platform-27/ee-supported-rhel9 as a component of Red Hat Ansible Automation Platform 2
- rhai/guidellm-rhel9 as a component of Red Hat OpenShift AI (RHOAI)
- rhoai/odh-feature-server-rhel9 as a component of Red Hat OpenShift AI (RHOAI)
- rhoai/odh-llama-stack-core-rhel9 as a component of Red Hat OpenShift AI (RHOAI)
- rhoai/odh-ogx-core-rhel9 as a component of Red Hat OpenShift AI (RHOAI)
- python3-h2 as a component of Red Hat OpenStack Platform 17.1
- python-h2.src as a component of Red Hat OpenStack Platform 18.0
- rhoso/openstack-tempest-all-rhel9 as a component of Red Hat OpenStack Platform 18.0
- Fixed
- 4.4.1.
- Action
- Use the product-specific evidence above. Patch only products with a verified fixed release, and keep every affected or under-investigation state without a matching fix in the remediation queue.
- Workaround
- Update the bundled h2 Python library to 4.4.1 or later. Where an immediate upgrade is not possible, avoid HTTP/2 to HTTP/1.1 downgrade in front-end proxies for affected services, or reject requests carrying more than one Host header at the edge.
04
Evidence and provenance
Published 6 Aug 2026 · Last source change 8 Aug 2026, 02:49 UTC · CWE-444 · Inconsistent Interpretation of HTTP Requests ('HTTP Request/Response Smuggling')
CVE recordCVE.org · 5.2
CVSS sourceCNA
EPSS source FIRST · tracked since 2026-08-14
?
The date BlackTree first stored a score for this CVE from the daily FIRST EPSS feed.European sourceENISA EUVD · EUVD-2026-54177
Product sourceVendor CSAF · Red Hat Product Security
Remediation sourceVendor CSAF · Red Hat Product Security
CWE sourceCNA
NVD statusNVD not scheduled
Core structured fields are present and their contributing authorities are shown above.
Material change intelligence
View recent updates ↗What changed after publication
- Affected versionsThe structured affected or fixed version information changed.CNA ↗
- Before
- < 4.4.1 · Fixed: 4.4.1.
- After
- h2: < 4.4.1 · Fixed: 4.4.1.
CVE published
Fixed release or patch reference recorded
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.