CVE-2025-6703: Mozilla Neqo DOS Vulnerability

CVE-2025-6703 Overview

CVE-2025-6703 is an improper input validation vulnerability [CWE-20] in Mozilla neqo, the Rust implementation of the QUIC transport protocol. A malformed input processed by the library triggers an unexploitable crash, resulting in a denial-of-service condition for the affected process. The issue affects neqo versions 0.4.24 through 0.13.2. Mozilla documented the flaw in security advisory GHSA-jfv6-x22w-grhf. The Exploit Prediction Scoring System (EPSS) rates the probability of exploitation at 0.213% (percentile 11.578), and the vulnerability is not listed in the CISA Known Exploited Vulnerabilities catalog.

Critical Impact
A remote attacker can send crafted QUIC traffic to a process using vulnerable neqo versions and cause an unexploitable crash, disrupting availability of the affected component.

Affected Products

Mozilla neqo 0.4.24 through 0.13.2
Rust applications embedding the neqo QUIC library
Downstream projects depending on vulnerable neqo crate versions

Discovery Timeline

2025-06-26 - CVE-2025-6703 published to the National Vulnerability Database
2026-06-17 - Last updated in NVD database

Technical Details for CVE-2025-6703

Vulnerability Analysis

The vulnerability resides in Mozilla neqo, a Rust-based implementation of the QUIC transport protocol used to support HTTP/3 and related workloads. neqo parses attacker-influenced network input during QUIC connection handling. When the library encounters certain malformed input, its validation logic fails to reject the input safely and instead reaches a runtime condition that terminates the process. The result is a crash without memory corruption or code execution primitives, categorizing the flaw as a pure availability issue. Because QUIC endpoints are reachable over the network and connection establishment does not require prior authentication, an unauthenticated remote peer can trigger the crash. Impact is limited to the availability of the affected process and any service it fronts.

Root Cause

The root cause is improper input validation [CWE-20] in neqo's handling of protocol-level input. The library does not fully constrain unexpected values before they reach code paths that assume validated state, leading the process to abort. Mozilla's advisory GHSA-jfv6-x22w-grhf tracks the fix in versions after 0.13.2.

Attack Vector

The attack vector is network-based. An attacker sends a crafted QUIC datagram or frame sequence to a listener or client built on a vulnerable neqo version. No authentication is required, and user interaction is limited to initiating or accepting a QUIC session. The outcome is a process crash rather than information disclosure or code execution. Refer to the Mozilla neqo GitHub Security Advisory GHSA-jfv6-x22w-grhf for protocol-level specifics.

Detection Methods for CVE-2025-6703

Indicators of Compromise

Unexpected process termination or panic messages in logs from services embedding neqo
Repeated QUIC connection resets or abrupt UDP session terminations from a single remote source
Crash dumps referencing neqo crate stack frames on QUIC endpoints

Detection Strategies

Inventory Rust builds and container images to identify binaries linked against neqo versions 0.4.24 through 0.13.2 using cargo tree or SBOM tooling.
Alert on repeated service restarts or panic events on hosts exposing QUIC (UDP/443 and other QUIC listeners).
Correlate crash telemetry with inbound UDP traffic patterns to identify anomalous senders targeting QUIC listeners.

Monitoring Recommendations

Ingest application crash logs and QUIC error counters into your SIEM for baseline and anomaly analysis.
Track dependency versions of neqo in CI/CD pipelines and flag builds that pin vulnerable versions.
Monitor upstream neqo releases and Mozilla advisories for follow-on fixes and regressions.

How to Mitigate CVE-2025-6703

Immediate Actions Required

Upgrade neqo to a version later than 0.13.2 in all affected Rust projects and rebuild dependent binaries.
Rebuild and redeploy downstream services and containers that statically link the fixed crate version.
Restrict exposure of QUIC endpoints to trusted networks where feasible until patched builds are deployed.

Patch Information

Mozilla published fix guidance in GitHub Security Advisory GHSA-jfv6-x22w-grhf. Update the neqo dependency to a release beyond 0.13.2 in Cargo.toml, run cargo update -p neqo, and validate the resolved version with cargo tree. No known exploit code is publicly available, and the vulnerability is not listed in the CISA KEV catalog.

Workarounds

Temporarily disable QUIC and HTTP/3 listeners in services built on vulnerable neqo versions and fall back to TCP-based HTTP/2.
Apply rate limiting and source filtering on UDP ports serving QUIC to reduce the blast radius of crash attempts.
Configure process supervisors such as systemd to automatically restart crashed services while patching is in progress.

bash

# Configuration example
# Update neqo to a fixed release and verify the resolved version
cargo update -p neqo
cargo tree -p neqo