Tesla vulnerable to multipart part smuggling via unescaped content-disposition values
Summary
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
1. Call Tesla.Multipart.add_file_content/4 with a filename containing \r\nX-Injected: evil.
2. POST the multipart body to any upstream via any Tesla adapter.
3. The upstream receives X-Injected: evil as a standalone header line on the affected part.
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.
Workarounds
Validate disposition parameter values before passing them to the multipart API, rejecting any value that contains \r, \n, or ".
Resources
- Introduction commit: https://github.com/elixir-tesla/tesla/commit/6ebfdb9abe9c6f119408045b933d82462decd351
- Patch commit: https://github.com/elixir-tesla/tesla/commit/bb1a2c3da2775924d96e3db8e315dcc4d5d2246e
