Summary
Tesla vulnerable to multipart part smuggling via unescaped content-disposition values
Tesla.Multipart.part_headers_for_disposition/1 interpolates Content-Disposition parameter values (field name, filename, and other opts) verbatim into the part header line without encoding or escaping any special characters. An attacker who controls a filename, field name, or other disposition parameter can use unescaped double-quotes to inject extra disposition key-value pairs, or CRLF sequences to inject additional part headers or prepend bytes to the part body.
Details
part_headers_for_disposition/1 in lib/tesla/multipart.ex formats each disposition parameter as k="v" with no sanitization. Values flow in from add_field/4 (the name argument), add_file/3 and add_file_content/4 (the filename argument and any disposition opts). A " in the value closes the quoted parameter early, allowing extra ; key="value" pairs to be appended. A \r\n ends the Content-Disposition header line entirely, with subsequent bytes interpreted as additional part headers (e.g. a forged Content-Type); a second \r\n ends the whole part header block and prepends attacker bytes to the part body.
The default-filename path in add_file/3 derives the name via Path.basename/1, which does not strip CR or LF, so any code that forwards a partially attacker-controlled file path is equally vulnerable.
PoC
- Call
Tesla.Multipart.add_file_content/4with a filename containing\r\nX-Injected: evil. - POST the multipart body to any upstream via any Tesla adapter.
- The upstream receives
X-Injected: evilas a standalone header line on the affected part.
Workarounds
Validate disposition parameter values before passing them to the multipart API, rejecting any value that contains \r, \n, or ".
Resources
Impact
Low severity (CVSS v4.0: 2.1). Any application using tesla 0.8.0 through 1.18.2 that passes untrusted input into add_field/4, add_file/3, or add_file_content/4 disposition parameters is affected. Consequences range from forging part-level headers to body prepending against lenient multipart parsers. 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.
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. Kodem's runtime-powered SCA identifies whether this CVE is reachable in your applications.
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Frequently Asked Questions
- What is CVE-2026-48598? CVE-2026-48598 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-2026-48598? tesla (erlang) versions >= 0.8.0, < 1.18.3 is affected.
- Is there a fix for CVE-2026-48598? Yes. CVE-2026-48598 is fixed in 1.18.3. Upgrade to this version or later.
- Is CVE-2026-48598 exploitable, and should I be worried? Whether CVE-2026-48598 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-2026-48598 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-2026-48598? Upgrade
teslato 1.18.3 or later.