CVE-2025-61770 Rack's unbounded multipart preamble buffering enables DoS (memory exhaustion)
Remote attackers can cause memory exhaustion and service crashes by sending large multipart preamble data to Rack applications.
- Exploited in the wild
- Not known
- Login needed
- Not needed
- Attackable
- Over the internet
- Victim action needed
- Not needed
- Fix
- Available
What is CVE-2025-61770?
CVE-2025-61770 is a high-severity denial of service vulnerability in rack, published on 7 Oct 2025. An attacker can exploit it over the internet without logging in, with no action from the victim.
Official description
Rack is a modular Ruby web server interface. In versions prior to 2.2.19, 3.1.17, and 3.2.2, `Rack::Multipart::Parser` buffers the entire multipart preamble (bytes before the first boundary) in memory without any size limit. A client can send a large preamble followed by a valid boundary, causing significant memory use and potential process termination due to out-of-memory (OOM) conditions. Remote attackers can trigger large transient memory spikes by including a long preamble in multipart/form-data requests. The impact scales with allowed request sizes and concurrency, potentially causing worker crashes or severe slowdown due to garbage collection. Versions 2.2.19, 3.1.17, and 3.2.2 enforce a preamble size limit (e.g., 16 KiB) or discard preamble data entirely. Workarounds include limiting total request body size at the proxy or web server level and monitoring memory and set per-process limits to prevent OOM conditions.
Weakness: CWE-400
Which versions are affected by CVE-2025-61770?
6 open source packages ship an affected version, including rack. It affects 1 product.
| Package | Affected | Fixed in |
|---|---|---|
rack RubyGems | All versions | 2.2.19 |
rack RubyGems | All versions | 2.2.19 |
rack RubyGems | From 3.1 | 3.1.17 |
rack RubyGems | From 3.1 | 3.1.17 |
rack RubyGems | From 3.2 | 3.2.2 |
rack RubyGems | From 3.2 | 3.2.2 |
Products include Rack Rack.
How do I fix CVE-2025-61770?
Upgrade rack to a fixed version for your release line: 2.2.19, 3.1.17 or 3.2.2.
Upgrade, then put limits on request size, rate and nesting depth so one client cannot exhaust the service. More about Denial of service.
Is CVE-2025-61770 being exploited?
There is no known exploitation of CVE-2025-61770 in the wild and no public exploit code.
Other vulnerabilities in rack
- CVE-2026-34785 Rack: Local file inclusion in `Rack::Static` via URL Prefix Matching
- CVE-2026-34827 Rack: Algorithmic-Complexity DoS in Rack::Multipart::Parser
- CVE-2026-34829 Rack: Denial of Service via Unbounded Multipart File Upload Without Content-Length
- CVE-2026-22860 Rack has a Directory Traversal via Rack:Directory
- CVE-2025-61919 Rack is vulnerable to a memory-exhaustion DoS through unbounded URL-encoded body parsing
Sources
Also known as CLEANSTART-2026-NL32966, CLEANSTART-2026-SK17501, DEBIAN-CVE-2025-61770, GHSA-p543-xpfm-54cp, MINI-3jpg-53cm-3hrg, MINI-4r99-jpr7-vfxr and 13 more.