CVE
Busybox: heap buffer overflow in the tftp client on fallback to the default block size
- Exploited
- No known exploitation
- Fix
- See vendor advisory
- Who is exposed
- Specific setups only
Do first: Check whether your setup matches what the bug needs. The TFTP client requests a custom small block size and the server replies with a standard-sized response The client has an active connection to an untrusted TFTP server using a non-default, reduced block size
- Last modified
- Whether it needs action now or is a reference record, by fixed rules, with why. Last checked is when cve.now last re-read it.What do these mean? →
- Reference
- Last checked
- 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? →
- 35/100 · 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? →
- Specific setups only
- 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
A heap buffer overflow in the TFTP client of BusyBox, a package shipped in Red Hat Enterprise Linux 6 and Red Hat Hardened Images, can be triggered remotely, without logging in, when a user is tricked into a transfer with a malicious TFTP server. An attacker who exploits it can run arbitrary code or crash the application, causing a denial of service.
Written by anthropic/claude-haiku-5.5 from the sources below. Verify against them.
Read the original description
A flaw was found in BusyBox. When the TFTP (Trivial File Transfer Protocol) client requests a custom small block size and receives a standard response, it falls back to the default block size without resizing the allocated memory buffer. A malicious TFTP server can exploit this issue by sending standard-sized network packets during a transfer, causing a heap buffer overflow. This flaw allows an attacker to achieve arbitrary code execution or cause a Denial of Service (DoS) by crashing the application.
Who should check
Teams running the BusyBox TFTP client with a reduced block size against untrusted servers, including Red Hat Enterprise Linux 6 and Red Hat Hardened Images users.
Read the source excerpt
“BusyBox is predominantly deployed within lightweight container base images and recovery environments”
cve.now Index
- Exploitation?/40Unknown. No CISA SSVC exploitation rating yet
- Exposure12/25Specific setups only, reachable over the network without a login
- Reach10/20Shipped in major Linux distributions
- Severity7.5/10CVSS 7.5
- No patch5/5No fixed version listed; see vendor advisory
What would change the score
- Would score about 85 if added to CISA KEV
- Would score about 80 if CISA's SSVC rates exploitation active
What to do
Check whether your setup matches what the bug needs.
The TFTP client requests a custom small block size and the server replies with a standard-sized response The client has an active connection to an untrusted TFTP server using a non-default, reduced block sizeWorkaround: The checked facts do not describe a workaround. Until a fix is available, avoid using the TFTP client against untrusted servers, since the issue requires a connection to one using a non-default, reduced block size.
Check your systems with an agent
Paste this into a coding or ops agent. It checks your systems read-only and reports a table per asset.
Check whether my systems are affected by CVE-2026-124737 (Busybox: heap buffer overflow in the tftp client on fallback to the default block size), using the cve.now data below.
Rules: work read-only. Do not run exploit code, proof-of-concept scripts or scanners that send attack payloads, and do not change any configuration, package, image or deployment. Report findings only.
What cve.now has for CVE-2026-124737 (entry last updated 2026-11-11):
Affected:
- Red Hat Red Hat Enterprise Linux 6: version range not reported
- Red Hat Red Hat Hardened Images: version range not reported
Fixed versions: none listed yet.
Conditions (all must be true for the bug to be hit):
- The TFTP client requests a custom small block size and the server replies with a standard-sized response (source: https://access.redhat.com/security/cve/cve-2026-124737)
- The client has an active connection to an untrusted TFTP server using a non-default, reduced block size (source: https://access.redhat.com/security/cve/cve-2026-124737)
Setup: the bug needs a feature or setting that is off by default or uncommon. Check whether each asset has it on.
Attacker needs: network access, no login, a user action.
CISA KEV: CVE-2026-124737 is not listed as of cve.now's last read.
This KEV status is for CVE-2026-124737 only. If you also check another CVE, look up its own KEV status; never assume it from this one.
How to check:
- Installed software: check what is actually running, not what a repo declares. Read the version from each host, container image and Kubernetes node (the binary's version output, the OS package database, image layers), and check whether the conditions above hold there. Distribution packages may carry a backported fix under an older version number; confirm against the distribution's own advisory.
Verify against the linked advisories before concluding, and cite the date you read each one:
- https://bugzilla.redhat.com/show_bug.cgi?id=2571607
- https://access.redhat.com/security/cve/CVE-2026-124737
- https://access.redhat.com/security/cve/cve-2026-124737
Do not conclude an asset is unaffected unless you checked it and the evidence covers both its version and the conditions above. If you could not check something, mark it unknown and say what is missing.
Report a table with one row per asset and these columns: Asset | Version or build | Status (affected, fixed, not applicable, unknown) | Compensating controls | Exposure (internet-facing, internal, none) | Evidence | Next action.
Entry: https://cve.now/CVE-2026-124737
For follow-up questions, use the cve.now MCP server at https://mcp.cve.now/mcp (no authentication), for example execute with name cve.get and arguments {"id": "CVE-2026-124737"}.Timeline
- CVE published
- First seen by cve.now
- Record last updated
Affected versions
- busybox
Version range not reported
- busybox
Version range not reported
Evidence 4
CVE record · 2
Establishes: The identifier and description.
- Record
- Record
Write-ups · 1
Establishes: Technical detail and context.
- Report
Other links · 1
- Link
Corrections
None so far.
Something wrong on this page? Report an error. Every fix is listed here.
Questions about CVE-2026-124737
Is CVE-2026-124737 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?
When the BusyBox TFTP client asks a server for a small custom block size and the server replies with a standard-sized response, the client can write past the end of a heap buffer. The flaw can crash the client process or, according to the advisory, allow arbitrary code execution.Source: access.redhat.com
Am I affected, and how do I check?
The facts list Red Hat Enterprise Linux 6 and Red Hat Hardened Images as affected products, with no version ranges given. The issue needs the TFTP client to be connected to an untrusted server using a non-default, reduced block size. To check, confirm whether the BusyBox tftp client is installed and whether it is used with such a server.Source: access.redhat.com
How do I fix CVE-2026-124737?
No fixed version is listed in the checked facts, so no upgrade target can be named here. Check the Red Hat advisory for the current fix status for these products.Source: access.redhat.com
What if there's no patch?
The checked facts do not describe a workaround. Until a fix is available, avoid using the TFTP client against untrusted servers, since the issue requires a connection to one using a non-default, reduced block size.Source: access.redhat.com