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Response snapshot Oct 27, 2026, 01:13 UTC

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VulnerabilityGHSA-f2hq-32vh-53cc

postal-mime: Quadratic CPU consumption when decoding a header of many joinable RFC 2047 base64 encoded words

Decoding a header that holds a long run of joinable RFC 2047 base64 encoded words, such as `=?utf-8?B?QUJD?=` repeated tens of thousands of times in a `Subject`, `From`, `To`, a filename or a description, took time quadratic in the number of words.

Published Updated

At a glance

Exploited
Not known
Severity
Not scored yet
Chance of exploitation
Not estimated
Sources
1 source

What to do

GitHub lists first patched versions: postal-mime: 4.0.8. Confirm the affected range and upgrade guidance in the exact advisory.

Read GitHub-reviewed advisory GHSA-f2hq-32vh-53cc (opens a new tab)

What happened

### Impact

Decoding a header that holds a long run of joinable RFC 2047 base64 encoded words, such as `=?utf-8?B?QUJD?=` repeated tens of thousands of times in a `Subject`, `From`, `To`, a filename or a description, took time quadratic in the number of words. The join check ran a regex over the text joined so far for every word, which flattens the accumulated string each time. A single well formed message of about 1.

6 MB blocked the event loop for one to twenty seconds depending on the host, and inline `message/rfc822` parts multiplied the cost, since each nested message carried its own header budget.

Any application that parses untrusted mail with postal-mime 3.0.0 through 4.0.7, including the exported `decodeWords()` helper, is affected. There is no confidentiality or integrity impact.

Continue reading

### Patches

Fixed in 4.0.8. The pad check now tracks the last appended word instead of testing the joined text, so decoding is linear in the header length. The same release also counts the parts and header bytes of inline `message/rfc822` sub-messages against the limits of the message as a whole, counts every header line for at least 64 bytes of `maxHeadersSize`, and adds a scaling test that measures every stage of the parser at two input sizes.

### Workarounds

Lower `maxHeadersSize` from its 2 MiB default to bound the cost per message, or parse untrusted mail in a worker that can be terminated.

### Credits

Reported by Felipe Fernández.

Affected package ranges reported by GitHub: postal-mime: >= 3.0.0, < 4.0.8

Related technology

Sources

  1. Primary source

    GitHub-reviewed advisory GHSA-f2hq-32vh-53cc (opens a new tab)

    The public advisory supplies the description, affected package/version ranges, and any listed CVSS metric. GitHub review is not local review.

    Published Checked

  2. Primary source

    GitHub public advisory API page (opens a new tab)

    The API page supplied GHSA-f2hq-32vh-53cc, its actual identifiers, source dates, and structured metrics.

    Published Date not suppliedChecked

Cite this record

For factual claims, cite the primary sources above; cite KEVWire for its compilation, dates, and classifications.

APA

KEVWire. (2026, October 23). postal-mime: Quadratic CPU consumption when decoding a header of many joinable RFC 2047 base64 encoded words. https://kevwire.com/record/GHSA-f2hq-32vh-53cc

MLA

"postal-mime: Quadratic CPU consumption when decoding a header of many joinable RFC 2047 base64 encoded words." KEVWire, 23 Oct. 2026, kevwire.com/record/GHSA-f2hq-32vh-53cc.

Chicago

KEVWire. "postal-mime: Quadratic CPU consumption when decoding a header of many joinable RFC 2047 base64 encoded words." Last modified October 23, 2026. https://kevwire.com/record/GHSA-f2hq-32vh-53cc.

Evidence summary (plain text)

postal-mime: Quadratic CPU consumption when decoding a header of many joinable RFC 2047 base64 encoded words (Oct 23, 2026).
Decoding a header that holds a long run of joinable RFC 2047 base64 encoded words, such as `=?utf-8?B?QUJD?=` repeated tens of thousands of times in a `Subject`, `From`, `To`, a filename or a description, took time quadratic in the number of words. The join check ran a regex over the text joined so far for every word, which flattens the accumulated string each time. A single well formed message of about 1.6 MB blocked the event loop for one to twenty seconds depending on the host, and inline `message/rfc822` parts multiplied the cost, since each nested message carried its own header budget.

Any application that parses untrusted mail with postal-mime 3.0.0 through 4.0.7, including the exported `decodeWords()` helper, is affected. There is no confidentiality or integrity impact.
Sources:
GitHub-reviewed advisory GHSA-f2hq-32vh-53cc: https://github.com/advisories/GHSA-f2hq-32vh-53cc
GitHub public advisory API page: https://api.github.com/advisories?type=reviewed&per_page=100&sort=updated&direction=desc
Record ID: GHSA-f2hq-32vh-53cc.
Record origin: Ingested record.
AI evidence: Not assessed.
Known exploitation: Not assessed.
Zero-day status: Not assessed. KEV membership alone does not establish zero-day status.
About this data

This record was ingested from a data source. Retrieved: Oct 26, 2026, 20:55 UTC. Ingestion is not an independent editorial investigation.

A CISA KEV listing documents known exploitation, not necessarily a zero-day; absence from the catalog does not prove a vulnerability has never been exploited.

Technology tags indicate possible relevance, not verified exposure. Confirm the affected product, version, and configuration against your own inventory; KEVWire does not scan your systems.

A personal reading checklist on this device, not verified remediation.

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