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CVE-2026-65636

YAML injection via unescaped newlines in ymlr document comments

Description

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.

Public exploit

An exploit or proof of concept is public: anyone can try it against an unpatched system. CISA's SSVC rates its exploitation: a proof of concept is public.

Timeline

  1. ID reserved

    By EEF

  2. · 9 days later

    Published

  3. · 1 day later

    Record updated

Changes after publication (a new score, a public exploit, EPSS crossing 1, 10 or 50 %, leaving KEV) are kept since November 2026; earlier ones are not known.

Scores

CVSS scores of CVE-2026-65636, by who scored it
Scored byCVSSScoreVector
EEFCNA (assigner)4.0Low2.1CVSS:4.0/AV:L/AC:L/AT:P/PR:N/UI:N/VC:N/VI:L/VA:N/SC:N/SI:L/SA:N
NVDNVD · Awaiting Analysis4.0Low2.1CVSS:4.0/AV:L/AC:L/AT:P/PR:N/UI:N/VC:N/VI:L/VA:N/SC:N/SI:L/SA:N/E:X/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/MVI:X/MVA:X/MSC:X/MSI:X/MSA:X/S:X/AU:X/R:X/V:X/RE:X/U:X
EPSS (exploitation in 30 days)
0.20% · top 91% of all CVEs (11 Nov 2026)
CISA SSVC
Exploitation: poc · Automatable: no · Technical impact: partial
Loading the EPSS history…

The list uses the assigner's score when there is one, then CISA's, then NVD's: Low 2.1.

Affected products

  • ufirstgroup · ymlr

    • affected 0.0.1 – < 5.1.6

  • ufirstgroup · ymlr

    • affected 0c11a86de83825e91c27cecaccb03f36416d8fe0 – < 42a0bf8b2af44b0e7c42d0b7044c8588ca5866dc

What to do

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.

References

References5

Sources

The same CVE in the primary databases and aggregators, to check against.

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Record last updated .