Summary
Tesla has CRLF injection in request Content-Type header via addcontenttype_param
Tesla.Multipart.add_content_type_param/2 appends caller-supplied strings to the multipart Content-Type header with no validation. A param value containing \r\n splits the header line, allowing an attacker who controls any content-type parameter (charset, boundary parameter, etc.) to inject arbitrary headers into the outbound HTTP request.
Details
add_content_type_param/2 in lib/tesla/multipart.ex stores the supplied string directly in multipart.content_type_params without any CR/LF check. headers/1 then joins all params with "; " and appends the result verbatim to the Content-Type header value. Because HTTP headers are delimited by \r\n, a param containing that sequence breaks out of the header field and introduces new header lines before the adapter writes the request to the socket.
The precondition is that untrusted input reaches add_content_type_param/2, which is the normal pattern for applications that accept user-supplied charset values, file type parameters, or any other content-type extension fields.
PoC
- Call
Tesla.Multipart.add_content_type_param/2with a value containing\r\nX-Injected: pwned. - Pass the resulting
Multipartstruct as the request body via any Tesla adapter. - The raw request on the wire contains
X-Injected: pwnedas a standalone header line.
Workarounds
Validate content-type parameter strings before passing them to Tesla.Multipart.add_content_type_param/2, rejecting any value that contains \r or \n.
Reesources
Impact
Low severity (CVSS v4.0: 2.1). Any application using tesla 0.8.0 through 1.18.2 that passes untrusted input into Tesla.Multipart.add_content_type_param/2 is affected. Consequences range from forging arbitrary outbound request headers to potential request smuggling against the upstream server. Fixed in tesla 1.18.3.
Affected versions
Security releases
Kodem intelligence
Severity tells you how bad this could be in the worst case. It does not tell you whether you are exposed. Exploitability and impact are functions of runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A vulnerable package can sit in your dependency tree and never run.
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Frequently Asked Questions
- What is CVE-2026-48596? CVE-2026-48596 is a low-severity security vulnerability in tesla (erlang), affecting versions >= 0.8.0, < 1.18.3. It is fixed in 1.18.3.
- Which versions of tesla are affected by CVE-2026-48596? tesla (erlang) versions >= 0.8.0, < 1.18.3 is affected.
- Is there a fix for CVE-2026-48596? Yes. CVE-2026-48596 is fixed in 1.18.3. Upgrade to this version or later.
- Is CVE-2026-48596 exploitable, and should I be worried? Whether CVE-2026-48596 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
- What actually determines whether CVE-2026-48596 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
- How do I fix CVE-2026-48596? Upgrade
teslato 1.18.3 or later.