# CVE-2026-55556: Rsyslog: Heap buffer overflow in imhttp plugin Basic Authentication handling

> An unauthenticated remote attacker can cause a denial of service in Rsyslog by sending oversized credentials to the optional imhttp module.

- **Severity:** High 8.2 (`CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:N/VI:N/VA:H/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:** Yes
- **Published:** 18 Sep 2026
- **Updated:** 22 Sep 2026
- **Also known as:** AZL-104994, DEBIAN-CVE-2026-55556, ECHO-295e-562f-90a0, GHSA-947w-69ph-mc2r

## What is CVE-2026-55556?

CVE-2026-55556 is a high-severity memory corruption vulnerability in rsyslog, published on 18 Sep 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.

### Official description

Rsyslog is a rocket-fast system for log processing. From 8.2110.0 until 8.2604.0, the optional imhttp module's parse_auth_header function in contrib/imhttp/imhttp.c allocates a zero-byte heap buffer with calloc(0, len) when an HTTP Basic Authorization value exceeds its fixed work buffer, then passes that pointer to apr_base64_decode. An unauthenticated remote attacker can send an oversized encoded credential to an imhttp endpoint configured for Basic Authentication, causing decoded data to overwrite adjacent heap memory before credential validation. Deployments that do not install, load, and use imhttp with Basic Authentication are not affected. The demonstrated impact is a process crash that interrupts log collection, and code execution has not been demonstrated. This issue is fixed in version 8.2604.0.

Weakness: CWE-122 (Heap-based Buffer Overflow)

## Which versions are affected by CVE-2026-55556?


## How do I fix CVE-2026-55556?

Upgrade to a fixed version.

## Is CVE-2026-55556 being exploited?

It is not confirmed as exploited in the wild, but public exploit code for CVE-2026-55556 exists.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-55556 and fix it if it is affected.

What it is: CVE-2026-55556 is a high-severity memory corruption vulnerability in rsyslog, published on 18 Sep 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
Exploitation: It is not confirmed as exploited in the wild, but public exploit code for CVE-2026-55556 exists.

Affected product: rsyslog. 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-2026-55556/ (Markdown: https://insights.sbomify.com/CVE-2026-55556/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-55556
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-55556
- OSV: https://osv.dev/vulnerability/CVE-2026-55556

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-55556/
Summaries are generated and may contain mistakes; the official description above is the record.