KEVWire

Security intelligence, with context.

Response snapshot Oct 27, 2026, 17:26 UTC

Back to the wire
Permalink
VulnerabilityCVE-2025-43859

h11 accepts some malformed Chunked-Encoding bodies

A leniency in h11's parsing of line terminators in chunked-coding message bodies can lead to request smuggling vulnerabilities under certain conditions. HTTP/1.1 Chunked-Encoding bodies are formatted as a sequence of "chunks", each of which consists of:

Published Updated

At a glance

Exploited
Not known
Severity
9.1 Critical (CVSS 3.1)
Chance of exploitation
Not estimated
Sources
1 source

What to do

GitHub lists first patched versions: h11: 0.16.0. Confirm the affected range and upgrade guidance in the exact advisory.

Read GitHub-reviewed advisory GHSA-vqfr-h8mv-ghfj (opens a new tab)

What happened

### Impact

A leniency in h11's parsing of line terminators in chunked-coding message bodies can lead to request smuggling vulnerabilities under certain conditions.

### Details

HTTP/1.1 Chunked-Encoding bodies are formatted as a sequence of "chunks", each of which consists of:

Continue reading

- chunk length - `\r\n` - `length` bytes of content - `\r\n`

In versions of h11 up to 0.14.0, h11 instead parsed them as:

- chunk length - `\r\n` - `length` bytes of content - any two bytes

i.e. it did not validate that the trailing `\r\n` bytes were correct, and if you put 2 bytes of garbage there it would be accepted, instead of correctly rejecting the body as malformed.

By itself this is harmless. However, suppose you have a proxy or reverse-proxy that tries to analyze HTTP requests, and your proxy has a _different_ bug in parsing Chunked-Encoding, acting as if the format is:

- chunk length - `\r\n` - `length` bytes of content - more bytes of content, as many as it takes until you find a `\r\n`

For example, [pound](https://github.com/graygnuorg/pound/pull/43) had this bug -- it can happen if an implementer uses a generic "read until end of line" helper to consumes the trailing `\r\n`.

In this case, h11 and your proxy may both accept the same stream of bytes, but interpret them differently. For example, consider the following HTTP request(s) (assume all line breaks are `\r\n`):

``` GET /one HTTP/1.1 Host: localhost Transfer-Encoding: chunked

5 AAAAAXX2 45 0

GET /two HTTP/1.1 Host: localhost Transfer-Encoding: chunked

0 ```

Here h11 will interpret it as two requests, one with body `AAAAA45` and one with an empty body, while our hypothetical buggy proxy will interpret it as a single request, with body `AAAAXX20\r\n\r\nGET /two ...`. And any time two HTTP processors both accept the same string of bytes but interpret them differently, you have the conditions for a "request smuggling" attack.

For example, if `/two` is a dangerous endpoint and the job of the reverse proxy is to stop requests from getting there, then an attacker could use a bytestream like the above to circumvent this protection.

Even worse, if our buggy reverse proxy receives two requests from different users:

``` GET /one HTTP/1.1 Host: localhost Transfer-Encoding: chunked

5 AAAAAXX999 0 ```

``` GET /two HTTP/1.1 Host: localhost Cookie: SESSION_KEY=abcdef... ```

...it will consider the first request to be complete and valid, and send both on to the h11-based web server over the same socket. The server will then see the two concatenated requests, and interpret them as _one_ request to `/one` whose body includes `/two`'s session key, potentially allowing one user to steal another's credentials.

### Patches

Fixed in h11 0.15.0.

### Workarounds

Since exploitation requires the combination of buggy h11 with a buggy (reverse) proxy, fixing either component is sufficient to mitigate this issue.

### Credits

Reported by Jeppe Bonde Weikop on 2025-01-09.

Affected package ranges reported by GitHub: h11: < 0.16.0

Scores and how they were measured

CVSS base severity

9.1

Version 3.1As of

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:N

Measurement source (opens a new tab)

How bad it is if exploited. Not a prediction of exploitation or proof you are exposed.

EPSS probability

Not available

A model's estimate of exploitation in the next 30 days, not certainty or your own risk.

Related technology

Sources

  1. Primary source

    GitHub-reviewed advisory GHSA-vqfr-h8mv-ghfj (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-vqfr-h8mv-ghfj, 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 27). h11 accepts some malformed Chunked-Encoding bodies (CVE-2025-43859). https://kevwire.com/record/CVE-2025-43859

MLA

"h11 accepts some malformed Chunked-Encoding bodies (CVE-2025-43859)." KEVWire, 27 Oct. 2026, kevwire.com/record/CVE-2025-43859.

Chicago

KEVWire. "h11 accepts some malformed Chunked-Encoding bodies (CVE-2025-43859)." Last modified October 27, 2026. https://kevwire.com/record/CVE-2025-43859.

Evidence summary (plain text)

h11 accepts some malformed Chunked-Encoding bodies (Apr 24, 2025).
A leniency in h11's parsing of line terminators in chunked-coding message bodies can lead to request smuggling vulnerabilities under certain conditions.

HTTP/1.1 Chunked-Encoding bodies are formatted as a sequence of "chunks", each of which consists of:
CVSS 9.1 (version 3.1; evidence as of Oct 27, 2026; https://github.com/advisories/GHSA-vqfr-h8mv-ghfj).
Sources:
GitHub-reviewed advisory GHSA-vqfr-h8mv-ghfj: https://github.com/advisories/GHSA-vqfr-h8mv-ghfj
GitHub public advisory API page: https://api.github.com/advisories?type=reviewed&per_page=100&sort=updated&direction=desc
Record ID: CVE-2025-43859.
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 27, 2026, 17:25 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.

Record JSON

Your account

Email alerts

Email alerts are not available yet. To follow your stack today, use its RSS feed, which honors your filters, or connect an AI agent through the API.

Suggest an improvement

Ideas, missing sources, corrections, and bugs are all welcome. No account needed. Suggestions are reviewed by a person before anything changes.

Record details

Record detail