GHSA-p543-xpfm-54cp CVE-2025-61770

Rack's unbounded multipart preamble buffering enables DoS (memory exhaustion)

High RubyGems

Severity: 7.5 / 10; High · CVSS 3.1
Exploit chance (EPSS): 0.93%; Chance of exploitation in the next 30 days. Higher than 60.61% of scored vulnerabilities.
Known exploited: No; Not in CISA's catalog
Weakness: CWE-400 Uncontrolled Resource Consumption
Fix: 2.2.19; Upgrade rack to this version or later

Description

Summary

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.

Details

While searching for the first boundary, the parser appends incoming data into a shared buffer (@sbuf.concat(content)) and scans for the boundary pattern:

@sbuf.scan_until(@body_regex)

If the boundary is not yet found, the parser continues buffering data indefinitely. There is no trimming or size cap on the preamble, allowing attackers to send arbitrary amounts of data before the first boundary.

Impact

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.

Mitigation

Upgrade: Use a patched version of Rack that enforces a preamble size limit (e.g., 16 KiB) or discards preamble data entirely per RFC 2046 § 5.1.1.
Workarounds:
- Limit total request body size at the proxy or web server level.
- Monitor memory and set per-process limits to prevent OOM conditions.

Affected versions

RubyGems

Package	Vulnerable	Fixed in
rack	`< 2.2.19`	`2.2.19`
rack	`>= 3.1, < 3.1.17`	`3.1.17`
rack	`>= 3.2, < 3.2.2`	`3.2.2`

How to fix

Move to the fixed version or later with your package manager, update the lockfile, and run your tests. If the package only arrives through another dependency, upgrade that dependency first. If none of its releases pulls a fixed version yet, force the version with your package manager's override.

rack RubyGems → `2.2.19`

bundler

bundle update rack # Gemfile: gem "rack", ">= 2.2.19"

Severity breakdown

CVSS 3.1 7.5

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H

Attack vector: Network
Attack complexity: Low
Privileges required: None
User interaction: None
Scope: Unchanged
Confidentiality: None
Integrity: None
Availability: High

Weakness

CWE-400: Uncontrolled Resource Consumption

The product does not properly control the allocation and maintenance of a limited resource.

How to prevent it

Architecture and Design: Design throttling mechanisms into the system architecture. The best protection is to limit the amount of resources that an unauthorized user can cause to be expended. A strong authentication and access control model will help prevent such attacks from occurring in the first place. The login application should be protected against DoS attacks as much as possible. Limiting the database access, perhaps by caching result sets, can help minimize the resources expended. To further limit the potential for a DoS attack, consider tracking the rate of requests received from users and blocking requests that exceed a defined rate threshold.
Architecture and Design: Mitigation of resource exhaustion attacks requires that the target system either: recognizes the attack and denies that user further access for a given amount of time, or uniformly throttles all requests in order to make it more difficult to consume resources more quickly than they can again be freed. The first of these solutions is an issue in itself though, since it may allow attackers to prevent the use of the system by a particular valid user. If the attacker impersonates the valid user, they may be able to prevent the user from accessing the server in question. The second solution is simply difficult to effectively institute -- and even when properly done, it does not provide a full solution. It simply makes the attack require more resources on the part of the attacker.
Architecture and Design: Ensure that protocols have specific limits of scale placed on them.
Implementation: Ensure that all failures in resource allocation place the system into a safe posture.

References

Advisories

nvd.nist.gov/vuln/detail/CVE-2025-61770

Packages

github.com/rack/rack

Timeline

Oct 7, 2025 Published
Oct 19, 2026 Last updated by the source
Nov 1, 2026 Latest EPSS score

Advisory	Severity	Why it's related
GHSA-g2pf-xv49-m2h5 Rack::Request accepts invalid Host characters, enabling host allowlist bypass	Medium	Also affects rack
GHSA-v6x5-cg8r-vv6x Rack's multipart header parsing allows Denial of Service via escape-heavy quoted parameters	High	Also affects rack
GHSA-qfgr-crr9-7r49 Rack: Forwarded Header semicolon injection enables Host and Scheme spoofing	Medium	Also affects rack
GHSA-rx22-g9mx-qrhv Rack's improper unfolding of folded multipart headers preserves CRLF in parsed parameter values	Medium	Also affects rack
GHSA-q2ww-5357-x388 Rack has Content-Length mismatch in Rack::Files error responses	Medium	Also affects rack
GHSA-qv7j-4883-hwh7 Rack::Sendfile header-based X-Accel-Mapping regex injection enables unauthorized X-Accel-Redirect	High	Also affects rack
GHSA-8vqr-qjwx-82mw Rack's multipart parsing without Content-Length header allows unbounded chunked file uploads	High	Also affects rack
GHSA-x8cg-fq8g-mxfx Rack's multipart byte range processing allows denial of service via excessive overlapping ranges	High	Also affects rack

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