# CVE-2026-74755: Apache bRPC: Integer Underflow in mcpack2pb Parser Leading to Denial of Service


- **Severity:** Not scored
- **Exploited in the wild:** Not known
- **Fix available:** No
- **Published:** 21 Oct 2026
- **Updated:** 21 Oct 2026
- **Also known as:** GHSA-9644-97m2-4fc3

## What is CVE-2026-74755?

CVE-2026-74755 is a vulnerability in Apache bRPC, published on 21 Oct 2026. 

### Official description

An integer underflow in the mcpack2pb component of Apache bRPC (version < 1.18.0) on all platforms allows remote attackers to crash a bRPC server by sending a request containing a string field with a zero value_size. The UnparsedValue::as_string() function computes `_size - 1` without validating `_size`; when value_size is 0 the expression underflows to SIZE_MAX, resize() throws std::length_error, and because the exception is not caught on the request parsing path the process aborts (SIGABRT).

Root Cause:
In src/mcpack2pb/parser.cpp, UnparsedValue::as_string() resizes the output string to `_size - 1`, where `_size` is the value_size of a string field taken directly from the network data. A value_size of 0 underflows the unsigned size_t, and the subsequent resize() throws an exception that is not caught.

Affected Scenarios:
bRPC servers that accept nshead frames wired to the ubrpc mcpack2 adaptor (for example the NsheadService + UbrpcMcpack2Adaptor deployment pattern) can be crashed by a single unauthenticated request. Stock open-source bRPC registers the ubrpc/nshead_mcpack protocols as client-only, so default server configurations are not affected.

How to Fix:
1. Upgrade bRPC to a version that includes this fix.
2. Apply the patch in PR  https://github.com/apache/brpc/pull/3450

Weakness: CWE-191 (Integer Underflow (Wrap or Wraparound))

## Which versions are affected by CVE-2026-74755?


## How do I fix CVE-2026-74755?

No fix has been published for CVE-2026-74755 yet. Limit exposure of the affected software until one is available.

## Is CVE-2026-74755 being exploited?

There is no known exploitation of CVE-2026-74755 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-74755 and fix it if it is affected.

What it is: CVE-2026-74755 is a vulnerability in Apache bRPC, published on 21 Oct 2026. 
Exploitation: There is no known exploitation of CVE-2026-74755 in the wild and no public exploit code.

Affected product: Apache bRPC. 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-2026-74755/ (Markdown: https://insights.sbomify.com/CVE-2026-74755/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-74755
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-74755
- OSV: https://osv.dev/vulnerability/CVE-2026-74755

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-74755/
Summaries are generated and may contain mistakes; the official description above is the record.