Microsoft System.Formats.Cbor (.NET): Denial of service via hash flooding
Summary
Microsoft's System.Formats.Cbor NuGet package, used by .NET, has an inefficient algorithm when processing CBOR map keys. Colliding keys can cause hash flooding during uniqueness checks. A remote, unauthenticated attacker can send crafted CBOR data to cause a denial of service. Only availability is affected, and exploitation requires that an application parses untrusted CBOR input.
Affected
- Microsoft System.Formats.Cbor NuGet package for .NET 8, 9, 10 and 11 (the advisory table lists 8.0.0-0 before 8.0.2 and the 9.0 line before 9.0.24)
- Exact affected ranges for the 10.x and 11.x lines: Unknown.
Fixed in
- 8.0 line: System.Formats.Cbor 8.0.2
- 9.0 line: System.Formats.Cbor 9.0.24 (runtime 9.0.23 alone does not fix it)
- 10.x and 11.x lines: Unknown. Check the vendor advisory before rollout.
Workaround
- No vendor workaround. Update the package.
- Until patched, limit exposure of services that parse untrusted CBOR input (own assessment, not a vendor recommendation).
Exploitation
- Microsoft rates exploitation as Less Likely and lists no public disclosure.
- No public proof of concept or CISA KEV entry found in the sources checked.
- The reporter is credited as Anonymous.
Our recommendation
- Check whether System.Formats.Cbor is referenced directly or transitively (for example with dotnet list package --include-transitive).
- Update the package to 8.0.2 or 9.0.24, and to the confirmed fix for 10.x and 11.x, then rebuild and redeploy affected applications.
- A runtime update alone is not sufficient; self-contained apps may ship their own copy.
- Prioritise internet-facing APIs and device gateways. Patch in the normal cycle, and sooner for public-facing services.
Information from Microsoft (Patch Tuesday November 2026)
Public exploits
PoC-in-GitHub, Metasploit and Exploit-DB list nothing for this CVE.
Research sources
- Microsoft Security Response Center: CVE-2026-118747 msrc.microsoft.com
- CVE Record: CVE-2026-118747 www.cve.org
- .NET announcement issue 501 (advisory table and root cause) github.com
- .NET and .NET Framework November 2026 servicing updates devblogs.microsoft.com
- Rapid7 Patch Tuesday November 2026 www.rapid7.com
- OffSeq threat radar: CVE-2026-118747 radar.offseq.com
Researched and written up in English and German by an AI (Claude by Anthropic) based on NVD, CISA, EPSS and the sources listed. The vendor information is authoritative. Researched on 11 Nov 2026, 05:32.
Original description (NVD)
Inefficient algorithmic complexity in .NET allows an unauthorized attacker to deny service over a network.
NVD references (1)
Sources
The tracker combines official, freely usable data sources – updated continuously:
- NVD (NIST) – CVEs, rating (CVSS), affected products
- CISA KEV – vulnerabilities confirmed by CISA as actively exploited
- CISA Vulnrichment – CISA assessment (SSVC) in the official CVE record
- FIRST EPSS – probability of exploitation
- Microsoft MSRC – Patch Tuesday: vulnerabilities, severity, updates
- PoC-in-GitHub – public proof-of-concept repositories
- Metasploit (Rapid7) – modules of the Metasploit Framework
- Exploit-DB (OffSec) – archive of public exploits (links only)
- Patch days: official vendor pages (linked per vendor) (Vendor patch days)
This product uses data from the NVD API but is not endorsed or certified by the NVD. EPSS data © FIRST.org.
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