Rack: Multipart parsing allows DoS via unbounded scan buffer growth
Any application that parses multipart requests with Rack is affected in its default configuration. Fixed in Rack 2.2.25, 3.1.23, and 3.2.8.
Written by anthropic/claude-haiku-5.5 from the sources below. Verify against them.
- Exploited
- No known exploitation
- Fix
- Fixed in 2.2.25, 3.1.23, 3.2.8
- Who is exposed
- Default setups
Do first: Check every install: default setups are affected.
- Last modified
- A 0-100 score for how much a CVE matters across the industry: exploitation, exposure, how widely the software is used, severity and whether a fix exists.What do these mean? →
- 30/100 · provisional
- How many real-world setups can hit the bug, from the vendor's advisory and CISA data. It does not know your setup.What do these mean? →
- Default setups
- Whether working attack code is public (Exploit-DB, Metasploit, Nuclei or GitHub). Public code makes attacks easier.What do these mean? →
- None on Exploit-DB, Metasploit or GitHub
Summary
Rack, a Ruby web server interface, has a flaw in how its multipart parser handles buffered data, which can let a remote, unauthenticated attacker send a crafted multipart request that makes a worker run out of memory and stop responding. The attack needs only a well-formed multipart body and affects applications that parse multipart requests.
Read the original description
Rack is a modular Ruby web server interface. Prior to 2.2.25, 3.1.23, and 3.2.8, Rack::Multipart::Parser can retain parsed data in its internal StringScanner buffer when reads end inside or immediately after multipart headers. An unauthenticated attacker can align many small parts with rack.multipart.buffer_size boundaries so the buffer grows with the request despite per-field, header, and preamble limits, causing a worker to exhaust memory. The attack requires only a well-formed multipart body and affects applications that parse multipart requests. This issue is fixed in versions 2.2.25, 3.1.23, and 3.2.8.
cve.now Index
- Exploitation?/40Unknown. No CISA SSVC exploitation rating yet
- Exposure22/25Default setups, reachable over the network without a login
- Reach?/20Unknown. No tracked technology or package matched
- Severity7.5/10CVSS 7.5
- No patch0/5No signal
What would change the score
- Would score about 70 if added to CISA KEV
- Would score about 65 if CISA's SSVC rates exploitation active
What to do
Check every install: default setups are affected.
Upgrade to a fixed version: 2.2.25, 3.1.23, 3.2.8.
Workaround: Limit the request body size at a reverse proxy or application boundary.
Timeline
- CVE published
- First seen by cve.now
Affected versions
- rack rack
fixed in 2.2.25; From 3.0 · fixed in 3.1.23; From 3.2 · fixed in 3.2.8
Evidence 4
- Advisory
- Repository
- Repository
- Repository
Corrections
None so far.
Something wrong on this page? Report an error. Every fix is listed here.
Questions about CVE-2026-125111
Is CVE-2026-125111 being exploited?
It is not in CISA's Known Exploited Vulnerabilities catalog, and CISA's SSVC assessment does not mark exploitation as active. That does not mean it has not been attacked: vendors and researchers can report attacks before CISA lists them. No public exploit code is tracked.Source: cve.now feed data
What does this mean?
A single crafted multipart request can make Rack's parser keep growing its scan buffer because it does not discard parsed bytes before reading more data. This can exhaust worker memory and cause a denial of service, and the request needs no authentication.Source: github.com
Which versions are affected by CVE-2026-125111?
rack: before 2.2.25; from 3.0 before 3.1.23; from 3.2 before 3.2.8.Source: cve.now feed data
Am I affected, and how do I check?
Rack versions before 2.2.25, versions from 3.0 up to before 3.1.23, and versions from 3.2 up to before 3.2.8 are affected. Applications that parse multipart requests with Rack are affected in the default configuration, so check the installed Rack version in the Gemfile.lock or with `bundle exec gem list rack`.Source: github.com
How do I fix CVE-2026-125111?
Update Rack to 2.2.25 on the 2.2 line, 3.1.23 on the 3.1 line, or 3.2.8 on the 3.2 line. Until the update is applied, limit the request body size at a reverse proxy or application boundary.Source: github.com