Skip to content

transport/fc.rs: panic attempting to send MAX_DATA with value larger max varint

Low
larseggert published GHSA-jfv6-x22w-grhf Jun 26, 2025

Package

cargo neqo (Rust)

Affected versions

<=0.13.2

Patched versions

0.13.3

Description

Summary

A remote can trick us into attempting to send a MAX_DATA with a value larger than the maximum varint, thus leading to a panic.

Reported in https://bugzilla.mozilla.org/show_bug.cgi?id=1965279.

Details

For connection flow control, Neqo simply sets a huge limit, i.e. the QUIC varint max value.

const LOCAL_MAX_DATA: u64 = 0x3FFF_FFFF_FFFF_FFFF; // 2^62-1

A remote can trick us into attempting to encode a value larger than the QUIC varint max value by consuming 1 or more bytes on the connection and then sending a DATA_BLOCKED frame.

Neqo will attempt to send a MAX_DATA connection flow control update with a value larger than the QUIC varint max value.

Impact

Remote QUIC server can trigger a panic in Neqo, thus crash the socket thread which thus crashes the main process.

Severity

Low

CVSS overall score

This score calculates overall vulnerability severity from 0 to 10 and is based on the Common Vulnerability Scoring System (CVSS).
/ 10

CVSS v3 base metrics

Attack vector
Network
Attack complexity
Low
Privileges required
Low
User interaction
Required
Scope
Unchanged
Confidentiality
None
Integrity
None
Availability
Low

CVSS v3 base metrics

Attack vector: More severe the more the remote (logically and physically) an attacker can be in order to exploit the vulnerability.
Attack complexity: More severe for the least complex attacks.
Privileges required: More severe if no privileges are required.
User interaction: More severe when no user interaction is required.
Scope: More severe when a scope change occurs, e.g. one vulnerable component impacts resources in components beyond its security scope.
Confidentiality: More severe when loss of data confidentiality is highest, measuring the level of data access available to an unauthorized user.
Integrity: More severe when loss of data integrity is the highest, measuring the consequence of data modification possible by an unauthorized user.
Availability: More severe when the loss of impacted component availability is highest.
CVSS:3.1/AV:N/AC:L/PR:L/UI:R/S:U/C:N/I:N/A:L

CVE ID

CVE-2025-6703

Weaknesses

No CWEs

Credits