Summary
A misbehaving or malicious server can trigger an assertion in a quic-go client (and crash the process) by sending a premature HANDSHAKE_DONE frame during the handshake.
Affected Versions
- All versions prior to v0.49.1 (for the 0.49 branch)
- Versions v0.50.0 to v0.54.0 (inclusive)
- Fixed in v0.49.1, v0.54.1, and v0.55.0 onward
Users are recommended to upgrade to the latest patched version in their respective maintenance branch or to v0.55.0 or later.
Details
For a regular 1-RTT handshake, QUIC uses three sets of keys to encrypt / decrypt QUIC packets:
- Initial keys (derived from a static key and the connection ID)
- Handshake keys (derived from the client's and server's key shares in the TLS handshake)
- 1-RTT keys (derived when the TLS handshake finishes)
On the client side, Initial keys are discarded when the first Handshake packet is sent. Handshake keys are discarded when the server's HANDSHAKE_DONE frame is received, as specified in section 4.9.2 of RFC 9001. Crucially, Initial keys are always dropped before Handshake keys in a standard handshake.
Due to packet reordering, it is possible to receive a packet with a higher encryption level before the key for that encryption level has been derived. For example, the server's Handshake packets (containing, among others, the TLS certificate) might arrive before the server's Initial packet (which contains the TLS ServerHello). In that case, the client queues the Handshake packets and decrypts them as soon as it has processed the ServerHello and derived Handshake keys.
After completion of the handshake, Initial and Handshake packets are not needed anymore and will be dropped. quic-go implements an assertion that no packets are queued after completion of the handshake.
A misbehaving or malicious server can trigger this assertion, and thereby cause a panic, by sending a HANDSHAKE_DONE frame before actually completing the handshake. In that case, Handshake keys would be dropped before Initial keys.
This can only happen if the server implementation is misbehaving: the server can only complete the handshake after receiving the client's TLS Finished message (which is sent in Handshake packets).
The Fix
quic-go needs to be able to handle misbehaving server implementations, including those that prematurely send a HANDSHAKE_DONE frame. We now discard Initial keys when receiving a HANDSHAKE_DONE frame, thereby correctly handling premature HANDSHAKE_DONE frames. The fix was implemented in https://github.com/quic-go/quic-go/pull/5354.
Impact
A misbehaving or malicious server can cause a denial-of-service (DoS) attack on the quic-go client by triggering an assertion failure, leading to a process crash. This requires no authentication and can be exploited during the handshake phase. Observed in the wild with certain server implementations (e.g. Solana's Firedancer QUIC).
CVE-2025-59530 has a CVSS score of 7.5 (High). The vector is network-reachable, no privileges required, and no user interaction. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. A fixed version is available (0.49.1, 0.54.1); upgrading removes the vulnerable code path.
Affected versions
Security releases
Kodem intelligence
Severity tells you how bad this could be in the worst case. It does not tell you whether you are exposed. Exploitability and impact are functions of runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A vulnerable package can sit in your dependency tree and never run.
Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter. Kodem's runtime-powered SCA identifies whether this CVE is reachable in your applications.
Remediation advice
github.com/quic-go/quic-go to 0.49.1 or later; github.com/quic-go/quic-go to 0.54.1 or later
Kodem Kai can prioritize this vulnerability in your dependency tree and generate a fix recommendation.
Frequently Asked Questions
- What is CVE-2025-59530? CVE-2025-59530 is a high-severity security vulnerability in github.com/quic-go/quic-go (go), affecting versions < 0.49.1. It is fixed in 0.49.1, 0.54.1.
- How severe is CVE-2025-59530? CVE-2025-59530 has a CVSS score of 7.5 (High). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
- Which versions of github.com/quic-go/quic-go are affected by CVE-2025-59530? github.com/quic-go/quic-go (go) versions < 0.49.1 is affected.
- Is there a fix for CVE-2025-59530? Yes. CVE-2025-59530 is fixed in 0.49.1, 0.54.1. Upgrade to this version or later.
- Is CVE-2025-59530 exploitable, and should I be worried? Whether CVE-2025-59530 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
- What actually determines whether CVE-2025-59530 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
- How do I fix CVE-2025-59530?
- Upgrade
github.com/quic-go/quic-goto 0.49.1 or later - Upgrade
github.com/quic-go/quic-goto 0.54.1 or later
- Upgrade