# CVE-2026-55951: httpc memory exhaustion via unbounded response header accumulation

Status: published. Published: 2026-09-01. Updated: 2026-11-11.

The Erlang/OTP httpc HTTP client does not enforce a limit on the total size of response headers received from a server. The max_header_size option defaults to nolimit, and httpc_response:parse_headers/6 accumulates every header into a list before the length check runs (which only fires after the terminating CRLF CRLF is received).

A malicious or compromised HTTP server can send an arbitrarily large number of headers, or headers with very large values, causing the client process to allocate unbounded memory until the system runs out of memory or the BEAM VM crashes. A proof-of-concept server sending 100,000 headers of roughly 4000 bytes each caused the client VM to allocate over 13 GB of memory in under 30 seconds.

Any application using httpc:request/4,5 to connect to untrusted servers is affected. No authentication is required: any server the client connects to (including via a redirect or man-in-the-middle) can trigger the exhaustion.

This issue affects OTP from OTP 17.0 before OTP 27.3.4.17, OTP 28.5.0.6, and OTP 29.0.6, corresponding to inets from 5.10 before 9.3.2.7, 9.6.2.3, and 9.7.2. Whether OTP before OTP 17.0, corresponding to inets before 5.10, is affected is unknown.

## Remediation

- workaround: * Restrict httpc to trusted servers only, and avoid using it to fetch resources from arbitrary or user-supplied URLs.
* Apply operating-system memory limits (cgroups,ulimit) to contain the blast radius of a runaway client process.
* Use an alternative HTTP client library that enforces a response header size limit. (source: CNA: EEF)
- fix: The reporting source identifies this reference as a patch. Check the advisory for affected versions and supported release branches. (source: CNA: EEF) — https://github.com/erlang/otp/commit/aba0fe8c2d700bf4ac94607cf7f00e53bbe4042d
- fix: The reporting source identifies this reference as a patch. Check the advisory for affected versions and supported release branches. (source: CNA: EEF) — https://github.com/erlang/otp/commit/e3be1cfe9f6cedd0cd20d9905e05601dfb31c8aa

## Affected products and versions

- Erlang / OTP (otp): ≥ 17.0; < * (affected, otp); unaffected at 27.3.4.17; unaffected at 28.5.0.6; unaffected at 29.0.6. Source: CNA: EEF
- Erlang / OTP (inets): ≥ 5.10; < * (affected, otp); unaffected at 9.3.2.7; unaffected at 9.6.2.3; unaffected at 9.7.2. Source: CNA: EEF
- Erlang / OTP (erlang/otp): ≥ 84adefa331c4159d432d22840663c38f155cd4c1; < * (affected, git); unaffected at aba0fe8c2d700bf4ac94607cf7f00e53bbe4042d; unaffected at e3be1cfe9f6cedd0cd20d9905e05601dfb31c8aa; ≥ aba0fe8c2d700bf4ac94607cf7f00e53bbe4042d; < * (unaffected, git); ≥ e3be1cfe9f6cedd0cd20d9905e05601dfb31c8aa; < * (unaffected, git). Source: CNA: EEF

## Severity and prioritization

- CVSS 4.0: 8.2 (high); CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N; source: CNA: EEF

- CISA KEV: No listing in collected sources

- EPSS: 0.69% as of 2026-11-10; percentile 53.29%

- SSVC: exploitation none; automatable no; technical impact partial; source ADP: CISA-ADP

- Weaknesses: CWE-770

## Exploit evidence

No exploit evidence in collected sources. This is not proof of no exploitation.

## References

- https://github.com/erlang/otp/security/advisories/GHSA-f9fw-mg7q-4g3x (CNA: EEF; vendor-advisory)

- https://cna.erlef.org/cves/CVE-2026-55951.html (CNA: EEF; related)

- https://osv.dev/vulnerability/EEF-CVE-2026-55951 (CNA: EEF; related)

- https://www.erlang.org/doc/system/versions.html#order-of-versions (CNA: EEF; x_version-scheme)

- https://github.com/erlang/otp/commit/aba0fe8c2d700bf4ac94607cf7f00e53bbe4042d (CNA: EEF; patch)

- https://github.com/erlang/otp/commit/e3be1cfe9f6cedd0cd20d9905e05601dfb31c8aa (CNA: EEF; patch)

## Provenance

- CNA: EEF: https://www.cve.org/CVERecord?id=CVE-2026-55951; retrieved 2026-11-11; source updated 2026-11-11

- FIRST EPSS: https://api.first.org/data/v1/epss?cve=CVE-2026-55951; retrieved 2026-11-11; source updated Not reported

Revision: f19e53a41e5fa5691a0068139fa69c359093d32b02026ef9985b2a6d5edffb63

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