CVE-2018-1000120 — curl
CRITICALCVE-2018-1000120 is a critical-severity (CVSS 9.8) Out-of-bounds Write vulnerability in curl. EPSS puts its 30-day exploitation probability at 11.6% (96th percentile). No vendor fix is recorded yet; mitigation options are listed below.
curl FTP path confusion leads to NIL byte out of bounds write
EPSS Exploitation Probability
EPSS (Exploit Prediction Scoring System) is a daily probability model maintained by FIRST.org. It estimates the likelihood a CVE will be exploited in production environments within the next 30 days, derived from real-world threat intelligence signals.
How urgent is this, really
CVE-2018-1000120 plotted by exploitation likelihood (EPSS) against impact (CVSS). The shaded corner — EPSS 50%+ and CVSS 7.0+ — is where this CVE doesn't sit, though severity or exploitability alone can still warrant action.
Where this sits among everything scored
Of 377,333 CVEs with a current EPSS score, this one falls in the 10–50% band (highlighted). Real counts from FIRST.org, not a sample — log-scaled since the landscape is heavily right-skewed.
Real-World Exposure
curlReal-time download stats are indexed for npm and PyPI packages. This vulnerability affects NuGet packages — download data is not available via public APIs for these ecosystems.
Description
curl can be coerced into writing a zero byte out of bounds.
This bug can trigger when curl is told to work on an FTP URL, with the setting to only issue a single CWD command (--ftp-method singlecwd or the libcurl alternative CURLOPT_FTP_FILEMETHOD).
curl then URL-decodes the given path, calls strlen() on the result and deducts the length of the file name part to find the end of the directory within the buffer. It then writes a zero byte on that index, in a buffer allocated on the heap.
If the directory part of the URL contains a %00 sequence, the directory length might end up shorter than the file name path, making the calculation size_t index = directory_len - filepart_len end up with a huge index variable for where the zero byte gets stored: heap_buffer[index] = 0. On several architectures that huge index will wrap and work as a negative value, thus overwriting memory before the intended heap buffer.
By using different file part lengths and putting the string %00 in different places in the URL, an attacker that can control what paths a curl-using application uses can write that zero byte on different indexes.
Affected Packages
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| .NETNuGet | curl | ≥ 7.12.3 | No fix |
Detection & mitigation playbook
Open-source dependencyDetect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for curl, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
Remediation status
No patched version of curl has shipped for CVE-2018-1000120 yet. Where your build allows, override or pin the dependency away from the vulnerable range, and apply any maintainer-recommended mitigation.
Mitigate without a patch
If you can't upgrade right away: gate or disable the affected feature, validate untrusted input at the boundary, and avoid passing attacker-controlled data into the vulnerable path. O3's runtime protection blocks exploitation in production as an interim safeguard until the upgrade lands.
How O3 protects you
O3 Security's impact-aware SCA analyses which vulnerable code paths your application actually calls, so a match like CVE-2018-1000120 can be triaged on real exposure rather than presence alone.
Tailored to CVE-2018-1000120. Runtime protection reduces exposure until a permanent patch is applied and verified — it complements patching, it doesn't replace it.
Fixing This On Your OS
If you run this on a Linux distribution, patch through your package manager against the distro's own security advisory below — it tracks the exact backported fix for your release, which can ship on a different timeline (and sometimes a different severity) than the upstream project.
| Product | Fixed in | Advisory |
|---|---|---|
| JBoss Core Services Apache HTTP Server 2.4.29 SP2 | jbcs-httpd24-curl | RHSA-2019:1543 |
| Red Hat Enterprise Linux 7 | curl-0:7.29.0-51.el7 | RHSA-2018:3157 |
| Red Hat Enterprise Linux 7.4 Advanced Update Support | curl-0:7.29.0-42.el7_4.2 | RHSA-2020:0594 |
| Red Hat Enterprise Linux 7.5 Extended Update Support | curl-0:7.29.0-46.el7_5.1 | RHSA-2020:0544 |
| Red Hat Software Collections for Red Hat Enterprise Linux 6 | httpd24-curl-0:7.61.1-1.el6 | RHSA-2018:3558 |
Frequently Asked Questions
Is CVE-2018-1000120 in your dependencies?
O3 Security finds CVE-2018-1000120 across NuGet dependencies, including transitive ones, and its impact-aware SCA ranks findings by whether your code actually calls the vulnerable path.