CVE-2025-6703Network attack vector

Process crash via oversized MAX_DATA encoding

Published Jun 26, 2025 · Updated Jun 26, 2025

Improper input validation in Mozilla Neqo 0.4.24 through 0.13.2 allows remote attackers to crash a QUIC process via crafted frames. ReceiverFlowControl::next_limit adds retired bytes to a default maximum of 2^62-1, then encodes the oversized result in a MAX_DATA frame without clamping it to the QUIC variable-integer limit. After a peer consumes at least one byte and sends DATA_BLOCKED, the encoding panic terminates the socket thread and main process; the path is reachable when Neqo acts as a client or server.

CVSS severity2.3
Low
EPSS probability0.23%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Improper input validation in Mozilla Neqo 0.4.24 through 0.13.2 allows remote attackers to crash a QUIC process via crafted frames. ReceiverFlowControl::next_limit adds retired bytes to a default maximum of 2^62-1, then encodes the oversized result in a MAX_DATA frame without clamping it to the QUIC variable-integer limit. After a peer consumes at least one byte and sends DATA_BLOCKED, the encoding panic terminates the socket thread and main process; the path is reachable when Neqo acts as a client or server.

The record

CVE
CVE-2025-6703
Published
Jun 26, 2025
Updated
Jun 26, 2025
Vendor
Mozilla
Product
Neqo
Classifications
CWE-20, CAPEC-227, T1499.004
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. Jun 26, 2025CVE publishedPublication date reported by the CVE source.
  2. Jun 26, 2025Record updatedLatest update available in the CVE record.

Exploitability

Present is not the same as exploitable

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Is a vulnerable build present?

Compare these published version ranges with your installed build and any vendor patches.

  1. Affected versionversion=0.4.24 <=0.13.2

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

Mozilla · Neqoversion=0.4.24 <=0.13.2

Published CVSS scores

2.3MozillaCVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:P/VC:N/VI:N/VA:L/SC:N/SI:N/SA:L/AU:Y/R:A/RE:L/U:Clear
3.5GitHubCVSS:3.1/AV:N/AC:L/PR:L/UI:R/S:U/C:N/I:N/A:L

CVSS describes severity. EPSS estimates exploitation probability.

Attacks

What attackers are doing with it

Daily unique IPs observed by Shadowserver honeypots for known exploited vulnerabilities (KEVs). Missing observations do not establish an absence of attacks.

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No observations available

Sep 9, 2026Sep 15, 2026
Latest reporting daySep 15, 2026
Latest daily unique IPsUnavailable
Prior 30-day averageUnavailable
SourceShadowserver honeypots (KEV)
Vectornetwork
Privilegesunauthenticated
Known exploitationUnconfirmed
Public exploitUnconfirmed

Weakness, pattern, technique

CWE-20Improper Input Validation
CAPEC-227Sustained Client Engagement
T1499.004Application or System Exploitation

Public exploit references

No public exploit references are available in this record.

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Technologies

Your stack

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Your stack

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