YAML injection via unescaped newlines in ymlr document comments
Summary
Improper Neutralization of CRLF Sequences vulnerability in ufirstgroup ymlr (Elixir.Ymlr module) allows attackers to inject arbitrary content into generated YAML documents through document comments. Ymlr.document!/2 interpolates each caller-supplied comment string into the output behind a single # prefix without validating it or escaping line breaks. Because a YAML comment is terminated by a line break, the first carriage return or line feed in the comment string ends the comment context and everything after it is emitted at column 0 of the document body.
An attacker who controls text that the host application passes as a comment can forge top-level mapping keys, override values the application itself set, and emit --- or ... markers that split the output into additional documents. Downstream consumers of the generated YAML, such as configuration loaders, deployment manifests, CI pipelines and data importers, parse the injected content as legitimate data. The same clause backs Ymlr.document/2, Ymlr.documents!/2 and Ymlr.documents/2, so every document encoding entry point is affected.
This vulnerability is associated with program files lib/ymlr.ex and program routines 'Elixir.Ymlr':document!/2, 'Elixir.Ymlr':documents!/2.
This issue affects ymlr from 0.0.1 before 5.1.6.
Workaround
Split any untrusted comment string on line breaks and pass the resulting list as the comment element of the {comment, data} tuple. Ymlr prefixes every list element with its own # marker, so no line can leave the comment context. Alternatively, reject comment strings that contain carriage return or line feed characters, or keep untrusted text out of comments entirely and place it in the encoded data instead, where the encoder quotes it.
Configuration
A deployment is vulnerable when the host application places attacker-controlled text in the comment position of a document, that is, as the first element of the {comment, data} tuple passed to Ymlr.document!/2, Ymlr.document/2, Ymlr.documents!/2 or Ymlr.documents/2. Callers that encode data without comments, or that use only static comment strings, are not exploitable.
