CVE-2026-71554 python-hyper · h2

h2 (Python HTTP/2): duplicate Host header forwarded, request smuggling

Vulnerability at a glance

Severity, exploitation likelihood & attack requirements

CVSS v3.1 severity
5.3Medium
EPSS · exploitation likelihood
0.418% Probability within 30 daysPercentile 35.0
EPSS trend
Unchanged Since 2026-10-14
Known exploitation
Not in KEV CISA KEV catalog Absence does not establish safety
Attack vector
NetworkRemotely reachable
Attack complexity
LowFewer obstacles to exploitation
Privileges required
NoneNo authentication required
User interaction
NoneNo user action needed
Exploit availability & maturity
Not scored Availability not established No explicit exploit maturity assessment View exploit evidence
How to read the colors & trend
Higher exposureElevatedModerateLower exposureUnknown / unassessed

CVSS colors follow its reported severity. Attack-factor colors describe each requirement, not an overall risk rating. Gray also covers absence from KEV; absence does not establish safety.

EPSS display bands: below 1% blue, 1–10% amber, 10–50% orange, 50% and above red. These are presentation bands, not FIRST severity categories. The trend compares daily EPSS probabilities, not percentile ranks; “pp” means percentage points. The sparkline shows relative movement, with its vertical scale fitted to the observed scores.

Already protected by existing Atomicorp rules No new CVE-specific rule required.

Atomicorp ModSecurity protection

Protected by Atomicorp rules

Atomicorp ModSecurity rules block attacks exploiting this vulnerability when blocking is enabled. Keep your rules current.

By targeting underlying attack techniques and CWE weakness classes, existing rules can protect against newly disclosed vulnerabilities. Rule changes are regression-tested against attack and false-positive corpora.

1 rule identified View rules & research details →

WAF protection applies to the tested attack paths; it does not change the CVSS or EPSS score. Keep your software patched and your Atomicorp rules current.

CISA SSVC assessment

CISA ADP · Assessed 2026-08-08 · SSVC 2.0.3

Exploitation
None reported
Automatable
Yes
Technical Impact
Partial

Stakeholder-Specific Vulnerability Categorization records CISA’s assessment of exploitation, automation, and technical impact. “None reported” reflects the assessment date; it does not establish that no exploit exists. View SSVC source details.

On this page

CVE Details

  • Protection status: Protected by Atomicorp rules · About protection statuses
  • CVE: CVE-2026-71554
  • Name: h2 (Python HTTP/2): duplicate Host header forwarded, request smuggling
  • Severity: Medium
  • CVSS: 5.3 (v3.1) (Medium)
  • CVSS version: 3.1
  • CVSS source: GitHub_M CNA
  • CVSS vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L
    • Attack vector: Network
    • Attack complexity: Low
    • Privileges required: None
    • User interaction: None
    • Scope: Unchanged
    • Confidentiality impact: None
    • Integrity impact: None
    • Availability impact: Low
  • CWE: CWE-444 — Inconsistent Interpretation of HTTP Requests (‘HTTP Request/Response Smuggling’)
  • Related CAPEC attack patterns (via MITRE CWE relationships): CAPEC-273 , CAPEC-33
  • Vendor: python-hyper
  • Vulnerable product: h2
  • CVE published: 2026-08-06
  • CVE last modified: 2026-09-18
  • EPSS: 0.0042; 35.0th percentile as of 2026-10-21 (FIRST )
  • Latest Update: 2026-10-14
  • Reviewed Rule IDs: 340364
  • Reviewed rules with current blocking actions: 340364
  • Request surfaces inspected: Request headers
Technical vulnerability details · CVSS assessments, exploit evidence & CPE data

CVSS assessments

CVSS v3.1 · security-advisories@github.com

  • Base score: 5.3
  • Base severity: MEDIUM
  • Exploitability subscore: 3.9
  • Impact subscore: 1.4
  • Assessment type: Secondary
  • Source identifier: security-advisories@github.com
  • Full vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L
    • Attack vector: Network
    • Attack complexity: Low
    • Privileges required: None
    • User interaction: None
    • Scope: Unchanged
    • Confidentiality impact: None
    • Integrity impact: None
    • Availability impact: Low
  • Exploit maturity: Not scored

Exploitability and impact subscores describe the vulnerability’s technical characteristics. Exploit maturity describes the reported state of exploitation; these are separate measures.

Assessment source: NVD record · CVE record

CISA SSVC assessments

These are CISA’s published decision points at the assessment time. They are not a complete organization-specific prioritization decision.

  • Provider: CISA ADP
  • Provider identifier: 134c704f-9b21-4f2e-91b3-4a467353bcc0
  • SSVC version: 2.0.3
  • Role: CISA Coordinator
  • Assessment timestamp: 2026-08-08T02:49:21.263516Z
  • Exploitation: none
  • Automatable: yes
  • Technical Impact: partial
  • Source: CISA ADP in the CVE record

Vulnerable CPE configurations

No vulnerable CPE configurations are supplied in the current NVD record. See the affected-product information and vendor guidance on this page.

Vulnerability Description

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.

  1. Maintain the reviewed Atomicorp rules. Keep your Atomicorp commercial rules current and confirm that the reviewed rules (340364 ) have not been locally disabled. Atomicorp commercial rules are enabled by default. If you use Atomic WAF, review the applicable policy and rule settings.

  2. Apply the vendor’s remediation. WAF protection is defense in depth and does not replace upgrading or following the affected vendor’s guidance.

    • Affected versions: python-hyper h2 version < 4.4.1.
    • No specific CNA/vendor solution or workaround is included in the current CVE Record . Review the vendor’s security advisories and supported-release guidance before making changes.

  3. Validate after changes. Confirm rule updates are current, review WAF events for the affected application, and test application behavior after vendor upgrades, mitigations, or local policy changes.

Research Notes

Atomicorp WAF protections are designed around attack techniques and exploit behaviors, allowing existing protections to apply across multiple vulnerabilities without requiring a unique rule for every CVE.

Public Vulnerability References

Test or Research Source

  • Source: https://github.com/python-hyper/h2/commit/292a40829feefda98c8509dcdbbb4a57af9bd6a6
  • Tags: manual-review
  • Research basis: Every supported web server rejects a request that carries more than one Host header before forwarding it, and Atomicorp rule 340364 enforces the same check on any platform that does not.
  • Notes: Platform-mitigated special case (Mike, 2026-10-14): the CVE needs two Host lines, or :authority plus a literal Host, to reach the vulnerable listener. Apache 2.4.62, Caddy 2.11.7 and Varnish 6.0/9.1 were measured rejecting that with 400 before ModSecurity runs; nginx per its source. A proxy forwards one Host downstream regardless. 340364 is defense in depth; a replay in which the request reaches ModSecurity and 340364 does not fire is the regression to alarm on.

Research Note Context

This research note documents exploit-technique analysis associated with this vulnerability and references Atomicorp WAF protections observed during engineering review, testing, or validation activities.

Atomicorp security controls are generally designed to identify and mitigate attack techniques and exploit behaviors rather than individual CVE identifiers. As a result, protections developed for one vulnerability may also apply to related attack variants and future vulnerabilities using similar techniques.

Learn More

Atomicorp develops commercially supported security technologies for web applications, servers, endpoints, workloads, and legacy enterprise platforms.

Organizations interested in protecting systems against exploit techniques associated with vulnerabilities such as this one can learn more about:

  • Atomic ModSecurity Rules: commercially maintained WAF rules for ModSecurity deployments, including protections for web attacks, bots, scanners, malware, exploitation attempts, and emerging vulnerability classes.
  • Atomic WAF: an enterprise web application firewall with Atomicorp rules, centralized management, reporting, authentication controls, and operational tooling.
  • Atomic ModSecurity Integrator: a special licensing option for MSPs, MSSPs, OEMs, VARs, reverse proxy providers, and other integrators who want to incorporate Atomicorp ModSecurity rule technology into their own products or services.
  • Atomic OSSEC: an enterprise OSSEC-based intrusion detection, compliance, and security monitoring platform.
  • Atomic Protector: workload protection for Linux, web, cloud, data center, and hybrid environments.

For additional information, product evaluations, or technical discussions, visit https://www.atomicorp.com/ or https://modsecurity.io/ .