GitHub fine-grained personal access token: format and detection

What it is

Fine-grained tokens are scoped to chosen repositories and permissions. Maskflare's Credentials pack has a detector for them next to the classic ghp_ one, masked by default.

In Maskflare this is the detector github_fine_grained_pat, labelled "GitHub fine grained PAT", category password or access key, in the Credentials pack.

Format

• The literal prefix github_pat_.
• 82 characters from letters, digits and underscore.

How Maskflare recognises it

Prefix, exact length of the body and the entropy floor of 3.0. The prefix itself contains underscores, so a keyword prefilter on github_pat_ keeps this detector from running on most text.

Why this page shows no sample key

A realistic sample key would look like a live credential to secret scanners and to anyone skimming the page, and an obviously fake one would not pass this detector's length and entropy rules. The format above is enough to recognise the key: the prefix, the length and the character set.

Regex and validation caveats

• There is no closing boundary: if more word characters follow the 82, only the first 82 are matched and the rest stays in the text.
• The classic ghp_ token is a different shape and different detector.

How to detect it in LLM traffic

Fine-grained tokens are often created for a single repository and then pasted into CI variables, agent configuration and scripts that end up in prompts.

Maskflare reads the whole request body, including the system prompt, earlier messages and tool arguments, and replaces each match with a token such as mf_mask_d7e0 before the request goes out. Coding agents are covered through hooks that mask tool inputs and results, so a key printed by a shell command does not travel onward in the next prompt. On ChatGPT, browser protection blocks prompts that contain API keys, tokens or private keys outright.

If you build the detection yourself

1. Scan the raw text rather than parsed fields: keys sit in code, JSON strings, headers, shell output and pasted logs, often with no label at all.
2. Match the prefix and the exact length for the GitHub fine-grained personal access token. Where the key has no fixed prefix, require the provider's name and an assignment next to the value instead of matching any long string.
3. Compute the Shannon entropy of the candidate and reject low values, and keep an allow list of placeholders (${KEY}, repeated characters, the alphabet run), because documentation and templates are full of them.
4. Mask the value before the text is sent anywhere. If the key was already sent, treat it as exposed and rotate it; masking later does not recall it.

How Maskflare handles it

• Protect now, Finance - Protect now, Healthcare, Developer and Monitor first: mask (a reversible token).

The available actions are mask (reversible token), redact, partial, hash, format-preserving encryption, generalize, alert and block. Masked values are kept encrypted for 30 days so that answers can be restored, then they expire; prompts and responses are not logged. The full list of detectors is in the detector reference.

Compliance profiles that include it

All 6 compliance profiles include the Credentials pack, and applying them turns this detector on: HIPAA Safe Harbor, GDPR - EU/EEA, UK GDPR, PCI DSS, US personal data and Switzerland - nDSG health data. A profile is a floor: it never turns anything off or weakens a rule that is already stricter.