How severe is CVE-2025-57804?

No CVSS score has been assigned to CVE-2025-57804 yet. Review the advisory details and affected package list to assess your exposure.

Which packages are affected by CVE-2025-57804?

CVE-2025-57804 affects the following packages: h2 (PyPI). Ecosystems affected: PyPI.

How do I fix CVE-2025-57804?

Update h2 to 4.3.0 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2025-57804 is resolved across your whole dependency graph.

How do I detect CVE-2025-57804 in my PyPI dependencies?

Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for h2, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.

How do I mitigate CVE-2025-57804 if there is no patch (or I can't update yet)?

If you can't upgrade right away: gate or disable the affected feature, validate untrusted input at the boundary, and avoid passing attacker-controlled data into the vulnerable path. O3's runtime protection blocks exploitation in production as an interim safeguard until the upgrade lands.

Is CVE-2025-57804 actively exploited in the wild?

No public exploit code has been indexed for CVE-2025-57804 yet. This does not mean the vulnerability cannot be exploited — absence of public exploits does not imply safety. Apply the recommended fix and use O3 Security to monitor your exposure.

What is the EPSS score for CVE-2025-57804?

CVE-2025-57804 has an EPSS (Exploit Prediction Scoring System) score of 1.9%, placing it in the 80th percentile of all CVEs. EPSS is maintained by FIRST.org and estimates the probability that a vulnerability will be exploited in the wild within the next 30 days. This score indicates relatively lower exploitation probability, though the CVSS severity should still guide your patching priority.

What type of vulnerability is CVE-2025-57804?

CVE-2025-57804 is classified as CWE-93 (CWE-93). This weakness type describe the underlying flaw category, which helps determine the potential impact and the right class of mitigation.

When was CVE-2025-57804 published, and has it been updated?

CVE-2025-57804 was published on August 25, 2025 and was last updated on October 28, 2026. Advisory data evolves as severity scores, affected ranges, and exploit intelligence are revised — always check the latest version of the advisory before acting.

CVE-2025-57804: h2 — Fixed in 4.3.0

Exploitation Status

No confirmed exploitation observed yet

CISA assesses this as automatable — exploitation doesn’t require manual, per-target effort, which raises the odds of mass scanning and opportunistic attacks.
CISA’s own triage has not observed active exploitation or public proof-of-concept code for this CVE as of its last assessment.

Exploitation and automatability from CISA’s SSVC triage for CVE-2025-57804.

EPSS Exploitation Probability

via FIRST.org ↗

1.9%probability of exploitation in next 30 days

Lower Risk0.00%

Lower risk than most CVEs80th percentile — riskier than 80% of all scored CVEsHighest risk

Probability of exploitation in the next 30 days, from FIRST.org EPSS.

Real-World Exposure

1 pkg affected

🐍h2

Real-time download stats are indexed for npm and PyPI packages. This vulnerability affects PyPI packages — download data is not available via public APIs for these ecosystems.

Description

h2 is a pure-Python implementation of a HTTP/2 protocol stack. Prior to version 4.3.0, an HTTP/2 request splitting vulnerability allows attackers to perform request smuggling attacks by injecting CRLF characters into headers. This occurs when servers downgrade HTTP/2 requests to HTTP/1.1 without properly validating header names/values, enabling attackers to manipulate request boundaries and bypass security controls. This issue has been patched in version 4.3.0.

Affected Packages

1 total 1 fixed

Ecosystem	Package	Vulnerable range	Fix
🐍PyPI	`h2`	all versions	4.3.0`pip install --upgrade 'h2==4.3.0'`

Detection & mitigation playbook

Open-source dependency

Detect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for h2, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
Fix
Update h2 to 4.3.0 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2025-57804 is resolved across your whole dependency graph.
Workarounds
If you can't upgrade right away: gate or disable the affected feature, validate untrusted input at the boundary, and avoid passing attacker-controlled data into the vulnerable path. O3's runtime protection blocks exploitation in production as an interim safeguard until the upgrade lands.

Fixing This On Your OS

If you run this on a Linux distribution, patch through your package manager against the distro's own security advisory below — it tracks the exact backported fix for your release, which can ship on a different timeline (and sometimes a different severity) than the upstream project.

Red HatModerate

This issue is classified as Moderate rather than Important because its impact is constrained to integrity violations through request injection, without directly compromising confidentiality or availability. The flaw arises only in environments where HTTP/2 traffic is downgraded to HTTP/1.1 by intermediaries, which is…

Workaround published by Red Hat

Mitigation is either unavailable or does not meet Red Hat Product Security standards for usability, deployment, applicability, or stability.

Source: Red Hat security advisory for CVE-2025-57804 (CC BY 4.0)

Frequently Asked Questions

h2 is a pure-Python implementation of a HTTP/2 protocol stack. Prior to version 4.3.0, an HTTP/2 request splitting vulnerability allows attackers to perform request smuggling attacks by injecting CRLF characters into headers. This occurs when servers downgrade HTTP/2 requests to HTTP/1.1 without properly validating header names/values, enabling attackers to manipulate request boundaries and bypass security controls. This issue has been patched in version 4.3.0.

O3 Security · Impact-Aware SCA

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CVE-2025-57804 — h2