Oh My Posh: Terminal escape sequence injection via unsanitized prompt segment data

Summary

Oh My Posh renders dynamic, potentially attacker-controlled strings (the current directory name, Git commit metadata, environment variable values, command output) into the prompt without neutralizing raw terminal control characters. An attacker who controls one of these values can inject ANSI/OSC escape sequences that the victim's terminal executes on every prompt render. (This is separate from the path-segment command-execution report; it has a different root cause and fix.)

Details

src/terminal/writer.go, write(s rune): the literal characters of rendered segment content are emitted to the output buffer, and the only neutralization is a lookup in formats.EscapeSequences, which per shell contains only the shell's prompt-length markers (\ for bash, % for zsh, nothing for fish/pwsh/cmd/nu). Raw C0/C1 control bytes (ESC 0x1b, BEL 0x07, CSI, OSC) are written verbatim.

Oh My Posh's own styling is emitted through separate paths (writeEscapedAnsiString, writeColorise, builder.WriteString(formats.Hyperlink...)), so any control rune reaching write(s rune) originates from rendered data. trimAnsi is already applied to the console title (FormatTitle) but not to the prompt body.

Attacker-controlled sources (both verified)

1. Current directory name (default config, Linux/macOS). Directory names may contain any byte except / and NUL, including 0x1b. The path segment renders the working directory in every theme.
2. Git commit subject / author / upstream URL (cross-platform, including Windows). Git.Commit() runs git log -1 --pretty=format:...su:%s... and exposes .Commit.Subject, .Commit.Author.Name/.Email and .RawUpstreamURL; Git imposes no restriction on these, so they can carry raw escape sequences.

PoC

Git commit-subject vector (config: a single git segment with template {{ .Commit.Subject }}):

printf 'feat: \033]0;HACKED\007\033]52;c;ZWNobyBQV05FRA==\007 update' > msg.txt
git commit --allow-empty -F msg.txt
oh-my-posh print primary --config poc.omp.json --shell fish | xxd

Output (excerpt) shows the attacker's OSC 0 (set title) and OSC 52 (clipboard write) passed through unmodified, wrapped only in Oh My Posh's colors:

...255m feat: 1b5d 303b 4841 434b 4544 07 1b5d 3532 3b63 3b5a 574e ... 07 ...
              ESC ] 0 ; H A C K E D  BEL  ESC ] 5 2 ; c ; <base64>  BEL

The directory-name vector reproduces identically via the path segment.

Impact

The terminal interprets the injected sequences on every render, enabling, per emulator: clipboard hijacking (OSC 52 write, so a command placed in the clipboard runs when the victim pastes), prompt/screen spoofing, window-title manipulation, and terminal denial of service. Delivery is remote (repo/archive/share); execution is local when the prompt renders after cd.

Suggested fix: neutralize C0/C1 control characters in untrusted segment data before it reaches the terminal, at minimum the path segment (folder names) and the git segment (Commit.Subject, Commit.Author.*, RawUpstreamURL). trimAnsi already exists and is applied to the title; extending equivalent handling to segment data closes the gap. Filtering control runes in write(s rune) would also work as defense in depth (Oh My Posh's own styling never flows through that function), optionally behind an opt-out for users who deliberately embed escapes in their own templates.