MediumCVE-2026-62949

    Ronf Asyncssh: Infinite Loop (CVE-2026-62949)

    Published Last modified Sources: OSV, Hersteller, CVE.org

    What is affected, what to do

    AsyncSSH is a Python package which provides an asynchronous client and server implementation of the SSHv2 protocol on top of the Python asyncio framework. Prior to 2.24.0, _process_channel_open and _process_channel_open_confirmation in asyncssh/connection.py accept a peer-supplied send_pktsize value of zero.

    Update Ronf Asyncssh to version 2.24.0.

    Affected
    • Ronf Asyncssh lt *..2.24.0
    Fixed in
    • Ronf Asyncssh 2.24.0

    Not sure whether and how you are affected? Ask a question about this vulnerability

    Metrics

    6.5

    Priority

    XIEM Effective CVSS: highest available CVSS rating from NVD and vendor data.

    Source: Vendor (CNA), CVSS v3

    6.5

    CVSS v3

    Severity Medium on the scale from 0 to 10.

    Source: OSV, Hersteller, CVE.org

    0.4 %

    EPSS score

    Low — model estimates < 1% exploitation likelihood.

    Source: FIRST EPSS, percentile 33.0

    Show all metrics
    Severity
    Medium
    Priority (0–325)

    The priority score is a consolidated metric that condenses several individual signals into a single, action-guiding ranking: technical severity (CVSS), exploitation likelihood (EPSS), known-exploited status (KEV) and the context of the affected asset. The aim is to answer the only truly important question in vulnerability management: what first?

    Read in glossary
    68.60 Methodology
    Exploit maturity

    An exploit is code or a technique that specifically uses a concrete vulnerability to force unintended behaviour — such as executing code or escalating privileges. It is the tool that turns a theoretical flaw into a practical attack.

    Read in glossary
    no public PoC known
    XIEM Effective CVSS

    XIEM® is the tiered managed-security operating model from NEOSEC and a registered trademark of NEOSEC GmbH. It extends classic SIEM — the central collection, normalisation and correlation of security-relevant events — with endpoint forensics, network visibility and deception technology, and consolidates them into three building tiers: Sentry, Orchestrate and Command.

    Read in glossary
    6.5 Methodology
    CVSS v3

    CVSS is the globally established, open standard for assessing the technical severity of a vulnerability on a scale from 0 to 10. It describes how easily a flaw can be exploited and what damage it can cause — but not how likely its actual exploitation is. CVSS and the complementary EPSS deliberately answer different questions.

    Read in glossary
    6.5
    EPSS score

    The EPSS score is the absolute model output for an individual vulnerability: the estimated probability of real-world exploitation within the next 30 days, expressed as a value between 0 and 1 (i.e. 0 % to 100 %). A score of 0.42 means „the model estimates a 42 % chance of real-world exploitation within 30 days“.

    Read in glossary
    0.4 %
    Low — model estimates < 1% exploitation likelihood.
    EPSS percentile

    The EPSS percentile is the relative position of a vulnerability within all EPSS-scored CVEs, expressed as a value between 0 and 1. A percentile of 0.90 means „this CVE has a higher EPSS score than 90 % of all other scored CVEs“. Unlike the EPSS score, it is not a probability — but a positioning within a ranking.

    Read in glossary
    33.0
    Elevated — this CVE ranks above at least 10% of all CVEs scored today.
    KEV status

    The KEV catalogue is a list maintained by the US authority CISA of vulnerabilities for which active exploitation in the wild has been confirmed. It answers not the question „how severe?“ but „is it already being attacked?“ — thereby sharply prioritising patching.

    Read in glossary
    —
    Published
    CISA assessment (SSVC)
    Exploitation: none known · not automatable · Technical impact: partial
    Source: CISA Vulnrichment, as of 17 September 2026
    CWE

    CWE is a catalogue of typical weakness types in software and hardware — such as flawed input validation, unsafe memory handling or poor authentication. Where a CVE names a concrete vulnerability in a concrete product, a CWE describes the underlying class of defect.

    Read in glossary
    CWE-835

    Detection rules

    No NEOSEC detection rule is available for this vulnerability yet.

    Classification under NIS2 and CRA

    Rule-based classification from exploitation status and severity. It does not replace a case-by-case assessment.

    NIS2

    NIS2 is EU Directive (EU) 2022/2555 to raise the level of cybersecurity across the Union. It substantially broadens obligations compared with the first NIS directive: more sectors, mandatory risk management, staged reporting duties and personal accountability of senior management.

    Read in glossary
    Handle the vulnerability in regular vulnerability handling under Art. 21(2)(e) NIS2.
    CRA

    The CRA is EU Regulation (EU) 2024/2847 setting horizontal cybersecurity requirements for „products with digital elements“ — from industrial controllers and software to connected consumer devices. For the first time manufacturers must ensure security across the entire product lifecycle, handle vulnerabilities and provide security updates.

    Read in glossary
    From 11.12.2027, Annex I Part II CRA requires manufacturers of affected products with digital elements to handle vulnerabilities without delay and provide security updates.
    ISO/IEC 27001

    ISO/IEC 27001 is the internationally leading standard for an information security management system (ISMS). It describes a risk-based, process-oriented framework to systematically plan, implement, review and improve information security.

    Read in glossary
    Control A.8.8, management of technical vulnerabilities: assess relevance to your systems, plan remediation and keep evidence.

    Description

    AsyncSSH is a Python package which provides an asynchronous client and server implementation of the SSHv2 protocol on top of the Python asyncio framework. Prior to 2.24.0, _process_channel_open and _process_channel_open_confirmation in asyncssh/connection.py accept a peer-supplied send_pktsize value of zero. When channel data reaches SSHChannel._flush_send_buf in asyncssh/channel.py, the zero value causes each loop iteration to slice and remove zero bytes without reducing the send window, leaving the synchronous loop permanently true with no await point. A malicious SSH server can trigger the client path through SSH_MSG_CHANNEL_OPEN_CONFIRMATION before the first channel write, while an authenticated client can trigger the server path through SSH_MSG_CHANNEL_OPEN and freeze every current and future connection handled by the process. This vulnerability is fixed in 2.24.0.

    Source: OSV (Open Source Vulnerabilities), GitHub Security Advisories, cvelistv5, NVD (NIST)

    Affected products

    The following versions are affected. Older releases of the same product line are also vulnerable unless stated otherwise.

    Affected products
    Affected versionsFixed in
    ronfasyncssh
    • < 2.24.0
    • 2.24.0
    pypiasyncssh
    • 1.6.2
    • 0.9.2
    • 2.10.0
    • 2.2.1
    • 2.13.0
    • 2.1.0
    • 1.15.1
    • 2.2.0
    • 2.5.0
    • 1.5.2
    • 1.5.5
    • 1.2.1
    • 1.17.0
    • 1.16.1
    • 1.0.1
    • 1.11.1
    • 0.8.2
    • 1.3.1
    • 2.10.1
    • 1.10.0
    • 0.8.4
    • 1.10.1
    • 1.6.1
    • 1.12.2
    • 2.4.2
    • 0.9.1
    • 0.8.3
    • 1.5.0
    • 1.11.0
    • 2.23.1
    • 2.3.0
    • 1.13.0
    • 1.13.2
    • 1.7.1
    • 1.16.0
    • 1.12.1
    • 2.21.0
    • 2.0.1
    • 1.0.0
    • 2.7.2
    • 1.3.2
    • 2.18.0
    • 2.0.0
    • 1.8.0
    • 2.20.0
    • 1.13.1
    • 1.9.0
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    Vendors and products

    Weakness classes (CWE)

    • CWE-835Loop with Unreachable Exit Condition ('Infinite Loop')

      Base

      The product contains an iteration or loop with an exit condition that cannot be reached, i.e., an infinite loop.

      Entry at MITRE CWE-835

    References & sources

    • github.comronf/asyncssh/security/advisories/GHSA-rw4j-r22c-9gc3x_refsource_CONFIRM
    • github.comronf/asyncssh/commit/756cbae5350789ce9735f15f704bae9b5a3608b8x_refsource_MISC
    • github.comronf/asyncssh/commit/9c354270c009285525e126721e8ed5fbed1f8a67x_refsource_MISC
    • github.comronf/asyncssh/releases/tag/v2.24.0x_refsource_MISC
    • NVD (NIST)vuln/detail/CVE-2026-62949advisory
    • github.comronf/asyncsshpackage
    • pypi.orgproject/asyncsshpackage
    • GitHub Security Advisoriesadvisories/GHSA-rw4j-r22c-9gc3advisory

    Changes to this CVE

    Who changed this CVE and when: NVD (analysis, affected products, CVSS), the CVE publisher (CNA) and additional providers such as CISA (ADP).

    1. Record modifiedCISA ADP

      CISA assessment (SSVC): exploitation: none known, automatable: no

      Show the original change
      Individual changes of this event as published by NVD
      TypeActionBeforeAfter
      SSVC assessmentadded
      —
      {"id":"CVE-2026-62949","role":"CISA Coordinator","options":[{"exploitation":"none"},{"automatable":"no"},{"technicalImpact":"partial"}],"version":"2.0.3","timestamp":"2026-09-17T15:01:41.382412Z"}

      Source identifier: 134c704f-9b21-4f2e-91b3-4a467353bcc0

    2. ReceivedGitHub

      CVSS V3.1: AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H

      Weakness type: CWE-835

      Description added or changed

      Show 8 original changes
      Individual changes of this event as published by NVD
      TypeActionBeforeAfter
      Descriptionadded
      —
      AsyncSSH is a Python package which provides an asynchronous client and server implementation of the SSHv2 protocol on top of the Python asyncio framework. Prior to 2.24.0, _process_channel_open and _process_channel_open_confirmation in asyncssh/connection.py accept a peer-supplied send_pktsize value of zero. When channel data reaches SSHChannel._flush_send_buf in asyncssh/channel.py, the zero value causes each loop iteration to slice and remove zero bytes without reducing the send window, leaving the synchronous loop permanently true with no await point. A malicious SSH server can trigger the client path through SSH_MSG_CHANNEL_OPEN_CONFIRMATION before the first channel write, while an authenticated client can trigger the server path through SSH_MSG_CHANNEL_OPEN and freeze every current and future connection handled by the process. This vulnerability is fixed in 2.24.0.
      CVSSadded
      —
      AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H
      Weakness type (CWE)added
      —
      CWE-835
      Affected versions (publisher)added
      —
      New affected value received. <a href="https://github.com/CVEProject/cvelistV5/blob/main/cves/2026/62xxx/CVE-2026-62949.json">CVE-2026-62949</a>
      Referenceadded
      —
      https://github.com/ronf/asyncssh/commit/756cbae5350789ce9735f15f704bae9b5a3608b8
      Referenceadded
      —
      https://github.com/ronf/asyncssh/commit/9c354270c009285525e126721e8ed5fbed1f8a67
      Referenceadded
      —
      https://github.com/ronf/asyncssh/releases/tag/v2.24.0
      Referenceadded
      —
      https://github.com/ronf/asyncssh/security/advisories/GHSA-rw4j-r22c-9gc3

      Source identifier: security-advisories@github.com

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