QuicheApplication · Cloudflare
CVE-2025-4821
Official description Straight from the sourceThe vendor's or NVD's own wording, published unedited. Authoritative, but often terse — it says what broke, rarely what to do.
NVD · uneditedImpact Cloudflare quiche was discovered to be vulnerable to incorrect congestion window growth, which could cause it to send data at a rate faster than the path might actually support. An unauthenticated remote attacker can exploit the vulnerability by first completing a handshake and initiating a congestion-controlled data transfer towards itself. Then, it could manipulate the victim's congestion control state by sending ACK frames covering a large range of packet numbers (including packet numbers that had never been sent); see RFC 9000 Section 19.3. The victim could grow the congestion window beyond typical expectations and allow more bytes in flight than the path might really support. In extreme cases, the window might grow beyond the limit of the internal variable's type, leading to an overflow panic. Patches quiche 0.24.4 is the earliest version containing the fix for this issue.
Technical summary Written by usOur analysis, written from the advisory, the CVSS vector and the affected-version data. It adds context the advisory leaves out, and never invents facts that are not in the source.
dbcve analysis · high confidenceCloudflare quiche incorrectly grows the congestion window when receiving ACK frames covering a large range of packet numbers, including unsent packets per RFC 9000 Section 19.3. An unauthenticated attacker who completes a handshake and sends manipulated ACK frames can cause the congestion window to expand beyond what the path supports, potentially triggering an overflow panic.
Verify against the referenced sources before acting — the references below are authoritative for this CVE, this summary is not.
Affected products & versions What the vendor confirmedThe version ranges the vendor confirmed as vulnerable. If your version sits inside a range here, treat yourself as exposed until you have upgraded.
NVD · CPE data
QuicheApplication
< 0.24.4CVSS breakdown How the score is builtThe industry scoring standard. It rates how the flaw is reached, what it takes to exploit, and what an attacker gains — the score is derived from those, not the other way round.
From the vector- Attack vector
- Network
- Complexity
- Low
- Privileges
- None
- User interaction
- None
- Scope
- Unchanged
- Confidentiality
- None
- Integrity
- None
- Availability
- High
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
Am I affected? How to checkSteps we derive from the advisory and the affected-version data, so you can decide whether this CVE reaches your setup. They are a guide, not a scan — your own configuration is the authority.
dbcve checksWork through these to decide whether this CVE applies to you.
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Identify the installed quiche versionLocate the quiche library in your environment (check dependency manifests like Cargo.lock, package managers, or bundled binaries) and retrieve the version number. Common methods include: running `grep -r quiche` in your dependency files, checking the library binary metadata, or querying your package manager for the installed quiche package version.Affected if The installed version is lower than 0.24.4 (e.g., 0.24.3, 0.24.0, or any version below 0.24.4)
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Confirm QUIC server functionality is enabledDetermine whether your application or service uses quiche to accept and handle incoming QUIC connections. Review your server configuration, process list, or startup parameters to verify that the QUIC listener or server mode is active.Affected if QUIC server functionality is enabled and processing incoming connections from untrusted network paths
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Verify the service processes ACK frames from peersInspect the application logic or traffic logs to confirm that incoming ACK frames from remote peers are being processed by the quiche library. This is typically active in any bidirectional QUIC communication where the client sends data.Affected if The service processes ACK frames from remote clients, which is standard in any QUIC connection where data flows in either direction
Your environment is affected if you are running any version of Cloudflare Quiche below 0.24.4 that is actively accepting QUIC connections and processing ACK frames from remote peers.
Generated from the published advisory. Verify against your own configuration.
Remediation Closing itWhat it takes to close this. Where a vendor fix exists we point at it; where none exists we say so plainly, and can build one. Effort estimates are scoped from the advisory, not from your codebase.
dbcve · scoped0.24.4
Upgrade quiche to version 0.24.4 or later to patch the congestion control vulnerability.
quiche 0.24.4
- 1. Identify all projects or applications that depend on the Cloudflare quiche library
- 2. For each project, locate the current quiche dependency version (e.g., in Cargo.toml for Rust projects, or package manager lock files)
- 3. Update the quiche dependency to version 0.24.4 or later
- 4. Rebuild the affected projects to ensure the new quiche version is linked
- 5. Run existing test suites to verify the upgrade does not break functionality
- 6. Deploy the updated applications
Generated from the published advisory — verify against the referenced sources before acting.
Fix this in Quiche
Scoped from the published advisory
- Consultation3.0 h
- Implementation6.0 h
- Testing6.0 h
- Review / QA3.0 h
An estimate, not a bill — we confirm scope with you before any work starts. Need it this week? Rush from $4,992.
Scan for this in your stack
Free · runs locallyCheck whether your project pulls in CVE-2025-4821 — or any other known-vulnerable package — straight from your lock files. Free and open source; it runs locally and uploads nothing.
References Go to the primary sourcePrimary sources — vendor advisories, patches and trackers. Where our summary and a reference disagree, the reference wins.
Primary sourcesPractitioner notes
ContributedPeer-ranked notes from engineers who’ve handled CVE-2025-4821 in production — separate from our analysis above.
The advisory tells you what broke. It rarely tells you what actually worked. If you’ve dealt with this one, that detail is what the next engineer is searching for.
- The version that genuinely resolved it — not the one the vendor claimed
- A config change or rule that shut the vector down
- A gotcha in the upgrade path that cost you an afternoon
No notes yet
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- Verified mitigations, workarounds, and config changes
- Version or environment caveats, and links to real fixes
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