VulnerabilityPublished 18 Jun 2025

CVE-2025-4820: Cloudflare quiche · Unthrottled Resource Allocation

Affects Cloudflare quicheSource CVE Database V5

Not known to be exploited · CVSS 5.3 Medium : upgrade to 0.24.4 when convenient.

Data as of 26 Oct 2026, 17:16 UTC

CVSS 3.1 base score

5.3Medium

How severe the flaw is on the 0–10 CVSS scale.

EPSS: 0.8% chance of exploitation in 30 days – higher than 58% of CVEsExploit Prediction Scoring System: the likelihood attackers will use this flaw soon.
Patch: Available – fixed in 0.24.4

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: None
Integrity: None
Availability: Low

Bold values are the worst case: easiest to exploit or largest impact.

Attack Vector: Network: The vulnerable component is bound to the network stack and the set of possible attackers extends beyond the other options listed, up to and including the entire Internet. Such a vulnerability is often termed 'remotely exploitable' and can be thought of as an attack being exploitable at the protocol level one or more network hops away (e.g., across one or more routers).
Attack Complexity: Low: Specialized access conditions or extenuating circumstances do not exist. An attacker can expect repeatable success against the vulnerable component.
Privileges Required: None: The attacker is unauthorized prior to attack, and therefore does not require any access to settings or files to carry out an attack.
User Interaction: None: The vulnerable system can be exploited without any interaction from any user.
Scope: Unchanged: An exploited vulnerability can only affect resources managed by the same security authority. In this case, the vulnerable component and the impacted component are either the same, or both are managed by the same security authority.
Confidentiality: None: There is no loss of confidentiality within the impacted component.
Integrity: None: There is no loss of integrity within the impacted component.
Availability: Low: Performance is reduced or there are interruptions in resource availability. Even if repeated exploitation of the vulnerability is possible, the attacker does not have the ability to completely deny service to legitimate users. The resources in the impacted component are either partially available all of the time, or fully available only some of the time, but overall there is no direct, serious consequence to the impacted component.

Definitions: FIRST CVSS v3.1 specification.

Exploitation evidence

Disclosed: evidence found.
CVE published 18 Jun 2025
Detectable: no evidence on record.
No public nuclei template indexed
Weaponised: no evidence on record.
No public exploit code indexed in Metasploit or Exploit-DB
Exploited in the wildExploited: no evidence on record.
Not in CISA KEV and no source reports exploitation
Observed by OffSeq MirageSeen by Mirage: no evidence on record.
No attack traffic recorded by OffSeq Mirage sensors

A missing step is not evidence of safety. Sources: cve.org (opens in a new tab), the Metasploit, Exploit-DB and nuclei-templates index as of 26 Oct 2026.

Disclosed: The CVE record is public on cve.org. Its publication is t = 0.
Detectable: A public nuclei template can find exposed instances, for defenders and attackers alike.
Weaponised: Public exploit code exists in Metasploit or Exploit-DB, or CISA SSVC or the CVSS exploit maturity reports a public proof of concept.
Exploited in the wild: CISA KEV lists it, CISA SSVC rates exploitation active, or a source reports attacks.
Observed by OffSeq Mirage: OffSeq Mirage honeypot sensors recorded attack traffic for this CVE, or found exposed hosts without attack traffic.

Moments are spaced evenly, not to scale; a dashed line marks a gap of six months or more. Deltas count from t = 0. Sources that give only a date (CISA KEV, Exploit-DB, or a time of exactly 00:00Z) read “same day” rather than hours. Metasploit modules carry the flaw’s disclosure date, not their release date, and nuclei templates carry no date, so both appear above but not on the timeline.

What to do

Upgrade github/cloudflare/quiche to 0.24.4.

Affected software

Product	Affected	Fixed in
crates.iogithub/cloudflare/quiche`pkg:cargo/github/cloudflare/quiche`Other threats	Affected <0.24.4	Fixed in 0.24.4

Check your own hosts for these packages with threat-finder (opens in a new tab), our free open-source scanner.

AI-suggested

Upgrade quiche to version 0.24.4 or later immediately to apply the official patch addressing the congestion window growth flaw.
For organizations using Cloudflare services, verify with Cloudflare that their infrastructure has been updated to the patched quiche version.
Implement network-level rate limiting and anomaly detection to identify and block unusual ACK frame patterns or excessive QUIC traffic that may indicate exploitation attempts.
Monitor QUIC traffic metrics for abnormal congestion window growth or unexpected data flow rates that could signal an ongoing attack.
Employ traffic shaping and quality of service (QoS) policies to prevent a single connection from monopolizing bandwidth or resources.
Use intrusion detection/prevention systems (IDS/IPS) with updated signatures capable of detecting opportunistic ACK attacks or malformed QUIC packets.
Engage in regular vulnerability scanning and penetration testing focusing on QUIC implementations to identify residual or related weaknesses.
Maintain incident response readiness specific to network-layer DoS scenarios involving QUIC traffic.
Collaborate with Cloudflare support and security teams for timely threat intelligence and mitigation guidance. These measures go beyond generic advice by focusing on traffic behavior analysis, network controls specific to QUIC, and vendor coordination.

Description

Impact 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 exercising an opportunistic ACK attack; see RFC 9000 Section 21.4. The victim could grow the congestion window beyond typical expectations and allow more bytes in flight than the path might really support. Patches quiche 0.24.4 is the earliest version containing the fix for this issue.

Impact

For European organizations, the primary impact of CVE-2025-4820 is the risk of denial-of-service conditions affecting services that rely on Cloudflare's quiche library for QUIC protocol handling. Since QUIC is fundamental to HTTP/3 and other latency-sensitive applications, exploitation could lead to degraded network performance, increased latency, or complete service outages. This can affect web services, APIs, and cloud-based applications that use Cloudflare infrastructure or embed quiche directly. The availability impact could disrupt business operations, customer access, and critical communications. While confidentiality and integrity are not directly impacted, the availability degradation can indirectly affect operational continuity and service-level agreements. European organizations with high reliance on Cloudflare's CDN, edge computing, or security services may face increased risk. Additionally, sectors such as finance, telecommunications, and government services, which depend on low-latency, high-availability network services, could experience significant operational disruptions. The vulnerability's unauthenticated nature and lack of required user interaction make it easier for attackers to attempt exploitation remotely, increasing the threat surface. However, the absence of known exploits in the wild currently limits immediate risk, though proactive mitigation is advised.

CWE-770Allocation of Resources Without Limits or Throttling (opens in a new tab)
Base weaknessLikelihood of exploit: HighCWE Top 25 (2025): #25
The product allocates a reusable resource or group of resources on behalf of an actor without imposing any intended restrictions on the size or number of resources that can be allocated.
Requirements
Clearly specify the minimum and maximum expectations for capabilities, and dictate which behaviors are acceptable when resource allocation reaches limits.
Architecture and Design
Limit the amount of resources that are accessible to unprivileged users. Set per-user limits for resources. Allow the system administrator to define these limits. Be careful to avoid CWE-410.
Architecture and Design
Design throttling mechanisms into the system architecture. The best protection is to limit the amount of resources that an unauthorized user can cause to be expended.
OWASP Cheat Sheets
Abuse Case (opens in a new tab)
OWASP ASVS 4.0.3
V8.1.4 (opens in a new tab)
V13.4.1 (opens in a new tab)
V11.1.4 (opens in a new tab)
Attack patterns
XML Ping of the Death (opens in a new tab)
Exponential Data Expansion (opens in a new tab)
Serialized Data Parameter Blowup (opens in a new tab)
Serialized Data with Nested Payloads (opens in a new tab)
Oversized Serialized Data Payloads (opens in a new tab)
From MITRE CWE, MITRE CAPEC and OWASP (Top 10, OpenCRE).

References

github.com/cloudflare/quiche/security/advisories/GHSA-2v9p-3p3h-w56j (opens in a new tab)

Community reviews

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Exploitation evidence

What to do

Affected software

Description

AI analysisMachine-generated, updated 18 Jun 2025, 16:16 UTC

WeaknessesCWE-770

Technical details

References

Community reviews

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