# CVE-2026-15999: AES-CCM decryption accepts zero or out-of-range tag length, bypassing authentication

> An on-path attacker can modify AES-CCM encrypted content without detection due to improper tag length validation in the Bouncy Castle C# library.

- **Severity:** High 8.2 (`CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:N/VI:H/VA:N/SC:N/SI:N/SA:N`)
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** No
- **Published:** 2 Oct 2026
- **Updated:** 2 Oct 2026
- **Also known as:** GHSA-499v-wjw6-fvpx

## What is CVE-2026-15999?

CVE-2026-15999 is a high-severity vulnerability in Legion of the Bouncy Castle Inc. bc-csharp, published on 2 Oct 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.

### Official description

Improper validation of integrity check value in the AES-CCM implementation (CcmParameters and CcmBlockCipher) in Legion of the Bouncy Castle Inc. bc-csharp before 2.7.0 allows an on-path attacker to modify CCM-encrypted content without detection via an AlgorithmIdentifier whose CCMParameters declare an authentication tag (aes-ICVlen) of zero or another length outside the RFC 5084 set, because CcmParameters accepted any value and CcmBlockCipher validated the tag length only when encrypting, so decryption compared a zero-length or very short tag. Affected paths include ParameterUtilities.GetCipherParameters, used by CmsEnvelopedData and CmsEnvelopedDataParser for EnvelopedData encrypted with AES-CCM, and any caller passing an unchecked tag length to CcmBlockCipher for decryption.

Weakness: CWE-354 (Improper Validation of Integrity Check Value)

## Which versions are affected by CVE-2026-15999?

It affects 1 product.


Products: Bouncycastle Bc-Csharp.

## How do I fix CVE-2026-15999?

No fix has been published for CVE-2026-15999 yet. Limit exposure of the affected software until one is available.

## Is CVE-2026-15999 being exploited?

There is no known exploitation of CVE-2026-15999 in the wild and no public exploit code.

## Other vulnerabilities in Legion of the Bouncy Castle Inc. bc-csharp

- [CVE-2026-63569](https://insights.sbomify.com/CVE-2026-63569/): MTI/A0 DHAgreement does not validate the peer's ephemeral value (critical 9.1)
- [CVE-2026-103600](https://insights.sbomify.com/CVE-2026-103600/): Unbounded ASN.1 nesting depth causes process-terminating stack overflow (high 8.7)
- [CVE-2026-103603](https://insights.sbomify.com/CVE-2026-103603/): Unbounded HSS public key level count allows huge array allocation during signature verification (high 8.7)
- [CVE-2026-103604](https://insights.sbomify.com/CVE-2026-103604/): Quadratic-time escaping when converting X.509 distinguished names to strings (high 8.7)
- [CVE-2026-16000](https://insights.sbomify.com/CVE-2026-16000/): KCcmBlockCipher (DSTU 7624 CCM) tag not bound to nonce when no associated data is used (high 8.7)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-15999 and fix it if it is affected.

What it is: CVE-2026-15999 is a high-severity vulnerability in Legion of the Bouncy Castle Inc. bc-csharp, published on 2 Oct 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
Exploitation: There is no known exploitation of CVE-2026-15999 in the wild and no public exploit code.

Affected product: Legion of the Bouncy Castle Inc. bc-csharp. No package-level version data is available, so check the vendor advisory linked from the reference page.

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/CVE-2026-15999/ (Markdown: https://insights.sbomify.com/CVE-2026-15999/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-15999
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-15999
- OSV: https://osv.dev/vulnerability/CVE-2026-15999

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-15999/
Summaries are generated and may contain mistakes; the official description above is the record.