CVE-2025-4821: Incorrect congestion window growth by invalid ACK ranges
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 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.
Affected Software
Event History
Frequently Asked Questions
What is the severity of CVE-2025-4821?
CVE-2025-4821 is classified as a critical vulnerability due to the potential for an unauthenticated remote attacker to exploit it.
How do I fix CVE-2025-4821?
To mitigate CVE-2025-4821, users should upgrade to the latest version of Cloudflare quiche, specifically beyond version 0.24.4.
What impact does CVE-2025-4821 have on network performance?
CVE-2025-4821 can lead to incorrect congestion window growth, causing the application to send data at an unsustainable rate, potentially degrading network performance.
Who is affected by CVE-2025-4821?
CVE-2025-4821 affects users running versions of Cloudflare quiche up to and including 0.24.4.
Can CVE-2025-4821 be exploited remotely?
Yes, CVE-2025-4821 can be exploited by unauthenticated remote attackers who complete a handshake with the affected service.