EEF-CVE-2026-86696 CVE-2026-86696

Unbounded atom creation from unsigned LogoutResponse status codes in esaml allows atom table exhaustion

Not rated yet Hex

Severity: 0.0 / 10; Unknown · CVSS 4.0
Exploit chance (EPSS): 0.66%; Chance of exploitation in the next 30 days. Higher than 51.57% of scored vulnerabilities.
Known exploited: No; Not in CISA's catalog
Weakness: CWE-770 Allocation of Resources Without Limits or Throttling
Fix: None yet; No fixed version published

Description

Summary

Allocation of Resources Without Limits or Throttling vulnerability in esaml in dropbox esaml allows an unauthenticated attacker to exhaust the Erlang VM atom table and crash the node via unsigned LogoutResponse messages.

esaml:status_code_map/1 in src/esaml.erl maps any StatusCode value it does not recognize to an atom with list_to_atom/1 on the last colon-separated segment of the URN. esaml_sp:validate_logout_response/2 verifies a signature only when the message carries one, so an unsigned LogoutResponse posted to the single-logout endpoint reaches esaml:decode_logout_response/1 and mints one permanent atom per distinct status value. Atoms are never reclaimed and the VM caps the atom table at about one million entries, so an attacker who posts that many distinct values aborts the node.

This issue affects esaml: from 0.1.0 onward.

Details

1. Input. The single-logout endpoint in esaml_cowboy:validate_logout/2 reads the request body, decodes the SAMLResponse form field with esaml_binding:decode_response/2, and passes a samlp:LogoutResponse document to esaml_sp:validate_logout_response/2.

2. No signature required. validate_logout_response/2 looks for a ds:Signature element and verifies it only when one is present. An unsigned document passes through unchanged to esaml:decode_logout_response/1, which requires only the Version and IssueInstant attributes before mapping the StatusCode value.

3. Interning. esaml:status_code_map/1 matches six known SAML status URNs to fixed atoms. Any other value starting with urn: falls through to a clause that interns the last colon-separated segment with list_to_atom/1. The attacker chooses that segment, so each request with a fresh value mints one permanent atom. The VM aborts when the atom table reaches its limit of about one million entries.

Proof of concept

Build an unsigned samlp:LogoutResponse document with Version="2.0", any IssueInstant, and a samlp:Status/samlp:StatusCode whose Value is a fresh URN such as urn:x:<unique token>.
Deflate and base64-encode it, and post it to the service provider's single-logout endpoint as the SAMLResponse form field with SAMLEncoding set to the DEFLATE URI.
esaml_sp:validate_logout_response/2 skips signature verification because none is present, esaml:decode_logout_response/1 maps the status through status_code_map/1, and the atom table grows by one entry per distinct token. The endpoint answers with an error, but the atom is never reclaimed. Repeat with fresh tokens until the VM aborts.

Impact

An unauthenticated attacker posts unsigned LogoutResponse messages with distinct status code URNs to a service provider's single-logout endpoint. Each message permanently consumes one atom table entry, and about one million of them abort the Erlang VM, denying service to every user of the service provider.

Configurations

Reachable only where the service provider exposes a single-logout endpoint that hands incoming LogoutResponse messages to esaml_sp:validate_logout_response/2, directly or through esaml_cowboy:validate_logout/2. The consume (ACS) path verifies signatures before decoding and does not reach the status code mapping.

Affected versions

Hex

Package	Vulnerable	Fixed in
esaml	`>= 0.1.0`	No fix yet

How to fix

No fixed version has been published yet. Until one is, remove or replace the package, or turn off or isolate the affected feature. This page updates when a fix ships.

Severity breakdown

CVSS 4.0 0.0

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

Attack vector: Network
Attack complexity: Low
Attack requirements: Present
Privileges required: None
User interaction: None
Confidentiality (vulnerable system): None
Integrity (vulnerable system): None
Availability (vulnerable system): High
Confidentiality (subsequent systems): None
Integrity (subsequent systems): None
Availability (subsequent systems): None

Weakness

CWE-770: Allocation of Resources Without Limits or Throttling

The product allocates a reusable resource or group of resources on behalf of an actor without imposing any intended restrictions on the size or number of resources that can be allocated.

How to prevent it

Requirements: Clearly specify the minimum and maximum expectations for capabilities, and dictate which behaviors are acceptable when resource allocation reaches limits.
Architecture and Design: Limit the amount of resources that are accessible to unprivileged users. Set per-user limits for resources. Allow the system administrator to define these limits. Be careful to avoid CWE-410.
Architecture and Design: Design throttling mechanisms into the system architecture. The best protection is to limit the amount of resources that an unauthorized user can cause to be expended. A strong authentication and access control model will help prevent such attacks from occurring in the first place, and it will help the administrator to identify who is committing the abuse. The login application should be protected against DoS attacks as much as possible. Limiting the database access, perhaps by caching result sets, can help minimize the resources expended. To further limit the potential for a DoS attack, consider tracking the rate of requests received from users and blocking requests that exceed a defined rate threshold.
Implementation: Assume all input is malicious. Use an "accept known good" input validation strategy, i.e., use a list of acceptable inputs that strictly conform to specifications. Reject any input that does not strictly conform to specifications, or transform it into something that does. When performing input validation, consider all potentially relevant properties, including length, type of input, the full range of acceptable values, missing or extra inputs, syntax, consistency across related fields, and conformance to business rules. As an example of business rule logic, "boat" may be syntactically valid because it only contains alphanumeric characters, but it is not valid if the input is only expected to contain colors such as "red" or "blue." Do not rely exclusively on looking for malicious or malformed inputs. This is likely to miss at least one undesirable input, especially if the code's environment changes. This can give attackers enough room to bypass the intended validation. However, denylists can be useful for detecting potential attacks or determining which inputs are so malformed that they should be rejected outright.
Architecture and Design: For any security checks that are performed on the client side, ensure that these checks are duplicated on the server side, in order to avoid CWE-602. Attackers can bypass the client-side checks by modifying values after the checks have been performed, or by changing the client to remove the client-side checks entirely. Then, these modified values would be submitted to the server.
Architecture and Design: Mitigation of resource exhaustion attacks requires that the target system either: recognizes the attack and denies that user further access for a given amount of time, typically by using increasing time delays uniformly throttles all requests in order to make it more difficult to consume resources more quickly than they can again be freed. The first of these solutions is an issue in itself though, since it may allow attackers to prevent the use of the system by a particular valid user. If the attacker impersonates the valid user, they may be able to prevent the user from accessing the server in question. The second solution can be difficult to effectively institute -- and even when properly done, it does not provide a full solution. It simply requires more resources on the part of the attacker.
Architecture and Design: Ensure that protocols have specific limits of scale placed on them.
Architecture and Design, Implementation: If the program must fail, ensure that it fails gracefully (fails closed). There may be a temptation to simply let the program fail poorly in cases such as low memory conditions, but an attacker may be able to assert control before the software has fully exited. Alternately, an uncontrolled failure could cause cascading problems with other downstream components; for example, the program could send a signal to a downstream process so the process immediately knows that a problem has occurred and has a better chance of recovery. Ensure that all failures in resource allocation place the system into a safe posture.
Operation, Architecture and Design: Use quotas or other resource-limiting settings provided by the operating system or environment. For example, when managing system resources in POSIX, setrlimit() can be used to set limits for certain types of resources, and getrlimit() can determine how many resources are available. However, these functions are not available on all operating systems. When the current levels get close to the maximum that is defined for the application (see CWE-770), then limit the allocation of further resources to privileged users; alternately, begin releasing resources for less-privileged users. While this mitigation may protect the system from attack, it will not necessarily stop attackers from adversely impacting other users. Ensure that the application performs the appropriate error checks and error handling in case resources become unavailable (CWE-703).

References

Packages

hex.pm/packages/esaml

Timeline

Oct 22, 2026 Published
Oct 30, 2026 Last updated by the source
Oct 31, 2026 Latest EPSS score

Advisory	Severity	Why it's related
EEF-CVE-2026-93471 Unbounded SAML message decompression in esaml allows memory exhaustion	Not rated yet	Also affects esaml
EEF-CVE-2026-105270 Unbounded atom creation while parsing SAML messages in esaml allows atom table exhaustion	Not rated yet	Also affects esaml
EEF-CVE-2026-28809 XXE in esaml SAML library allows local file read and potential SSRF	Medium	Also affects esaml
GHSA-62m5-6wh8-28x7 pypdf has possible long runtimes/large memory usage for encrypted PDF files	High	Same weakness, CWE-770
GHSA-9wfm-jcj8-jfv9 russh's SSH agent server is susceptible to unbound allocations via repeated SSH_AGENT_CONSTRAIN_CONFIRM messages	Low	Same weakness, CWE-770
GHSA-4p9f-7p9m-5vfg russh's SSH client is susceptible to unbounded allocations via EXT_INFO packets	Medium	Same weakness, CWE-770
GHSA-x3rh-gjgm-fmqc pypdf has possible large memory usage when parsing cross-reference streams	High	Same weakness, CWE-770
GHSA-8397-8qf6-f2gm vLLM: Phi-4 multimodal audio encoder materializes and retains full attention matrices: a single max-length audio OOM-crashes the vLLM engine	Medium	Same weakness, CWE-770

Credits: Jonatan Männchen / EEF, Quinn Wilton

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